Ebook Thyroid diseases - Pathogen esis, diagnosis, and treatment: Part 1

Part 1 book “Thyroid diseases - Pathogenesis, diagnosis, and treatment” has contents: Regulation of thyroid function, synthesis, and function of thyroid hormones, tests of thyroid function, thyroid autoantibodies, nonisotopic thyroid imaging, nontoxic goiter, thyroid nodule, acute and subacute thyroiditis,… and other contents.

Series Editor: Andrea Lenzi

Series Co-Editor: Emmanuele A Jannini

Thyroid

Diseases

Paolo Vitti

Laszlo Hegedüs Editors

Pathogenesis, Diagnosis, and Treatment

but continuously new science is the study of the various hormones and their actionsand disorders in the body The matter of Endocrinology are the glands, i.e., theorgans that produce hormones, active on the metabolism, reproduction, food absorp-tion and utilization, growth and development, behavior control, and several othercomplex functions of the organisms Since hormones interact, affect, regulate, andcontrol virtually all body functions, Endocrinology not only is a very complexscience, multidisciplinary in nature, but is one with the highest scientific turnover.Knowledge in the Endocrinological sciences is continuously changing and growing.

In fact, thefield of Endocrinology and Metabolism is one where the highest number

of scientific publications continuously flourishes The number of scientific journalsdealing with hormones and the regulation of body chemistry is dramatically high.Furthermore, Endocrinology is directly related to genetics, neurology, immunology,rheumatology, gastroenterology, nephrology, orthopedics, cardiology, oncology,gland surgery, psychology, psychiatry, internal medicine, and basic sciences Allthese fields are interested in updates in Endocrinology The aim of the MRW inEndocrinology is to update the Endocrinological matter using the knowledge of thebest experts in each section of Endocrinology: basic endocrinology, neuroendocri-nology, endocrinological oncology, pancreas with diabetes and other metabolicdisorders, thyroid, parathyroid and bone metabolism, adrenals and endocrine hyper-tension, sexuality, reproduction, and behavior

More information about this series athttp://www.springer.com/series/14021

Paolo Vitti • Laszlo Hegedüs

Editors

Thyroid Diseases

Pathogenesis, Diagnosis, and Treatment

With 117 Figures and 101 Tables

Odense University HospitalOdense, Denmark

ISBN 978-3-319-45012-4 ISBN 978-3-319-45013-1 (eBook)

ISBN 978-3-319-46284-4 (print and electronic bundle)

https://doi.org/10.1007/978-3-319-45013-1

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# Springer International Publishing AG, part of Springer Nature 2018

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Is there an unmet need for a new MRW series in Endocrinology and Metabolism? Itmight not seem so! The vast number of existing textbooks, monographs andscientific journals suggest that the field of hormones (from genetic, molecular,biochemical and translational to physiological, behavioral, and clinical aspects) isone of the largest in biomedicine, producing a simply huge scientific output.However, we are sure that this new Series will be of interest for scientists, academics,students, physicians and specialists alike.

The knowledge in Endocrinology and Metabolism almost limited to thetwo main (from an epidemiological perspective) diseases, namely hypo/hyperthy-roidism and diabetes mellitus, now seems outdated and closer to the interests

of the general practitioner than to those of the specialist This has led to ogy and metabolism being increasingly considered as a subsection of internalmedicine rather than an autonomous specialization But endocrinology is muchmore than this

endocrinol-We are proposing this series as the manifesto for“Endocrinology 2.0”, ing thefields of medicine in which hormones play a major part but which, for varioushistorical and cultural reasons, have thus far been “ignored” by endocrinologists.Hence, this MRW comprises “traditional” (but no less important or investigated)topics: from the molecular actions of hormones to the pathophysiology and man-agement of pituitary, thyroid, adrenal, pancreatic and gonadal diseases, as well asless common arguments Endocrinology 2.0 is, in fact, the science of hormones, but

embrac-it is also the medicine of sexualembrac-ity and reproduction, the medicine of genderdifferences and the medicine of wellbeing These aspects of Endocrinology have

to date been considered of little interest, as they are young and relatively unexploredsciences But this is no longer the case The large scientific production in these fieldscoupled with the impressive social interest of patients in these topics is stimulating anew and fascinating challenge for Endocrinology

The aim of the MRW in Endocrinology is thus to update the subject with theknowledge of the best experts in eachfield: basic endocrinology, neuroendocrinol-ogy, endocrinological oncology, pancreatic disorders, diabetes and other metabolicdisorders, thyroid, parathyroid and bone metabolism, adrenal and endocrine

v

hypertension, sexuality, reproduction and behavior We are sure that this ambitiousaim, covering for thefirst time the whole spectrum of Endocrinology 2.0, will befulfilled in this vast Springer MRW in Endocrinology Series.

Andrea Lenzi, M.D.Series EditorEmmanuele A Jannini, M.D

Series Co-Editor

Despite the availability of a number of endocrine textbooks, we believe that there is aneed for a comprehensive volume on the thyroid, which can be used by trainees,general endocrinologist, and experts in thefield alike The present one, available as abook, but also electronically, and intended to be updated online whenever develop-ments warrant this, in our mindsfills a void Covering the majority of clinicallyrelevant phenotypes, and thyroid physiology as well as pathogenesis, diagnosis, andtreatment, we have been fortunate to profit from the dedicated service of a number ofrecognized and authoritative experts Challengingly, being the first in a series ofvolumes intended to eventually cover all of endocrinology, there has been littleprecedence to learn from and to use as a point de référence.

The original idea for this volume was nurtured in the garden of the Italian Society

of Endocrinology and was then developed together with another European asco-editor The cutting-edge information is provided by a bouquet of mainlyEuropean colleagues but also some from the USA, Canada, and Australia Withthis in mind, we can only hope that the many instruments, or at least their players,orchestrated a tune that to the readers appears as one harmonious melody and not acacophony of contradictions and overlaps The two conductors have agreed andfollowed the same score, and a strong friendship has developed over the nearly

3 years from conception to delivering afinal product

The first of seven sections covers regulation of thyroid function, synthesis ofthyroid hormones, and their mechanism of action To be followed by thyroid testsand imaging and moving into thyroid diseases highlighting basic concepts, clinicaldiagnoses, and management in greater detail The solitary thyroid nodule andmultinodular goiter constitute the second part, while the next covers Hashimoto’sand the other types of thyroiditis The fourth section deals with different types ofhypothyroidism, while thyrotoxicosis and hyperthyroidism, autoimmune andnon-autoimmune, are covered in thefifth section Here, also the newest development

in the diagnosis and management of the very challenging and complex Graves’ophthalmopathy is given much attention The section on thyroid carcinomas, inaddition to describing the various histiotypes, discusses pathogenesis and the newestmedical therapies as well as the most recently elucidated molecular mechanisms andgenetic defects In the last section, a number of other conditions influencing thyroidfunction, or inducing thyroid dysfunction, come in focus Pregnancy, nonthyroidal

vii

illness, and the ever expanding and complicated effects of drugs and other stances that influence and interfere with thyroid function are offered competentattention A separate pharmacopeia has not been deemed necessary However,pharmacological treatments are included in the chapters where relevant.

sub-Just a few weeks after delivering his chapter on“Regulation of Thyroid Function,Synthesis and Function of Thyroid Hormone” the chocking message of the untimelypassing away of professor Theo J Visser reached us It is with sadness, but also greataffection and respect, that we dedicate this book to the memory of a gentle,humorous giant of a researcher and colleague

Needless to say, we are indebted to all our colleagues who have kindly andgenerously dedicated time and enthusiasm to contribute so competently to thistext As ever, our thanks go to our families for their patience and support duringour absence with this endeavor at mind

Pisa and Odense

April 2018

Paolo Vitti, M.D., Ph.D.Laszlo Hegedüs, M.D., D.M.Sc

Part I Thyroid Physiology and Laboratory Evaluation of the

Thyroid 1

1 Regulation of Thyroid Function, Synthesis, and Function of

Thyroid Hormones 3Theo J Visser

2 Tests of Thyroid Function 33Giovanni Ceccarini, Ferruccio Santini, and Paolo Vitti

3 Thyroid Autoantibodies 57

R A Ajjan and A P Weetman

4 Nonisotopic Thyroid Imaging 89

E Papini, R Guglielmi, G Bizzarri, and A Frasoldati

Part II Goiter and Thyroid Nodule 125

5 Nontoxic Goiter 127Steen Joop Bonnema and Laszlo Hegedüs

6 Thyroid Nodule 165Markus Eszlinger, Laszlo Hegedüs, and Ralf Paschke

Part III Thyroiditis 203

7 Hashimoto’s Thyroiditis 205Wilmar M Wiersinga

8 Postpartum Thyroiditis and Silent Thyroiditis 249Lakdasa D Premawardhana, Onyebuchi E Okosieme, and John H

Lazarus

9 Acute and Subacute Thyroiditis 277Karen M Rothacker and John P Walsh

ix

Part IV Hypothyroidism 299

10 Classification and Etiopathogenesis of Hypothyroidism 301Luca Chiovato, Stefano Mariotti, and Flavia Magri

11 Congenital Hypothyroidism 333Caterina Di Cosmo and Massimo Tonacchera

12 Central Hypothyroidism 373Andrea Lania, Claudia Giavoli, and Paolo Beck-Peccoz

13 Diagnosis and Treatment of Hypothyroidism 391Suhel Ashraff and Salman Razvi

Part V Hyperthyroidism and Thyrotoxicosis 427

14 Graves’ Disease 429Catherine Napier and Simon H S Pearce

15 Graves’ Ophthalmopathy 451Claudio Marcocci and Terry J Smith

16 Treatment of Graves’ Disease 489Luigi Bartalena

17 Toxic Adenoma and Multinodular Toxic Goiter 513Massimo Tonacchera and Dagmar Führer

Part VI Thyroid Carcinoma 541

18 Pathogenesis of Thyroid Carcinoma 543Massimo Santoro and Francesca Carlomagno

19 Differentiated Thyroid Carcinoma of Follicular Origin 563Furio Pacini, Maria Grazia Castagna, and Martin Schlumberger

20 Medullary Carcinoma 589Rossella Elisei and Barbara Jarzab

21 Anaplastic and Other Forms of Thyroid Carcinoma 629Leonard Wartofsky

22 New (Medical) Treatment for Thyroid Carcinoma 645Sebastiano Filetti and Steven I Sherman

Part VII Other Conditions Influencing Thyroid Function or InducingThyroid Dysfunction 671

23 Thyroid Physiology and Thyroid Diseases in Pregnancy 673Bijay Vaidya and Shiao-Yng Chan

24 Non-thyroidal Illness 709Theodora Pappa and Maria Alevizaki

25 Drugs and Other Substances Interfering with Thyroid

Function 733Lucia Montanelli, Salvatore Benvenga, Laszlo Hegedüs,

Paolo Vitti, Francesco Latrofa, and Leonidas H Duntas

Index 763

Professor Laszlo Hegedüs, 65, is trained in gen, Denmark In 1992, having defended his thesis, hebecame a consultant physician at the Department ofEndocrinology in Odense, Denmark Here he built up

Copenha-a thyroid reseCopenha-arch group Copenha-and becCopenha-ame Copenha-a Full Professor

in 2006

Publications, more than 480, include chapters inleading endocrine textbooks and include diagnosticimaging; radioiodine therapy; ultrasound-guided diag-nosis and treatment of thyroid diseases; environmentaland hereditary aspects of the regulation of thyroid func-tion and size; environmental and genetic background forthyroid disorders utilizing Danish twins; and late effects

of thyroid diseases Developing and utilizing ThyPRO

to evaluate thyroid-related quality of life and the role ofselenium in thyroid diseases are also in focus

Dr Hegedüs has served as editor of Clinical crinology and as editorial board member of a number ofjournals He has mentored around 50 individualsdefending their academic theses He is President-Elect

Endo-of the European Thyroid Association and a former ident of the Danish Thyroid Association He is exten-sively used at endocrine and thyroid teaching coursesand has given more than 200 invited talks ProfessorHegedüs is the recipient of several prizes, including theGeorge Murray lecture and the Pitt-Rivers lecture fromthe British Thyroid Association and the Frontiers inScience Award from the American Association ofClinical Endocrinologists

Pres-xiii

Paolo Vitti is Full Professor of Endocrinology andDirector of the School of Endocrinology at the MedicalSchool, University of Pisa, Italy, and Chief of Endocri-nology at the University Hospital of Pisa Dr Vitti is apast member of the Executive Committee of theEuropean Thyroid Association and a past Secretary ofthe Italian Thyroid Association He was appointed withthe European Thyroid Association – Merck SeronoPrize 2014 He has been a member of the InternationalCouncil for the Control of Iodine Deficiency Disorders(ICCIDD) since 1996 and an ICCIDD board membersince 2003 He has also been Deputy-Regional Coordi-nator for the ICCIDD in West Central Europe since 2001and a member of expert WHO/ICCIDD teams for exter-nal reviews in Kenya, Macedonia, and Peru He iscurrently the President of Italian Society of Endocrinol-ogy He is the author of over 150 articles, most of whichhave been published in authoritative internationaljournals, and of a number of chapters of internationaland national endocrine textbooks.

R A Ajjan Division of Cardiovascular and Diabetes Research, University ofLeeds, Leeds Institute for Cardiovascular and Metabolic Medicine (LICAMM),Leeds, UK

Maria Alevizaki Endocrine Unit, Department of Medical Therapeutics, AlexandraHospital, School of Medicine, National Kapodistrian University, Athens, GreeceSuhel Ashraff Institute of Genetic Medicine, Central Parkway, Newcastle Univer-sity, Newcastle upon Tyne, UK

Luigi Bartalena Department of Medicine and Surgery, University of Insubria,Endocrine Unit, ASST dei Sette Laghi, Ospedale di Circolo, Varese, Italy

Paolo Beck-Peccoz University of Milan, Milano, Italy

Salvatore Benvenga Department of Clinical and Experimental Medicine, Master

Interdepartmental Program of Molecular and Clinical Endocrinology, and Women’sEndocrine Health, University of Messina and A.O.U Policlinico G Martino,Messina, Italy

G Bizzarri Department of Diagnostic Imaging, Ospedale Regina Apostolorum,Albano (Rome), Italy

Steen Joop Bonnema Department of Endocrinology, Odense University Hospital,

xv

Luca Chiovato Department of Internal Medicine and Medical Therapy, University

of Pavia - Unit of Internal Medicine and Endocrinology, IRCCS ICS Maugeri, Pavia,Italy

Caterina Di Cosmo Department of Clinical and Experimental Medicine, nology Unit, University Hospital of Pisa, University of Pisa, Pisa, Italy

Endocri-Leonidas H Duntas Evgenideion Hospital, Unit of Endocrinology, Diabetes andMetabolism, University of Athens, Athens, Greece

Rossella Elisei Endocrine Unit, Department of Clinical and ExperimentalMedicine, University Hospital, University of Pisa, Pisa, Italy

Markus Eszlinger Department of Oncology and Arnie Charbonneau CancerInstitute, Cumming School of Medicine, University of Calgary, Calgary, AB,Canada

Sebastiano Filetti Department of Internal Medicine and Medical Specialties, versity of Rome Sapienza, Rome, Italy

Uni-A Frasoldati Department of Endocrinology, Ospedale Santa Maria Nuova IRCCS,Reggio Emilia, Italy

Dagmar Führer Department of Endocrinology, Diabetes and Metabolism, sity Hospital Essen, University Duisburg-Essen, Essen, Germany

Univer-Claudia Giavoli Endocrinology and Metabolic Diseases Unit, Fondazione IRCCS

Cà Granda-Ospedale Maggiore, Milano, Italy

R Guglielmi Department of Endocrinology and Metabolism, Ospedale ReginaApostolorum, Albano (Rome), Italy

Laszlo Hegedüs Department of Endocrinology and Metabolism, Odense sity Hospital, Odense, Denmark

Univer-Barbara Jarzab Department of Nuclear Medicine and Endocrine Oncology, MariaSklodowska-Curie Memorial Institute – Cancer Center, Gliwice Branch, Gliwice,Poland

Andrea Lania Department of Medical Sciences, Humanitas University andEndocrinology Unit, Humanitas Research Hospital, Rozzano, Italy

Francesco Latrofa Department of Clinical and Experimental Medicine, nology Unit 1, University Hospital of Pisa, Pisa, Italy

Endocri-John H Lazarus Thyroid Research Group, Institute of Molecular and tal Medicine, School of Medicine, Cardiff University, Cardiff, UK

Experimen-Flavia Magri Department of Internal Medicine and Medical Therapy, University ofPavia - Unit of Internal Medicine and Endocrinology, IRCCS ICS Maugeri, Pavia,Italy

Claudio Marcocci Department of Clinical and Experimental Medicine, University

of Pisa, Pisa, Italy

Stefano Mariotti Departmet of Medical Sciences and Public Health, University ofCagliari, Monserrato, CA, Italy

Lucia Montanelli Department of Clinical and Experimental Medicine, nology Unit 1, University Hospital of Pisa, Pisa, Italy

Endocri-Catherine Napier Institute of Genetic Medicine, Newcastle University, Newcastleupon Tyne, UK

Onyebuchi E Okosieme Endocrine and Diabetes Department, Prince CharlesHospital, Cwm Taf University Health Board, Merthyr Tydfil, UK

Thyroid Research Group, Institute of Molecular and Experimental Medicine, School

of Medicine, Cardiff University, Cardiff, UK

Furio Pacini Department of Medical, Surgical and Neurological Sciences, University

of Siena, Siena, Italy

E Papini Department of Endocrinology and Metabolism, Ospedale ReginaApostolorum, Albano (Rome), Italy

Theodora Pappa Section of Endocrinology, Diabetes and Metabolism, ment of Medicine, The University of Chicago, Chicago, IL, USA

Depart-Ralf Paschke Section of Endocrinology and Metabolism, Departments of cine, Oncology, Biochemistry and Molecular Biology and Arnie CharbonneauCancer Institute, Cumming School of Medicine, University of Calgary, Calgary,

Martin Schlumberger Department of Nuclear Medicine and Endocrine Oncology,Gustave Roussy and Université Paris Saclay, Villejuif, France

Steven I Sherman Department of Endocrine Neoplasia and Hormonal Disorders,University of Texas MD Anderson Cancer Center, Houston, TX, USA

Terry J Smith Department of Ophthalmology and Visual Sciences Kellogg EyeCenter and Division of Metabolism, Endocrinology, and Diabetes, Department ofInternal Medicine, University of Michigan Medical School, Ann Arbor, MI, USAMassimo Tonacchera Department of Clinical and Experimental Medicine,Endocrinology Unit, University Hospital of Pisa, University of Pisa, Pisa, ItalyBijay Vaidya Department of Endocrinology, Royal Devon and Exeter Hospital,University of Exeter Medical School, Exeter, UK

Theo J Visser Department of Internal Medicine, Erasmus University MedicalCenter, Rotterdam, The Netherlands

Paolo Vitti Department of Endocrinology, University of Pisa, Pisa, Italy

John P Walsh Department of Endocrinology and Diabetes, Sir Charles GairdnerHospital, Nedlands, WA, Australia

School of Medicine and Pharmacology, The University of Western Australia,Crawley, WA, Australia

Leonard Wartofsky Department of Endocrinology, MedStar Washington HospitalCenter, Georgetown University School of Medicine, Washington, DC, USA

A P Weetman School of Medicine and Biomedical Sciences, University of field, Sheffield, UK

Shef-Wilmar M Wiersinga Department of Endocrinology and Metabolism, AcademicMedical Center, University of Amsterdam, Amsterdam, The Netherlands

Thyroid Physiology and Laboratory Evaluation

of the Thyroid

Regulation of Thyroid Function, Synthesis,

Theo J Visser

Contents

Introduction 4

Regulation of Thyroid Function 5

TRH 5

TSH 7

Thyrostimulin 8

Biosynthesis of Thyroid Hormone 8

Iodide Uptake 9

Thyroid Hormone Synthesis 10

Formation of Iodothyronines 11

Release of Thyroid Hormone 11

Inhibitors of Thyroid Hormone Production and/or Secretion 12

Transport of Thyroid Hormone 13

Plasma Transport 13

Tissue Transport 14

Metabolism of Thyroid Hormone 17

Deiodination 17

Alanine Side Chain Modification 20

Sulfation 21

Glucuronidation 21

Thyroid Hormone Actions 22

Mechanism of T3 Action 22

T3 Inhibition of TSH and TRH Gene Expression 25

References 26

This chapter is adapted from Visser ( 2011 ).

T J Visser ( * )

Department of Internal Medicine, Erasmus University Medical Center, Rotterdam, The Netherlands e-mail: t.j.visser@erasmusmc.nl

# Springer International Publishing AG, part of Springer Nature 2018

P Vitti, L Hegedüs (eds.), Thyroid Diseases, Endocrinology,

https://doi.org/10.1007/978-3-319-45013-1_1

3

Thyroid hormone (TH) is a common name for the two products secreted bythyroid follicles, namely the prohormone thyroxine (T4) as the major product andthe active hormone triiodothyronine (T3) as the minor product Most T3 isproduced by deiodination of T4 in peripheral tissues TH is essential for thedevelopment of different tissues, in particular the central nervous system, and forthe function of the tissues throughout life The secretion of thyroid hormone isregulated within the hypothalamus-pituitary thyroid axis, but the biologicalactivity of thyroid hormone is largely regulated at the level of the target tissues.This chapter covers various aspects of (a) the neuroendocrine regulation ofthyroid function, (b) the biosynthesis of thyroid hormone, in particular T4, (c) thelocal regulation of bioactive TH in target tissues, and (d) the mechanisms bywhich T3 exerts its biological activities

of thyroid hormone (TH) in target tissues largely depends on the supply of plasmaT4 and T3, the activity of transporters mediating the cellular uptake and/or efflux

of these hormones, as well as the activity of deiodinases catalyzing the activation

or inactivation of TH Thyroid function is under positive control of stimulating hormone (TSH), also called thyrotropin In turn, TSH secretion fromthe anterior pituitary is stimulated by the hypothalamic factor thyrotropin-releasing hormone (TRH) TSH secretion is downregulated by negative feedbackaction of TH on the hypothalamus and the pituitary The contribution of locallyproduced T3 versus uptake of plasma T3 is much greater for some tissues such asthe brain and the pituitary than for other tissues Although serum TSH is animportant parameter for the diagnosis of thyroid dysfunction, it is not representa-tive for the thyroid state of all tissues In this chapter various aspects will bediscussed of (a) the neuroendocrine regulation of thyroid function, (b) the biosyn-thesis of TH (i.e., the prohormone T4), (c) the local regulation of bioactive TH intarget tissues, and (d) the mechanisms by which T3 exerts its biological activities

thyroid-A schematic overview of the hypothalamus-pituitary-thyroid-periphery axis ispresented in Fig.1

Regulation of Thyroid Function

TRH

TRH is a tripeptide, pyroglutamyl-histidyl-prolineamide (pGlu-His-ProNH2) in whichthe C-terminal carboxyl group is blocked by amidation and the N-terminalα-aminogroup is blocked by cyclization (Joseph-Bravo et al.2015) Besides stimulating TSHsecretion, TRH also stimulates prolactin secretion from the anterior pituitary TRH isnot only produced in the hypothalamus but is widely distributed throughout the centralnervous system It is also detected in the posterior pituitary and in different peripheraltissues, such as the heart, adrenal, ovary, testis, uterus, and placenta

Hypophysiotropic TRH is produced in neurons, the cell bodies of which are located

in the paraventricular nucleus (PVN) of the hypothalamus (Fliers et al 2006) Thesynthesis of TRH involves the production of a large precursor protein (proTRH).Human proTRH consists of 242 amino acids and contains 6 copies of the TRHprogenitor sequence Gln-His-Pro-Gly,flanked at both sides by pairs of basic aminoacids (Arg, Lys) Cleavage of proTRH at the basic amino acids by prohormoneconvertases and further removal of remaining basic residues by carboxypeptidases result

in the liberation of the progenitor sequences A specific glutaminyl cyclase catalyzes theformation of the pGlu ring, and peptidylglycine-α-amidating-monooxygenase (PAM)converts Pro-Gly to ProNH2, ultimately yielding mature TRH (Perello and Nillni2007).The processing of proTRH takes place in vesicles which transport mature TRH andintervening peptides along the axons of the TRH neurons to the median eminence (ME),where they are released into the portal vessels of the hypophyseal stalk Some of theintervening peptides may have biological activities (Perello and Nillni2007)

TRH is transported through the hypophyseal stalk to the anterior lobe of thepituitary, where it stimulates the production and secretion of TSH (and prolactin).These actions of TRH are initiated by its binding to the TRH receptor (TRHR) which

Fig 1 Schematic of the

regulation of the production

and metabolism of thyroid

hormone in the

hypothalamus-pituitary-thyroid-periphery

axis, showing the liver as a

major T3-producing tissue

is expressed on both the thyrotroph (TSH-producing cell) and the lactotroph(prolactin-producing cell) (Sun et al.2003) This receptor belongs to the family of Gprotein-coupled receptors, characteristically containing seven transmembranedomains Human TRHR is a protein consisting of 398 amino acids, and binding ofTRH induces a change in its interaction with the trimeric G protein, resulting in thestimulation of phospholipase C (PLC) activity The activated PLC catalyzes thehydrolysis of phosphatidylinositol 4,5-diphosphate to the second messengers inositol1,4,5-trisphosphate and diacylglycerol, which initiate a cascade of reactions, including

an increase in cellular Ca2+levels and protein kinase C (PKC) activity, that ultimatelystimulates the synthesis as well as the release of TSH (and prolactin) (Joseph-Bravo

et al.2016) Stimulation of TSHβ gene expression by TRH depends on the presence ofthe pituitary-specific transcription factor POU1F1, previously known as Pit-1

In addition to the anterior pituitary, TRHR is also expressed in different peripheraltissues, including the brain, heart, ovary, uterus, and thyroid Some of these tissuesalso express TRH, suggesting that TRH has also actions outside the anterior pituitary.The expression of TRH and TRHR in different brain regions underlies the function ofTRH as a neurotransmitter or neuromodulator Centrally mediated actions of TRHinclude neurobehavioral, hemodynamic, and gastrointestinal effects In rats and mice,these central activities are mediated by a separate TRH receptor, Trhr2 Rat and mouseTrhr1 corresponds with human TRHR (Sun et al.2003)

TRH is rapidly degraded in blood and in different tissues, in particular the brain,pituitary, liver, and lung Although multiple enzymes are involved, an important role

is played by the TRH-degrading ectoenzyme TRHDE, also called pyroglutamylpeptidase II, which catalyzes the cleavage of the pGlu-His bond in TRH (Heuer et al

1998) This enzyme has been characterized as a zinc-containing metalloproteinasewhich in humans consists of 1024 amino acids It has a single transmembranedomain and is inserted in the plasma membrane such that most of the protein isexposed on the cell surface (ectopeptidase) Enzymatic cleavage of the protein close

to the cell membrane releases most of the protein in a soluble and enzymaticallyactive form into the circulation Plasma TRHDE is derived mostly from the liver Inthe brain and the pituitary, TRHDE is located in close vicinity of the TRH receptor,where it plays an important role in the local regulation of TRH bioavailability.Interestingly, TRHDE activity in the pituitary and in plasma is increased in hyper-thyroidism and decreased in hypothyroidism which may contribute to the negativefeedback control of TSH secretion by TH (Heuer et al.1998)

The synthesis and release of hypothalamic TRH is importantly regulated bynegative feedback action of TH (Joseph-Bravo et al 2015; Mariotti and Beck-Peccoz2016) Since TH stimulates energy expenditure and thermogenesis, it is notsurprising that cold exposure induces hypothalamic TRH secretion with consequentstimulation of the HPT axis The activity of the HPT axis is also regulated by feeding,with an important role for leptin Fasting is associated with a central downregulation

of the HPT axis, which is prevented in animal studies by administration of leptin(Joseph-Bravo et al.2015; Mullur et al.2014)

Bi-allelic inactivating mutations in TRH or TRHR are obvious causes of genital central hypothyroidism, but no patients have been identified with central

con-hypothyroidism caused by mutations in TRH, and only three families have beenreported with central hypothyroidism caused by mutations in TRHR (Bonomi et al.

2009; Koulouri et al.2016; Garcia et al.2017)

TSH

TSH is a glycoprotein produced by the thyrotropic cells of the anterior pituitary.Like the other hypophyseal hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), it is composed of two subunits The α subunit isidentical and the β subunit is homologous among the three hormones (Mariottiand Beck-Peccoz 2016) Although hormone specificity is determined by the βsubunit, heterodimerization with theα subunit is required for biological activity.Human TSH consists of 205 amino acids, 92 in theα subunit and 113 in the βsubunit It has a molecular weight of 28 kDa, 20% of which is contributed by threecomplex carbohydrate groups: two on theα subunit and one on the ß subunit Thestructure of these carbohydrate groups is important for the biological activity ofTSH and is dependent on the stimulation of the thyrotroph by TRH (Mariotti andBeck-Peccoz2016)

In addition to the stimulation by TRH and negative feedback by TH, TSHproduction and secretion is also subject to negative regulation by hypothalamicsomatostatin and dopamine and by cortisol The inhibitory effect of cortisol is exerted

at both the hypothalamic and pituitary level (Mariotti and Beck-Peccoz2016).TSH binds to a specific TSH receptor (TSHR) located in the plasma membrane ofthe follicular cell Like the TRH receptor, this is also a G protein-coupled receptorwhich, in humans, is a protein consisting of 764 amino acids with an exceptionallylong extracellular N-terminal domain (Kleinau et al.2017) TSHR is preferentiallycoupled to a Gsα subunit of the trimeric G protein Binding of TSH to its receptorinduces the dissociation of the G protein subunits, resulting in the activation of themembrane-bound adenylate cyclase and, thus, in the stimulation of cAMP formation

as second messenger The increased cAMP levels activate several cellular processes,ultimately resulting in an increased production and secretion of TH (Maenhaut et al

2015) In particular, the expression of genes coding for key players in TH production(e.g., the iodide transporter, thyroglobulin, and thyroid peroxidase) is increasedthrough mechanisms which also involve different thyroid-specific transcriptionfactors such as TTF1 (NKX2-1), TTF2 (FOXE1), and PAX8 At high TSH concen-trations, TSHR also couples to the Gqα subunit, resulting in the activation of thephosphoinositide pathway, which is also involved in the regulation of thyroidfunction and growth (Maenhaut et al.2015)

As discussed elsewhere in this volume, hyperthyroidism is often caused by anautoimmune process in which TSHR-stimulating antibodies play an important role.Hyperthyroidism may also be caused by a hyper-functioning adenoma In mostpatients with a toxic adenoma, somatic mutations have been identified in TSHR,which result in the constitutive activation of this receptor (Davies et al.2005) Inother patients, somatic mutations have been found in the Gα subunit which result

in the constitutive activation of the G protein in the absence of TSH Together,mutations in TSHR and Gsα account for the majority of toxic thyroid adenomas.Also, germline activating TSHR mutations have been identified in patients withcongenital, non-autoimmune hyperthyroidism Conversely, germline inactivatingTSHR mutations have been described in patients with TSH resistance (Persani

et al.2010) Often, TSH resistance is partial, and patients are clinically euthyroid asthe diminished TSHR function is compensated by increased plasma TSH levels.However, complete absence of functional TSHR causes severe congenital hypo-thyroidism associated with thyroid hypoplasia (Persani et al.2010)

Several patients have been reported with congenital central hypothyroidismcaused by bi-allelic inactivating mutations in TSHβ (Nicholas et al 2017) SinceTSHR has some basal constitutive activity in the absence of TSH, one would expectthat patients who lack functional TSH are only moderately hypothyroid However,this is refuted by the identification of patients with severe congenital hypothyroidismcaused by bi-allelic inactivating TSHβ mutations (Nicholas et al.2017)

LH and FSH receptors (Okada et al 2006) In mice, overexpression of GPHB5induces hyperthyroidism, but deletion of GPHB5 has little effect on serum TH levels(Okada et al 2006) In humans, GPHA2 and GPHB5 are expressed in varioustissues, where GPHA2 mRNA levels are usually much higher than GPHB5mRNA levels, and by far the highest GPHA2 expression is observed in the pancreas

In the human anterior pituitary, GHPA2 and GHPB5 are co-expressed incorticotrophs (Okada et al 2006) In mice, thyrostimulin may have a biologicalfunction in bone development, but the physiological role of thyrostimulin in humansremains to be established

Biosynthesis of Thyroid Hormone

The functional unit of the thyroid gland is the follicle, composed of a single layer ofepithelial cells surrounding a colloidal lumen in which TH is produced and stored.This section is a brief overview of the steps involved in the production and secretion

of TH, schematically presented in Fig.2 An extensive overview has been publishedrecently (Carvalho and Dupuy2017)

Iodide Uptake

Iodine is an essential trace element required for the synthesis of TH It is not surprising,therefore, that the basolateral membrane of the follicular cell contains an active trans-porter that mediates uptake of Itogether with Na+ The human Na/I symporter (NIS,SLC5A5) has been characterized as a protein consisting of 618 amino acids and

13 transmembrane domains (Ravera et al.2017; Carvalho and Dupuy2017; Targovnik

et al.2017) NIS transports Iand Na+ in a stoichiometry of 1:2, indicating that Itransport is electrogenic and driven by the Na+gradient across the plasma membrane.TSH stimulates the expression of the NIS gene to such an extent that the intracellulariodide concentration may be up to 500-fold higher than its extracellular level The activity

of NIS also strictly requires its interaction with the KCNQ1-KCNE2 K+ channel inthyroid cells, but the exact mechanism of this interaction is unknown (Ravera et al.2017).Various NIS mutations have been identified in patients with congenital hypothyroidism(Targovnik et al.2017; Ravera et al.2017; Carvalho and Dupuy2017)

NIS is not completely specific for iodide but also binds other anions, some of whichare even transported An important example is perchlorate (ClO4 ) which potentlyinhibits iodide uptake by NIS, an effect utilized in the perchlorate discharge test usedfor the diagnosis of an organification defect, i.e., impaired incorporation of iodine in Tg(Targovnik et al.2017) Perchlorate inhibits the uptake but not the release of iodide fromthe thyroid Therefore, if perchlorate is administered after a dose of radioactive iodide, itwill provoke a marked release of radioactivity from the thyroid in case of an organificationdefect but not from a normal thyroid gland Importantly, perchlorate is also an environ-mental pollutant with potential thyroid disrupting activity (Leung et al.2014)

Fig 2 Schematic of a thyroid follicular cell and important steps in the synthesis of thyroid hormone

Pertechnetate (TcO4 ) is another anion transported by NIS, and this observation

is utilized in the scanning of the thyroid gland using radioactive 99mTcO4  Ofcourse, the latter is not incorporated into thyroglobulin and, thus, cannot be used

to test the hormone production capacity of the thyroid

It is not sufficient that iodide is transported across the basolateral plasma membrane

of thyroid cells Since TH production takes place at the luminal surface of the apicalmembrane, iodide also has to pass this membrane, and this involves at least twoproteins: pendrin (PDS, SLC26A4) and anoctamin (ANO1, TMEM16A) (Silveiraand Kopp2015) Pendrin is the protein mutated in patients with Pendred’s syndrome(Wemeau and Kopp2017), which comprises deafness due to a cochlear defect andhypothyroidism due to an organification defect (positive perchlorate discharge test).Pendrin functions as a chloride-iodide exchanger in the thyroid and as a bicarbonate-chloride exchanger in the cochlea (Silveira and Kopp2015) ANO1 is a Ca2+-activatedchloride (iodide) channel Efflux of iodide from thyroid cells is acutely stimulated byTSH, which may involve recruitment and/or activation of these apical iodide exporters

Thyroid Hormone Synthesis

Thyroglobulin (Tg) is an exceptionally large glycoprotein consisting of two identicalsubunits Each mature subunit in human Tg contains 2748 amino acids and has amolecular weight of ~330 kDa (Targovnik et al.2017; Carvalho and Dupuy2017).The Tg gene is located on human chromosome 8q24.2-q24.3; it covers about 300 kb

of genomic DNA and consists of 48 exons Many Tg mutations have been identified

in patients with congenital hypothyroidism (Targovnik et al.2017)

DUOX2 is a large and complex glycoprotein embedded in the apical membrane

of the thyrocyte (Muzza and Fugazzola2017; Carvalho and Dupuy2017) Maturehuman DUOX2 contains 1527 amino acids and has 7 transmembrane domains, anNADPH-binding domain, an FAD-binding domain, a heme-binding domain, 2 cal-cium-binding EF hands, and a peroxidase domain It catalyzes the oxidation ofNADPH from the cytoplasm and delivers its product H2O2to the luminal surface

of the membrane The heme group is the site of H2O2generation, and its locationwithin transmembrane domainsfits with the vectorial (enzyme/transport) function ofDUOX2 Functional expression of DUOX2 requires the presence of the maturationfactor DUOXA2, a protein consisting of 320 amino acids and 5 putative transmem-brane domains (Carvalho and Dupuy 2017; Muzza and Fugazzola 2017) TheDUOX2 and DUOXA2 genes are clustered together with the homologous DUOX1and DUOXA1 genes on human chromosome 15q15 A large number of DUOX2mutations and a small number of mutations in DUOXA2 have been reported inpatients with congenital hypothyroidism DUOX1 and DUOXA1 are expressed atlower levels in the thyroid and do not (fully) compensate for the loss of functionmutations in DUOX2 and DUOXA2 (Muzza and Fugazzola2017)

Mature TPO is a glycoprotein consisting of 919 amino acids and featuring a singletransmembrane domain A short C-terminal domain is located in the cytoplasm, butmost of the protein is exposed on the luminal surface of the apical membrane which

also contains a heme-binding domain, the active center of the enzyme (Targovnik et al.

2017; Carvalho and Dupuy2017) Functional TPO exists as a homodimer where thesubunits are linked through a disulfide bond The human TPO gene covers about

150 kb on chromosome 2p25, distributed over 17 exons In addition to full-lengthTPO (TPO1), other isoforms are generated by alternative splicing (TPO2–5), some ofwhich retain enzyme activity Over 100 TPO mutations have been identified inpatients with congenital hypothyroidism (Targovnik et al.2017)

Formation of Iodothyronines

TH synthesis takes place at the luminal surface of the apical membrane within thescaffold of the Tg molecule and occurs in two steps which are both catalyzed byTPO: (1) the iodination of Tyr residues and (2) the coupling of iodotyrosines withformation of iodothyronines (Carvalho and Dupuy2017; Taurog et al 1996) Theprosthetic heme group of TPO undergoes a two-electron oxidation by H2O2(sup-

one-electron oxidation reaction, by which it is converted to compound II, or atwo-electron oxidation by which native TPO is regenerated TPO catalyzes thetwo-electron oxidation of Ito I+by H2O2with subsequent electrophilic substitution

of Tyr residues in Tg, producing 3-iodotyrosine (monoiodotyrosine, MIT) tution of MIT residues with a second iodine produces 3,5-diiodotyrosine (DIT).Coupling of two suitably positioned iodotyrosines results in the formation of aniodothyronine residue at the site of the acceptor iodotyrosine and a dehydroalanineresidue at the site of the donor iodotyrosine This involves the one-electron oxidation

Substi-of each donor and acceptor iodotyrosine residue, generating radicals that rapidlycombine to produce an iodothyronine residue T4 is generated by the reaction of twoDIT residues, and T3 is generated by reaction of an acceptor DIT with a donor MITresidue MIT does not seem to function as an acceptor residue, since thyroidalproduction of rT3 (and 3,30-T2) is negligible (Carvalho and Dupuy2017)

Although Tyr is the building block of TH, the Tyr content of Tg is not greater thanthat of most other proteins Of the 67 Tyr residues per Tg subunit, 20–25 areavailable for iodination, but the capacity for iodothyronine formation is limited.Each Tg subunit has only four hormonogenic acceptor Tyr residues that can ulti-mately be transformed into iodothyronines, localized at positions 5, 1291, 2554, and

2747 of mature Tg (Carvalho and Dupuy2017; Targovnik et al.2017) However, atnormal levels of iodination, the average yield is 1–1.5 molecule of T4 and 0.1molecule of T3 per Tg subunit They are stored within the Tg scaffold in thefollicular lumen until their secretion is required

Release of Thyroid Hormone

In response to TSH stimulation, Tg is resorbed from the lumen by both macro- andmicropinocytosis (Carvalho and Dupuy2017; Botta et al.2017) Macropinocytosis or

fluid endocytosis involves the formation of large pseudopodia which engulfTg-containing colloid, resulting in the formation of large cytoplasmic vesicles (colloiddroplets) Micropinocytosis concerns the receptor-mediated endocytosis of Tg, whichmay involve different receptor proteins such as megalin, a very large (600 kDa)cargo protein located in the apical membrane of different cell types, includingthyrocytes It is believed that Tg-containing vesicles generated by receptor-mediatedendocytosis largely undergo transcytosis or recycling, whereas vesicles generated byfluid endocytosis fuse with lysosomes, generating so-called phagolysosomes (Botta

et al.2017) In these vesicles, Tg is hydrolyzed by lysosomal proteases, cathepsins(Friedrichs et al.2003), resulting in the liberation of T4, a small amount of T3, as well

as excess MIT and DIT molecules The iodotyrosines are exported from thephagolysomes by a so-called system h amino acid transporter, which may very wellrepresent the L-type amino acid transporter LAT1 and/or LAT2 (Andersson et al

1990; Zevenbergen et al.2015; Krause and Hinz2017) This provides the access ofMIT and DIT to iodotyrosine dehalogenase (DEHAL1 or IYD) located in the endo-plasmic reticulum which catalyzes their deiodination by NADH (Gnidehou et al

2004; Moreno et al.2008) The iodide is reutilized for iodination of Tg

Human IYD is a homodimer of a 289-amino acid protein containing an N-terminalmembrane anchor and a conserved nitroreductase domain with an FMN-binding site(Gnidehou et al.2004; Moreno et al.2008) The IYD gene is located on chromosome6q24-q25 and consists offive exons Since IYD lacks an NADH-binding sequence,iodotyrosine deiodinase activity requires the involvement of a reductase, which hasnot yet been identified Low levels of IYD are also expressed in the liver and kidney.Little is yet known about the exact mechanism of T4 (and T3) secretion Possibly,this also involves their release form the phagolysosomes through the system h(L-type) transporter(s) Subsequently, T4 and T3 are secreted via transporters located

in the basolateral membrane, and recent evidence suggests an important role for the

TH transporter MCT8 herein Some T4 is converted before secretion to T3 byiodothyronine deiodinases present in the thyrocyte (see below)

In an average human subject, T4 and T3 are secreted in a ratio of about 15:1, i.e.,about 100μg (130 nmol) T4 and 6 μg (9 nmol) T3 per day The latter represents

20% of daily total T3 production Hence, most T3 is produced by deiodination ofT4 in peripheral tissues (Bianco et al.2002)

Inhibitors of Thyroid Hormone Production and/or Secretion

Administration of a large amount of iodide usually results in an acute but transientdecrease in TH secretion (Maenhaut et al.2015) The mechanism of this inhibition of

TH secretion by excess iodide is not understood Excess iodide also results in theinhibition of TH synthesis, a phenomenon known as the Wolff-Chaikoff effect(Maenhaut et al.2015) The mechanism appears to involve, among other things,the formation of an iodinated lipid (iodolactone) that inhibits several steps in THsynthesis This includes the inhibition of NIS, resulting in a decrease in the intra-cellular iodide concentration and, thus, a decrease in iodolactone formation,

relieving the inhibited hormone synthesis This escape from the Wolff-Chaikoffeffect occurs despite the continued administration of excess iodide.

Thiourea derivatives have been known since the pioneering work of Astwood in the1940s as potent inhibitors of TH synthesis (Astwood1984) Two of these, methimazole(MMI) and 6-propyl-2-thiouracil (PTU), are widely used in the medical treatment ofpatients with hyperthyroidism Their antithyroid activity is based on the potent inhibi-tion of TPO, the mechanism of which depends on the available iodide concentration(Taurog2000) In the presence of iodide, the thiourea inhibitors compete with the Tyrresidues in Tg for the TPO-I+iodination complex, preventing the formation of TH.MMI is a more potent inhibitor of TPO than PTU (Taurog2000), and lower doses ofMMI (or the prodrug carbimazole) are required for the treatment of hyperthyroidismcompared with PTU Besides inhibiting TH synthesis by TPO, PTU also inhibitsconversion of T4 to T3 by the type 1 iodothyronine deiodinase (D1) located not only

in the thyroid but also in the liver and kidney (see below) In contrast, MMI does notaffect D1 activity

Transport of Thyroid Hormone

Plasma Transport

In plasma, TH is bound to three proteins, thyroxine-binding globulin (TBG), thyretin (TTR), and albumin (Benvenga2012; Refetoff2015a,b) Human TBG is a

trans-54 kDa glycoprotein produced in the liver, consisting of 395 amino acids and

4 carbohydrate residues The TBG (SERPINA7) gene is located on human some Xq22.3, spans5.5 kb, and contains five exons Of all plasma TH transportproteins, TBG has the highest affinity for T4 (Kd0.1 nM) but also the lowest serumconcentration (15 mg/l) (Benvenga2012; Refetoff2015a,b)

chromo-TTR is a non-glycosylated protein composed of 4 identical subunits, consisting each

of 127 amino acids The TTR gene is located on human chromosome 18q12.1, covers

7 kb, and contains four exons (Richardson2007; Benvenga2012; Refetoff2015a,b).TTR has a cigar-shaped structure with two identical binding channels, each formed bytwo symmetrically positioned subunits, with ligand entry sites at opposite ends of theTTR molecule TTR binds thefirst T4 molecule with a higher affinity (Kd10 nM) thanthe second T4 molecule, and its plasma concentration amounts to250 mg/l PlasmaTTR is produced in the liver, but the protein is also expressed in the choroid plexus andthe placenta, where it may be involved in plasma-cerebrospinalfluid and maternal-fetalT4 transfer, respectively TTR also binds retinol-binding protein and thus also plays animportant role in vitamin A transport (Richardson2007)

Albumin has multiple low-affinity binding sites for TH, with Kdvalues for T4 of

1–10 μM, but it has by far the highest plasma concentration (40 g/l) (Benvenga2012;Refetoff2015a,b) Iodothyronines also bind to lipoproteins, in particular high-densitylipoprotein (HDL) Although the proportion of plasma T4 and T3 bound to lipoproteins

is low compared with the other plasma transport proteins, it may be important to target

TH specifically to lipoprotein receptor-expressing tissues (Benvenga2012)

The consequence of the different concentrations and affinities of the TH-bindingproteins is that in healthy humans75% of plasma T4 is bound to TBG, 15% toalbumin, and10% to TTR (Benvenga 2012; Refetoff2015a,b) The total bindingcapacity of these proteins is so high that only0.02% of plasma T4 is free (nonprotein-bound) The affinity of T3 for the different proteins is one-tenth of that of T4 Therefore,the free T3 fraction in plasma amounts to0.2% Although mean normal plasma totalT4 (100 nmol/l) and T3 (2 nmol/l) levels differ about 50-fold, the difference in meannormal FT4 (20 pmol/l) and FT3 (5 pmol/l) levels is only about fourfold rT3 bindswith intermediate affinity to the plasma proteins (Benvenga2012; Refetoff2015a,b).Since the plasma FT4 and FT3 concentrations determine the tissue availability of

TH, they are more important indices of thyroid state than total plasma T4 and T3levels Both concentration and TH-binding affinity of the different plasma proteinsare influenced by a variety of (patho)physiological factors (Benvenga2012) Since itbinds most TH in plasma, variations in TBG concentration are more important thanvariations in TTR or albumin concentrations Inherited TBG excess is a rare phe-nomenon caused by TBG gene duplication Inherited TBG deficiency is caused bymutations in the TBG gene, resulting in a decreased T4 affinity or protein stability.More severe TBG gene defects result in a complete lack of serum TBG in affectedmales (Refetoff 2015a, b) Plasma TBG levels are also influenced by variousendogenous and exogenous factors Notably, plasma TBG levels are increased byestrogens, whereas they are decreased by androgens In addition, different endoge-nous factors, such as free fatty acids, and drugs, such as salicylates, competitivelyinhibit T4 binding to TBG (Benvenga2012; Refetoff2015a,b)

A large number of mutations have also been identified in the TTR gene, some ofwhich cause a decrease in T4 binding affinity, whereas others (e.g., Ala109Thr,Thr119Met) result in an increased T4 affinity (Saraiva 2001) More importantly,TTR mutations often cause familial amyloidotic polyneuropathy (FAP), involvingthe deposition of insoluble TTRfibrils in nerves or the heart (Saraiva2001).Finally, also binding of TH to albumin is subject to genetic variation Especially, aspecific increase in the binding of T4 to albumin occurs in some otherwise healthysubjects, which often leads to the false diagnosis of hyperthyroidism if inadequatemethods for analysis of plasma FT4 are used (Benvenga 2012; Refetoff 2015a, b).This“familial dysalbuminemic hyperthyroxinemia” (FDH) is caused by polymorphisms

in the albumin gene, in particular p.R218H, resulting in a marked increase in T4 affinity.Perturbation of plasma iodothyronine binding provokes an adaptation of thehypothalamus-pituitary-thyroid axis to maintain normal FT4 and FT3 concentra-tions Therefore, measurement of plasma TSH and FT4 rather than total T4 levels isthe cornerstone of the diagnosis of thyroid disorders

Tissue Transport

The biological activity of TH is importantly regulated at the level of the targettissues, involving plasma membrane transporters which facilitate the cellular uptakeand/or efflux of iodothyronines as well as deiodinases which catalyze the activationand/or inactivation of the hormone (Fig.3)

Despite decades of research and evidence published by different laboratoriessuggesting the involvement of carrier-mediated mechanisms, it was believed for along time that TH crosses the plasma membrane by passive diffusion A detailedreview of these early studies of cellular TH transport was published in 2001(Hennemann et al 2001) Since then several transporters from different proteinfamilies were identified which are capable of transporting iodothyronines Thisincludes several members of the organic anion transporting polypeptide (OATP)family (Abe et al 2002; Hagenbuch 2007), the Na-taurocholate cotransportingpolypeptide (NTCP) (Friesema et al 1999), the L-type amino acid transportersLAT1 and LAT2 (Taylor and Ritchie2007; Zevenbergen et al.2015; Krause andHinz 2017), and the monocarboxylate transporters MCT8 (Friesema et al 2003,

2006) and MCT10 (Friesema et al.2008) Detailed reviews on TH transporters havebeen published recently (Visser et al.2011; Schweizer et al.2014; Bernal et al.2015)

Of the above transporters, only NTCP (SLC10A1) transports its substrates in an

Na+-dependent manner (Friesema et al.1999; Anwer and Stieger2014) It is sively expressed in the liver and plays an important role in hepatic uptake of (conju-gated) bile acids (Slijepcevic et al 2017) Human NTCP is a 349-amino acidglycoprotein containing nine transmembrane domains The SLC10A1 gene comprisesfive exons and is located on chromosome 14q24.1 The SLC10 family contains sevenmembers, but only NTCP is capable of transporting (sulfated) iodothyronines (Visser

exclu-et al.2011) NTCP may be important for the liver targeting of TH analogues such aseprotirome (Kersseboom et al.2017) Interestingly, NTCP also plays an important role

as a receptor for the entry of hepatitis B and D virus in liver cells (Li and Urban2016).The human OATP family consists of 11 members, most of which have beenshown to transport iodothyronine derivatives (Hagenbuch 2007; Stieger andHagenbuch 2014; van der Deure et al 2010) In general they are multi-specific,transporting a variety of ligands, not only anionic but also neutral and even cationiccompounds OATPs are glycoproteins containing ~700 amino acids and 12 trans-membrane domains The human OATP1 subfamily contains four members(OATP1A2, 1B1, 1B3, 1C1) with interesting properties They are encoded bygenes clustering on chromosome12p12, each containing 14–15 exons OATP1B1and 1B3 are expressed only in the liver and show preferential transport of sulfatedover non-sulfated iodothyronines OATP1A2 also effectively transports non-sulfatedFig 3 Pathways of thyroid

hormone metabolism

T4 and T3 and is expressed in different tissues, including the liver, kidney, intestine,and brain OATP1C1 is the most interesting transporter in this subfamily, showing ahigh preference for T4 as the ligand and almost exclusive expression in the brain, inparticular in astrocytes and the choroid plexus (blood-CSF barrier) (Pizzagalli et al.

2002; Bernal et al.2015) In astrocytes, OATP1C1 is crucial for the conversion of T4

to T3 by the type 2 deiodinase expressed in these cells (see below)

T4 and T3 are also transported by two members of the heterodimeric amino acidtransporters, LAT1 and LAT2 (Taylor and Ritchie2007; Zevenbergen et al.2015;Krause and Hinz2017) These transporters are glycoproteins consisting of two sub-units: a heavy chain and a light chain In humans, there are two possible heavy chains(SLC3A1, SLC3A2) and 13 possible light chains (SLC7A1–11, SLC7A13,SLC7A14) The heavy chains contain a single transmembrane domain, and the lightchains contain 12–14 transmembrane domains LAT1 is composed of theSLC3A2–SLC7A5 and LAT2 of the SLC3A2–SLC7A8 subunits These transportersare expressed in various tissues, and the expression of LAT1 is particularly stimulated

in activated immune and cancer cells Both LAT1 and LAT2 are obligate exchangers,facilitating the bidirectional transport of a variety of aliphatic and aromatic amino acids

as well as iodothyronines across the plasma membrane (Taylor and Ritchie2007).Two important TH transporters belong to the monocarboxylate transporter (MCT)family, named such since the first four transporters characterized in this family(MCT1–4) were found to transport monocarboxylates such as lactate and pyruvate(Halestrap2013) The MCT (SLC16) family contains 14 members; in addition toMCT1–4, MCT7 (Hugo et al.2012) and MCT11 (Rusu et al.2017) have also beenidentified as monocarboxylate transporters, while carnitine is the physiologicalsubstrate for MCT9 (Kolz et al.2009) and creatine for MCT12 (Abplanalp et al

2013) MCT10 facilitates the transport of aromatic amino acids and iodothyronines(Kim et al.2002; Friesema et al.2008), while MCT8 (SLC16A2) only transportsiodothyronines (Friesema et al.2003,2006)

The prevalent form of human MCT8 consists of 539 amino acids and MCT10consists of 515 amino acids Like the other MCTs, they contain 12 transmembranedomains However, unlike most other MCTs, they are not glycosylated, and they also

do not require ancillary proteins for functional expression MCT8 and MCT10 arehighly homologous proteins, in particular in their transmembrane domains, explainingtheir similar substrate specificities They have identical gene structures; the MCT8gene is located on human chromosome Xq13.2, and the MCT10 gene is located onchromosome 6q21-q22 Both consist of six exons and five introns, with a large

~100 kbfirst intron MCT8 and MCT10 show wide but different tissue distributions.MCT8 and MCT10 are the most active and specific TH transporters known todate (Friesema et al.2006,2008) MCT8 is importantly expressed in the brain, where

it is localized in the endothelial cells of the blood-brain barrier, in the choroid plexus,and in neurons in different brain regions Males with hemizygous MCT8 mutationssuffer from severe psychomotor retardation, known as the Allan-Herndon-Dudleysyndrome (AHDS), associated with low (F)T4 and elevated (F)T3 levels (Heuer andVisser 2009; Visser et al 2007,2008) As TH is crucial for brain development,AHDS is thought to be caused by impaired transport of T4 (and T3) into the brain

during important stages of development However, MCT8 is also expressed in thethyroid and in peripheral tissues In the thyroid, MCT8 is involved in TH secretion,and inactivation of MCT8 results in an increased residence time of T4 in the thyroid,with a consequent increase in local conversion of T4 to T3 There is also evidence forincreased T4 to T3 conversion in the liver and kidney.

Metabolism of Thyroid Hormone

Deiodination

The thyroid gland of a healthy human subject normally produces predominantly theprohormone T4 and only a small amount of the bioactive hormone T3 It is generallyaccepted that in humans80% of circulating T3 is produced by enzymatic outer ringdeiodination (ORD) of T4 in peripheral tissues (Peeters and Visser2017; Gereben

et al.2008; van der Spek et al.2017) Alternatively, inner ring deiodination (IRD) ofT4 produces the inactive metabolite rT3, thyroidal secretion of which is negligible(Figs.3and4) Deiodination is also an important pathway by which T3 and rT3 arefurther metabolized T3 largely undergoes IRD to the inactive compound 3,30-T2,which is also produced by ORD of rT3 Thus, the bioactivity of TH is determined to

an important extent by the enzyme activities responsible for the ORD (activation) orIRD (inactivation) of iodothyronines

Three iodothyronine deiodinases (D1–3) are involved in the reductivedeiodination of TH (Figs.3 and4) (Gereben et al 2008; Peeters and Visser 2017;van der Spek et al.2017) They are homologous proteins consisting of ~250–300amino acids, with a single transmembrane domain located at the N-terminus The

Fig 4 Schematic of the regulation of the nuclear availability of thyroid hormone in a target cell by transporters and deiodinases

deiodinases are inserted in cellular membranes such that the major part of the protein

is exposed to the cytoplasm This is consistent with the reductive nature of thecytoplasmic compartment required for the deiodination process However, somecontroversy exists regarding the topography of D3 as some studies suggest that itsactive site is exposed on the cell surface (Wajner et al 2011) The differentdeiodinases require thiols as cofactor Although reduced glutathione (GSH) is themost abundant intracellular thiol, its activity is very low compared with the unnaturalthiol dithiothreitol (DTT) which is often used in in vitro studies Alternative endog-enous cofactors include dihydrolipoamide, glutaredoxin, and thioredoxin Probablyall three deiodinases are functionally expressed as homodimers (Gereben et al.2008).The most remarkable feature of all three deiodinases is the presence of aselenocysteine (Sec) residue in the center of the amino acid sequence As in otherselenoproteins, this Sec residue is encoded by a UGA triplet which in mRNAs fornon-selenoproteins functions as a translation stop codon The translation of the UGAcodon into Sec requires the presence of a particular stem-loop structure in the

30-untranslated region of the mRNA, termed Sec insertion sequence (SECIS) ment, Sec-tRNA, and a number of cellular proteins, including SECIS-bindingprotein (SBP2) A SECIS element has been identified in the mRNA of alldeiodinases (Gereben et al.2008)

ele-The DIO1 gene coding for human D1 is located on chromosome 1p32.3 and consists

of four exons D1 is expressed predominantly in the liver, kidneys, and thyroid (Gereben

et al.2008; van der Spek et al.2017; Peeters and Visser2017) It catalyzes the ORDand/or IRD of a variety of iodothyronine derivatives with a preference for rT3 andiodothyronine sulfates In the presence of the artificial cofactor DTT, D1 displays high

Kmand Vmaxvalues D1 is rapidly inactivated by iodoacetate and gold thioglucose due

to reaction with the reactive Sec residue D1 activity is also potently anduncompetitively inhibited by PTU Together with the ping-pong-type cofactor-dependent enzyme kinetics, thesefindings suggest that the catalytic mechanism of D1involves the transfer of an I+ion from the substrate to the selenolate (Se) group of theenzyme, generating a selenenyl iodide (SeI) intermediate which is reduced back tonative enzyme by thiols or converted into a dead-end complex by PTU

Hepatic and renal D1 contribute importantly to the production of plasma T3 andthe clearance of plasma rT3 D1 activity in the liver and kidney is increased inhyperthyroidism and decreased in hypothyroidism, representing the regulation of D1activity by T3 at the transcriptional level (Zhang et al.1998)

The DIO2 gene coding for human D2 is located on chromosome 14q31.1 andconsists of two exons D2 is expressed primarily in the brain, anterior pituitary,brown adipose tissue, and thyroid and to some extent also in skeletal muscle(Gereben et al.2008; Larsen2009) In the brain, D2 mRNA has been localized inastrocytes and in particular also in tanycytes lining the third ventricle in the hypo-thalamic region (Werneck de Castro et al 2015) D2 is a low-Km, low-capacityenzyme possessing only ORD activity, with a preference for T4 over rT3 as thesubstrate (van der Spek et al.2017; Peeters and Visser2017; Gereben et al.2008).The amount of T3 in the brain, pituitary, and brown adipose tissue is derived to alarge extent from local conversion of T4 by D2 and to a minor extent from plasma

T3 The enzyme located in the anterior pituitary and the hypothalamus is veryimportant for the negative feedback regulation of TSH and TRH secretion by T4.

In general, D2 activity is increased in hypothyroidism and decreased in thyroidism This is explained in part by substrate-induced inactivation of the enzyme

hyper-by T4 and rT3 involving the ubiquitin-proteasome system (Gereben et al 2008).However, inhibition of D2 mRNA levels by T3 has also been demonstrated in thebrain and pituitary The substrate (T4, rT3) and product (T3)-dependent down-regulation of D2 activity is important to maintain brain T3 levels in the face ofchanging plasma TH levels However, D2 in the hypothalamus is largely protectedfrom substrate-induced enzyme inactivation, allowing proper negative feedbackregulation of the HPT axis at the hypothalamic level (Werneck de Castro et al.2015)

In mammals, D2 mRNA contains a second UGA codon just upstream of a UAAstop codon (Gereben et al.2008) It is unknown to what extent this second UGAcodon specifies the incorporation of a second Sec residue or acts as a translation stopcodon The amino acid sequence downstream of this second Sec is not required forenzyme activity (Salvatore et al.1999)

The DIO3 gene coding for human D3 is located on chromosome 14q32.31 andconsists of a single exon D3 activity has been detected in different human tissues,i.e., brain, skin, liver, and intestine, where activities are much higher in the fetal than

in the adult stage (Gereben et al 2008) D3 is also abundantly expressed in theplacenta and the pregnant uterus D3 has only IRD activity, catalyzing the inactiva-tion of T4 and T3 with intermediate Kmand Vmaxvalues D3 in tissues such as thebrain is thought to play a role in the regulation of intracellular T3 levels, while itspresence in placenta, pregnant uterus, and fetal tissues may serve to protect devel-oping organs against undue exposure to active TH Indeed, fetal plasma contains lowT3 (and high rT3) concentrations However, local D2-mediated T3 production fromT4 is crucial for brain development Also in adult subjects, D3 appears to be animportant site for clearance of plasma T3 and production of plasma rT3 In the brain,but not in placenta, D3 activity is increased in hyperthyroidism and decreased inhypothyroidism, which at least in the brain is associated with parallel changes in D3mRNA levels (Gereben et al.2008)

Since D1–3 are selenoproteins, selenium deficiency would be expected to result

in reduced D1–3 activities in different tissues, but this is only observed for D1 in theliver and kidney and not for D2 or D3 activities in other tissues (Kohrle2005) Thismay be explained by findings that the selenium state of different tissues variesgreatly in Se-deficient animals In addition, the efficiency of the SECIS element tofacilitate read-through of the UGA codon may differ among selenoproteins, whichcould result in the preferred incorporation of Sec into D2 or D3 over otherselenoproteins

Substitution of Sec by Cys in the three deiodinases results in a large reduction ofenzyme activity, and replacement with Leu or Ala completely inactivates theenzymes, indicating that Sec is indeed the catalytic center of the deiodinases(Gereben et al.2008; Peeters and Visser2017) However, the catalytic mechanismsappear to differ between the deiodinases In contrast to the ping-pong-type enzymekinetics of D1, both D2 and D3 show sequential-type enzyme kinetics D2 and D3

are also much less sensitive to inhibition by iodoacetate, gold thioglucose, and inparticular, PTU (Gereben et al.2008) Interestingly, the amino acid two positionsdownstream of the catalytic Sec residue (Ser in D1, Pro in D2 and D3) plays animportant role in determining the reactivity of the catalytic Sec residue and itssensitivity for these inhibitors (Gereben et al.2008; Peeters and Visser2017).Patients with mutations in any one of the deiodinases have not been reported so far.However, patients have been identified with high serum T4, low T3, and mostlyelevated TSH levels and various other symptoms, associated with mutations in SBP2(Dumitrescu et al.2005; Schoenmakers et al.2010) The changes in serum T4 and T3levels suggest that impaired SBP2 function has a greater impact on tissue ORD than ontissue IRD activity, although it is not clear which deiodinase is most affected by SBP2

deficiency The relatively high serum TSH despite elevated T4 levels suggest that atleast D2 activity is diminished in these patients Patients with SBP2 mutations sufferfrom a multisystem disorder, reflecting the impaired synthesis of various other impor-tant selenoproteins In total 25 selenoproteins are encoded by the human genome,many of which play an important role in tissue antioxidant defense Recently, anotherpatient has been reported with a mutation in the Sec-tRNA, showing a similarphenotype as patients with SBP2 mutations (Schoenmakers et al.2016)

Alanine Side Chain Modification

Triiodothyroacetic acid (Triac, TA3) and tetraiodothyroacetic acid (Tetrac, TA4) aremetabolites with interesting biological properties (Groeneweg et al 2017; Davis

et al.2016) TA3 shows equally high affinity for the nuclear T3 receptors (and in theinvertebrate amphioxus even much higher affinity (Holzer et al.2017)) as T3 itself,and TA4 has anti-tumor activity by blocking αvβ3 integrin TA3 and TA4 areproduced by metabolism of the alanine side chain of T3 and T4, respectively,although TA3 is also generated by enzymatic ORD of TA4 These metaboliteshave been identified in early studies after administration of 125

I-labeled T4 or T3

to human subjects or experimental animals, as well as in vitro after incubation withtissue preparations, in particular kidney (Groeneweg et al.2017) However, exactlyhow iodothyronines are converted to the acetic acid metabolites has not been settled,although two possible pathways have been suggested (Fig.4)

The first pathway implies the decarboxylation of iodothyronines to thecorresponding iodothyronamines with subsequent conversion by monoamineoxidase-like enzymes to the iodothyroacetic acid derivatives In consideration ofthe iodothyronine structure, it is logical to hypothesize that the first reaction iscatalyzed by aromatic amino acid decarboxylase (AADC) also known as DOPAdecarboxylase (DDC) However, studies utilizing recombinant AADC have failed toobserve decarboxylation of any iodothyronine (Hoefig et al.2012) A subsequentstudy indicated relatively slow conversion of 3,5-T2 to 3,5-diiodothyronamine(3,5-T2AM) by ornithine decarboxylase (ODC) but decarboxylation of otheriodothyronines is negligible (Hoefig et al.2015) 3-Iodothyronamine (3T1AM) hasreceived much attention recently as it exerts highly interesting pharmacological

effects, including bradycardia and hypothermia (Scanlan et al.2004; Hoefig et al.

2016) The physiological relevance of the iodothyronamines, however, remains to beestablished Although iodothyronamines are converted to the acetic acid metabolites

by monoamine oxidase-like enzyme(s) (Wood et al 2009), the relevance of thispathway for the conversion of T4 to TA4 and of T3 to TA3 is uncertain as there is noevidence that T4 and T3 actually undergo decarboxylation

The alternative route for production of TA4 and TA3 involves a number of steps,thefirst of which concerns the conversion of T4 and T3 to their pyruvate metabolites,TK4 and TK3, respectively This reaction has been documented using tyrosineamino transferase (TAT) in the presence ofα-ketoglutarate as the co-substrate andpyridoxal-50-phosphate as the cofactor (Nakano1967) It has been suggested early

on that further conversion of the pyruvate metabolites TK4 and TK3 to the aceticacid metabolites TA4 and TA3 may proceed via other intermediates with lactateand/or acetaldehyde side chains (Wilkinson1957), but this remains to be established

Sulfation

Iodothyronines also undergo conjugation of the phenolic hydroxyl group with sulfate

or glucuronic acid (Fig.4) Sulfation and glucuronidation increase the water ity of substrates, facilitating their biliary and/or urinary clearance However,iodothyronine sulfate levels are normally very low in plasma, bile, and urine, asthese conjugates are rapidly degraded by D1, suggesting that sulfate conjugation is aprimary step leading to the irreversible inactivation of TH (Kester and Visser2005;

solubil-Wu et al.2005) Plasma levels (and biliary excretion) of iodothyronine sulfates areincreased if D1 activity is inhibited by drugs such as PTU, and during fetal develop-ment, non-thyroidal illness and fasting Under these conditions, T3S may function as

a reservoir of inactive hormone from which active T3 may be recovered by action oftissue sulfatases and bacterial sulfatases in the intestine

Sulfotransferases represent a family of enzymes with a monomer molecular weight

of34 kDa, located in the cytoplasm of different tissues, in particular the liver, kidney,intestine, and brain They catalyze the transfer of sulfate from 30-phosphoadenosine-

50-phosphosulfate (PAPS) to usually a hydroxyl group of the substrate Different phenolsulfotransferases have been identified with significant activity toward iodothyronines,including human SULT1A1, SULT1A2, SULT1A3, SULT1B1, and SULT1C2 (Kesterand Visser 2005) They show substrate preference for 3,30-T2 > T3 > rT3 > T4.Surprisingly, iodothyronines, in particular rT3 and T4, are also sulfated by humanestrogen sulfotransferase (SULT1E1) (Kester and Visser 2005) Different humanSULTs also catalyze the sulfation of iodothyronamines (Pietsch et al.2007)

Glucuronidation

In contrast to the sulfates, iodothyronine glucuronides are rapidly excreted in thebile However, this is not an irreversible pathway of hormone disposal, since after

hydrolysis of the glucuronides by bacterial ß-glucuronidases in the intestine, part ofthe liberated iodothyronines is reabsorbed, constituting an enterohepatic cycle(Wu et al 2005; Peeters and Visser 2017) Nevertheless, about 20% of daily T4production appears in the feces, probably via biliary excretion of glucuronideconjugates Glucuronidation is catalyzed by UDP-glucuronyltransferases (UGTs)using UDP-glucuronic acid (UDPGA) as cofactor UGTs are localized in the endo-plasmic reticulum of predominantly the liver, kidney, and intestine.

Glucuronidation of T4 and T3 is catalyzed by different members of the UGT1Afamily, i.e., UGT1A1, UGT1A3, and UGT1A7–10 Usually, this involves theglucuronidation of the hydroxyl group (Fig.4), but human UGT1A3 also catalyzesthe glucuronidation of the side-chain carboxyl group, with formation of so-calledacyl glucuronides (Kato et al.2008) Interestingly, TA4 and TA3 are glucuronidated

in human liver much more rapidly than T4 and T3, and this occurs largely by acylglucuronidation (Moreno et al.1994)

T4-glucuronidating UGTs by different classes of compounds, including barbiturates,fibrates, and PCBs (Visser et al 1993; Hood et al 2003) This may result in ahypothyroid state when the thyroid gland is not capable of compensating for theincreased hormone loss In humans thyroid function may be affected by induction ofT4 glucuronidation by antiepileptics, but overt hypothyroidism is rare (Benedetti

et al.2005) Administration of such drugs to LT4-substituted hypothyroid patientsmay necessitate an increase in the LT4 substitution dose

Thyroid Hormone Actions

TH is critical for the development of different tissues, such as brain (Bernal2015),intestine (Sirakov and Plateroti2011), bone (Bassett and Williams2016), skeletalmuscle (Salvatore et al.2014), and the auditory system (Ng et al.2013) However,

TH is also important for the maintenance of tissue function throughout life and is acrucial factor in the regulation of protein, carbohydrate, and lipid metabolism as well

as for thermogenesis (Mullur et al.2014; Bianco and McAninch2013; Vaitkus et al

2015) The interested reader is referred to the cited literature for extensive reviews ofthese TH actions We will focus here on the most important mechanism of TH actionmediated by binding of T3 to its nuclear receptors

Mechanism of T3 Action

Most biological actions of T3 are initiated by its binding to nuclear T3 receptors(TRs) (Yen2001; Mullur et al.2014; Brent2012; Mendoza and Hollenberg2017).These proteins are members of the superfamily of ligand-dependent transcriptionfactors, which also includes the receptors for steroids (e.g., cortisol, estradiol,testosterone), 1,25-dihydroxyvitamin D3, retinoic acid, and 9-cis-retinoic acid Thelatter retinoid X receptor (RXR) is an important member of this gene family, as it

forms functional heterodimers with a number of other nuclear receptors, includingTRs Two TR genes have been identified; the THRA gene is located on humanchromosome 17q21.1 and the THRB gene on human chromosome 3p24.2 Byalternative exon utilization of both genes, four major receptor isoforms, i.e., TRα1,TRα2, TRβ1, and TRβ2, are generated which consist of 410–514 amino acids(Fig.5) Although the THRB gene (150 kb) is much larger than the THRA gene(30 kb), their coding sequences show a high degree of homology (Yen2001; Brent

2012; Mullur et al.2014; Mendoza and Hollenberg2017)

As in the other members of the nuclear receptor family, functional key domainshave been recognized in the TRs, in particular the DNA-binding domain (DBD),which is100 amino acids long, and the ligand-binding domain (LBD) which is

250 amino acids in length The amino acid sequences of the TRα and TRβsubtypes are most homologous in their DBD and LBD and least homologous attheir N terminus The latter contains the ligand-independent activation function

1 (AF1) domain, while an AF2 domain necessary for homo- and heterodimerizationand ligand-dependent activation is located at the C-terminus The short sequencebetween the DBD and the LBD is referred to as the hinge region

The structural difference between TRα1 and TRα2 is located at the C-terminus ofthe proteins, with completely different sequences for the last 40 and 122 amino acids,respectively The alteration in the LBD of TRα2 is associated with a complete loss ofT3 binding Therefore, this splice variant is not a bonafide T3 receptor although it isreferred to as TRα2 TRα2 may have a weak negative effect on the action of T3mediated by the other TRs but, more importantly, its production signifies a negativeeffect on T3 action as it goes at the expense of TRα1 The N-terminal 106 aminoacids of TRβ1 and 159 amino acids of TRβ2 differ almost completely due toutilization of alternative promoters and transcription start sites This is important inrelation to the tissue-specific expression of TRβ2 vs TRβ1 (see below), but it is notclear to what extent the different N-terminal domains of TRβ2 are associated withdistinct molecular mechanisms of action

The different TR isoforms show distinct tissue distributions (8112-114) TRα1 isthe predominant T3 receptor expressed in the brain, heart, and bone, whereas TRβ1Fig 5 Domain structures of the different T3 receptor (TR) isoforms The TR α2 variant is incapable

of binding T3 DBD DNA-binding domain, LBD ligand-binding domain