REVIEW ARTICLE Open Access

A clinician’s guide to understandingresistance to thyroid hormone due toreceptor mutations in the TRα and TRβisoformsBrijesh K. Singh and Paul M. Yen*

Abstract

There are two genes that express the major thyroid hormone receptor isoforms. Mutations in both these geneshave given rise to Resistance to Thyroid Hormone (RTH) syndromes (RTHβ, RTHα) that can have variablephenotypes for mutations of the same receptor isoform as well as between the two receptor isoforms. In general,the relative tissue-specific distribution of TRβ and TRα determine RTH in different tissues for each form of RTH.These differences highlight some of the isoform-specific roles of each TR isoform. The diagnosis of RTH ischallenging for the clinician but should be considered whenever a patient presents with unexplained elevatedserum free T4 (fT4) and unsuppressed TSH levels, as well as decreased serum free T4/T3 ratio. Here we provide aguide for the clinician to diagnose and treat both types of RTH.

Keywords: Resistance to thyroid hormone, Thyroid hormone receptors, Dominant negative activity, Thyroidstimulating hormone, Human mutation

BackgroundFuller Albright first showed that pseudohypoparathyr-oidism represented a form of hormone resistance syn-drome 75 years ago [1]. Since then, others have usedclinical, biochemical, and molecular studies to identifymany examples of hormone resistance with mutations intheir corresponding receptors [18]. Indeed, hormone re-sistance due to mutations in many nuclear hormone re-ceptors (NRs) such as the estrogen, glucocorticoid,peroxisome proliferator activator, and vitamin D recep-tors have been identified in affected individuals [53].Similarly, numerous cases of resistance to thyroid hor-mone (RTH) and the corresponding mutations in thegenes encoding human thyroid hormone receptors (TRs)have been reported.In this current review, we will focus on a brief descrip-

tion of TRs and thyroid hormone (TH) action, as well asnew clinical, biochemical, and molecular insights into

RTH obtained from patients harboring mutations in thetwo TR isoforms, TRβ and TRα. After the recent identi-fication of RTH in patients with mutations in the THRAgene, a new nomenclature was adopted to distinguishbetween types of RTH due to specific TR isoforms(please see below) [34]. The RTH syndromes due to TRβand TRα are now called RTHβ and RTHα, respectively.Since RTHβ was identified and studied almost 50 yearsbefore the identification of RTHα (even though the pre-cise mechanism for the former was not known at thetime) [35], we will discuss RTHβ first. Mutations in theTH transporter, MCT8, and selenoprotein mutationsthat affect intracellular TH concentration but do notaffect the function of TRs also have been identified. Formore details on these syndromes, the reader is referredto several excellent recent reviews [15, 47]. New insightson the two forms of RTH have led to better understand-ing of the roles of the two TR isoforms on the functionof different tissues as well as the regulation of the hypo-thalamic, pituitary, and thyroid (HPT) axis. ConsideringRTHβ and RTHα as potential diagnoses for abnormalthyroid function tests requires a rational approach for

* Correspondence: paul.yen@duke-nus.edu.sgLaboratory of Hormonal Regulation, Cardiovascular and Metabolic DisordersProgram, Duke-NUS Graduate Medical School, 8 College Road, Singapore169857, Singapore

© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Singh and Yen Clinical Diabetes and Endocrinology (2017) 3:8 DOI 10.1186/s40842-017-0046-z

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distinguishing these syndromes from other causes of in-appropriate TSH secretion and low serum T4/T3 ratio,respectively, and will be discussed in more detail later inthis article.

Thyroid hormone actionTHs are involved in the regulation of metabolism, prolif-eration, and growth of most tissues [5, 12, 28]. SerumTH levels are tightly controlled by the HPT axis to de-liver appropriate amounts of TH to target tissues. Thetwo major THs (T3 and T4) are iodothyrosines synthe-sized by the thyroid gland under the control ofthyrotropin/thyroid stimulating hormone (TSH), aglycoprotein heterodimer that is produced by the pituit-ary gland. TSH, in turn, is regulated by thyrotropin re-leasing hormone (TRH), a tripeptide generated by thehypothalamus that is released into its own portal systemto reach the pituitary. Both the production of TRH andTSH are under negative feedback control determined bythe circulating free TH concentrations. Circulating THs,particularly T4, are mostly bound to transport proteinssuch as thyroxine-binding globulin (TBG), transthyretin(TTR), and albumin (HSA, human serum albumin).TBG binds 75% of serum T4 whereas TTR and HSAbind approximately 20% and 5%, respectively.Although T4 is the major secreted form, T3 is signifi-

cantly more potent than T4 and binds to TRs with 10-fold higher affinity [21, 25]. Thus, T3 is considered theactive form of the hormone whereas T4 serves primarilyas a less active precursor. After delivery to target tissues,THs utilize transporters (e.g., MCT8, MCT10, andOATP1C1) to cross the cell membrane and enter thecell [9]. THs then are metabolized by the iodothyroninedeiodinases (Dio1, Dio2, and Dio3), a subfamily of sele-noproteins [8]. The deiodinases serve as additional con-trol points for TH action by regulating serum andintracellular TH concentrations. In particular, activationof TH is mediated by Dio1 and Dio2 conversion of T4 to

T3 whereas inactivation of TH is regulated by Dio3 con-version to metabolites such as reverse triiodothyronine(rT3) and diiodothyronine (T2) [44].

Thyroid hormone receptorsTRs belong to the nuclear receptor (NR) family that in-cludes the steroid hormone, vitamin D, peroxisome pro-liferator activator, and retinoic acid receptors. Unlikepeptide- or protein-binding receptors that are located onthe cellular membrane, NRs are intracellular and bind totheir cognate hormones either in the cytoplasm (steroidhormones) or nucleus (TH, vitamin D, retinoic acid)[53]. After binding to hormone, they have the ability tobind to hormone response elements (HREs) located inthe promoter regions of target genes. As such, NRs canbe considered hormone-inducible transcription factors.There are two major THR genes, THRA and THRB, thatare expressed in a tissue-specific manner [12]. Twomajor THRA receptor splice variants (TRα1 and TRα2)are encoded by the THRA gene (Fig. 1a) and two majorTHRB isoforms (TRβ1 and TRβ2) are generated by alter-nate promoter choice on the THRB gene (Fig. 1b). TRα1is highly expressed in the heart, bone, and skeletalmuscle whereas TRα2 is widely expressed throughoutthe whole body. The alternative splicing of the THRAmRNA transcript leads to changes in the carboxy-terminus sequence of TRα2 that renders it incapable ofbinding to TH. It is possible that TRα2 may regulate al-ternative splicing of the THRA gene or may interferewith TRα1 action at the protein level. TRβ1 is predomin-ately expressed in brain, liver and kidney whereas TRβ2is found in the pituitary, retina, and cochlea. TRα1,TRβ1, and TRβ2 bind T3 with similar affinity.TRs have a modular structure, with a central DNA-

binding domain and a C-terminal ligand-binding domain[5, 12, 28]. They typically will bind to DNA as heterodi-mers with another nuclear hormone receptor familymember, retinoid X receptors (RXRs) (Fig. 2). These

Fig. 1 Alternative splicing and translation give rise to multiple TRα (a) and TRβ (b) isoforms

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heterodimers can recognize specific DNA sequences,thyroid hormone response elements (TREs), located inthe promoter regions of target genes. TREs typically arecomposed of two-half sites, most often organized as dir-ect repeats, separated by 4 nucleotides (consensus DR4:5′(A/G)GG(A/T)CANNNN(A/G)GG(A/T)CA 3′). TRsbind in a head to tail orientation with the upstream 5′half site of DR4 bound by RXR and the downstream3′ half site by TR. Interestingly, both unliganded andliganded TRs can bind to TREs; however, ligand bind-ing to TRs induces conformational changes in the re-ceptor that facilitate the recruitment of co-activatorswith histone acetyltransferase (HAT) and methyltrans-ferase activity to induce conformational changes atspecific chromatin sites in the promoters ofpositively-regulated target genes. These changes gen-erate a permissive local chromatin environment thatenables the binding and recruitment of the generaltranscriptional machinery (Fig. 2a) to the transcrip-tional start site and initiate transcription. In the ab-sence of TH, TRs also can bind to TREs but theyrecruit co-repressors with histone deacetylase (HDAC)activity instead of co-activators/ HATs owing to their

different conformation in the unliganded state. Theco-repressor complex alters its surrounding chromatinstructure by removing acetyl groups from histones toinduce a conformational change in the histone struc-ture that inhibits the binding of RNA polymerase II,and results in a decrease in target gene transcription(Fig. 2b). TREs can be located near or far from tran-scriptional start sites. The co-activator or co-repressortranscriptional complexes bound to them can interactco-operatively with multiple TR/TRE complexes inthe promoter region to further regulate transcription.Taken together, this model suggests that TR/RXR het-erodimer binding to the TRE and its recruitment ofco-activators/corepressors play important roles in TH-mediated gene transcription (see below). Recently,using a method to examine TR binding throughoutthe whole genome, chromatin immunoprecipitationsequencing (ChIP-Seq), it was found that TRs canbind to DNA with sequences that do not resembleTREs and in non-promoter regions [4, 33]. Thus, it islikely that TRs interact with other transcription fac-tors or chromatin via protein-protein interactions atthese sites. There also is evidence that TH also may

Fig. 2 Role of co-activator and co-repressor recruitment in positively-regulated target genes. a For positively-regulated target genes, in the presenceof T3, co-activators (Co-A) and histone acetyl transferases (HAT) are recruited by the T3-bound TR/RXR heterodimer sitting on the thyroid hormoneresponse element (TRE). This leads to histone acetylation and chromatin nearby changes to a more open conformation to facilitate recruitment of RNApol II to the TATA box region. Subsequently another co-activator complex, TH receptor-associated protein/vitamin D receptor interacting proteincomplex (TRAP/DRIP comp), is recruited by ligand-bound TR/RXR and RNA polymerase II complex to activate transcription. b For positively-regulatedtarget genes in the absence of T3, TR/RXR has a different conformation than its T3-bound state, and has poor affinity for co-activator complexes.Instead, it recruits a co-repressor complex (Co-R) with histone deacetylase activity (HDAC). This leads to histone deacetylation and formation of amore closed chromatin conformation that does not allow RNA pol II binding to the promoter and thus “represses” transcription. c In some negatively-regulated target genes, in the presence of ligand, co-repressor and HDAC are recruited by TR/RXR sitting on the TRE. This leads to decreased histoneacetylation and a more closed chromatin conformation that prevents RNA pol II binding to the promoter of the target gene, and thus negativelyregulates transcription in the presence of T3. Please see text for more details

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bind with low affinity to other non-TR proteins inthe cell to mediate novel actions; but so far, thesemechanisms are poorly understood [12].The transcription of approximately half of all target

genes is negatively regulated either indirectly (throughthe activation/increased expression of repressor tran-scription factors) or directly by TRs (Fig. 1c) [29]. Cur-rently, the mechanism for negative regulation by TRsstill is not well understood. TSHβ and the CGA are twonegatively-regulated target genes that are expressed inpituitary thyrotrophs. They generate two proteins, thy-roid stimulating hormone β (TSHβ) and the commonglycoprotein hormone α-subunit protein (α-GSU), thatdimerize with each other to form TSH. Studies inpituitary-specific TR knockout mice suggest that theTRβ2 is the major isoform that controls the TH-mediated negative regulation of these target genes in thepituitary [51].

Resistance to thyroid hormone βClinical featuresRTHβ is a rare disorder characterized by elevated levelsof circulating free thyroid hormones, inappropriatelynormal or elevated TSH secretion, and decreased per-ipheral tissue responses to iodothyronine action (Fig. 3a)[7, 30, 36]. The incidence of RTHβ is estimated to be 1

case per 50,000 live births with affected individuals iden-tified in Europe, Asia, and North and South America. Sofar, over 160 different mutations in TRβ have been found inRTHβ patients from more than 350 families. RTHβ followsan autosomal dominant inheritance pattern in families(80%) but also can be found sporadically in affected individ-uals with no other family history of RTHβ (20%) [36]. Pa-tients with RTHβ typically have a heterozygous mutation inthe THRB allele leading to the expression of a defectiveTRβ that has dominant negative activity on the transcrip-tional activities of the TRs encoded by the other normalTHRB allele and the two normal THRA alleles [7, 36].Major exceptions to this pattern were the first reportedRTHβ kindred in which an autosomal recessive pattern ofinheritance was observed [35]. The affected patients laterwere shown to harbor homozygous mutations in bothTHRB alleles that generated a severely truncated, non-functional form of TRβ [36].In the clinical setting, RTHβ often is detected at child-

birth during neonatal screening for congenital thyroiddysfunction when abnormal levels of T4, TSH, or bothare identified, and further diagnostic testing undertaken.However, RTHβ also can go undetected due to itsheterogenous presentation and variable symptoms [7,36]. Goiter frequently is the main clinical finding thatprompts the physician to order thyroid function testsand further investigations. In many cases, the high THlevels can compensate for tissue resistance; thus, affectedindividuals may appear to be clinically euthyroid. How-ever, upon closer inspection, the compensation may beincomplete, and hypothyroidism is found in tissues thatexpress predominantly TRβ (see below) such as the liver,kidney, and lung. Additionally, high endogenous TH levelssometimes can produce hyperthyroid effects, particularlyin tissues that express predominantly TRα such as theheart and bone [7, 36]. These tissues do not express muchmutant TRβ so it is likely that they are responding to thehigh circulating concentrations of TH [56].In addition to goiter, the most common presenting

signs and symptoms in patients with RTHβ are shortstature, attention deficit disorder, and resting tachycardiaalthough some patients may be entirely asymptomatic(Table 1). Moreover, the phenotypes and severity of THdysfunction frequently vary among affected individualsexpressing the same THRB mutation. Importantly, thisvariability in clinical phenotype may even occur amongdifferent affected family members with the same THRBmutation [7, 36]. These observations suggest that othergenetic and epigenetic modifiers may affect the expres-sion/penetrance of the RTHβ phenotype.

Differential diagnosisThere are other clinical conditions of inappropriate TSHexpression with increased serum T4, and they should be

Fig. 3 Models for RTHβ and RTHα. a RTHβ occurs in tissuesexpressing TRβ and causes a rise in serum T3 and T4 levels due toimpaired negative feedback of the hypothalamic/pituitary/thyroidaxis. b RTHα occurs in tissues expressing predominantly TRα (bone,gut, and heart) and causes symptoms shown in Table 1. In contrastto RTHβ, there is no defect in the negative feedback of the HPT axisby TH. However, patients have increased serum T4/ T3 ratiossuggesting that a downstream effect such as increased deiodinase 1(Dio 1) activity may occur

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considered when attempting to make the diagnosis ofRTHβ in a particular patient [36, 48]. First, there areseveral conditions or situations that can cause an ap-parent increase in serum T4 with detectable TSHlevels. These include: increased serum binding pro-teins (e.g., thyroxine binding globulin), abnormalserum binding proteins with altered binding affinityfor THs (e.g., familial dysalbuminemic hyperthyroxine-mia (FDH) and transthyretin variant), and anti-TSHor T4 antibodies. Measurement of serum free T4levels, particularly by equilibrium dialysis and pre-clearance of anti-TSH/T4 autoantibodies before hor-mone measurements usually can distinguish thesepossiblities from RTHβ. Serum fT3 also should benormal in these cases. Additionally, it is important toevaluate family members for symptoms associatedwith RTHβ. Uncovering similar abnormalities in thy-roid function tests among siblings and parents willprovide important clues for the diagnosis of RTHβsince 80–90% case of RTHβ are familial.

Next, it is important to consider transient causes forelevated serum T4 and detectable TSH levels such as:systemic illness (sick euthyroid syndrome), acute psychi-atric disorders, the neonatal period when there is a sud-den burst of T4 release post-natally before fullequilibration of the HPT axis, and early thyroxine re-placement therapy in hypothyroid patients. Additionally,certain drugs can cause abnormal thyroid function teststhat resemble those seen in RTHβ. Amiodarone, oralcontrast agents, and β-blockers interfere with the con-version of T4 to T3 by inhibiting the enzymatic activityof Dio1. Serum TSH may be in the normal range inthese patients. Amphetamines stimulate TRH releaseacutely leading to increased serum TSH and TH levels.Heparin induces lipoprotein lipase activity to increaseserum free fatty levels that can interfere with TH bindingto serum transport proteins.The remaining other major cause for « inappropriate

»TSH secretion with elevated serum T4 levels is TSH-secreting pituitary adenoma (TSHoma). Several import-ant diagnostic tests are helpful for distinguishing be-tween RTHβ and this condition: pituitary MRI(abnormal in TSHoma) and the common glycoproteinα-subunit hormone subunit (α-GSU) /TSH ratio. In thelatter diagnostic measurement, there can be an inappro-priately elevated secretion of common α-GSU in TSHo-mas such that the α-GSU /TSH ratio is elevated relativeto TSH (α-GSU (μg/l)/TSH (mU/l)] × 10 > 1.0 in TSHo-mas) due to dysregulated over-secretion of α-GSU. How-ever, this ratio may need to considered with cautionwhen the circulating levels of other pituitary glycopro-teins, particularly luteinizing hormone and follicle stimu-lating hormone, are elevated in post-menopausal womenand can give a spuriously high ratio. Although not rou-tinely used in the U.S. outside the academic setting, ap-proximately 90% patients with RTHβ had normal orincreased (similar to hypothyroid) TSH responses toTRH stimulation (200 μg bolus intravenously, samplingat 0, 20, 60, 90 and 120 min) whereas patients withTSHomas typically had high basal levels and only 39%responded to TRH [36, 41]. The reason for the occur-rence of normal vs. increased TSH responses to TRH inpatients with RTHβ may be that some patients have «compensated » pituitary response to the higher circulat-ing TH levels whereas some patients do not, and thushave relative pituitary hypothyroidism. When TRHstimulation tests were performed after 3 days of T3suppression at 50, 100, and 200 μg/days, euthyroidpatients had suppressed TSH levels at 50 μg T3/dayand almost all RTHβ patients also had some degreeof TSH level suppression at 200 μg T3 /day, albeit toa lesser degree than euthyroid patients since moststill had some residual TSH response at that dose ofT3. In contrast, only 25% patients with TSHomas had

Table 1 Clinical features and diagnostic tests for RTHβ andRTHαRTHβ RTHα

Typical Clinical Features

-Goiter -Bradycardia

-Resting tachycardia -Neurodevelopmental delay

-Osteoporosis -Anaemia

-Short stature -Skeletal dysplasia

-Attention deficit disorder -Dysmorphia

-Family history (80%) -Constipation

Diagnostic Tests

-Increased fT3, fT4(Rule out antibody interference)

-Decreased T4/T3 ratio

-Normal/Elevated TSH -Normal TSH

-Normal dialyzed free T4 -Exon sequencing of TRα

-Rule out autoimmune thyroiditis(anti-thyroid peroxidase,thyroglobulin, and TSHreceptor antibodies)

-Check serum markers THhyperfunction (increasedSHBG, ferritin, pro-collagen-1-N-terminal peptide (PINP)and decreased cholesterolin hyperthyroidism butnormal in RTH)

-Check serum a-GSU andcompare with TSH (α-GSU(μg/l)/TSH (mU/l)]× 10 > 1.0 (suggests TSHoma)

-Consider pituitary MRI(rule out TSHoma)

-Exon sequencing of TRβ

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any significant suppression of TSH levels after highdose T3 treatment [36, 41].Measurement of metabolic markers of thyroid hor-

mone action such as serum SGOT, SGPT, cholesterol,triglycerides, ferritin, osteocalcin, creatine phosphoki-nase (CPK), and sex hormone binding globulin (SHBG),also can be helpful in determining peripheral resistance.Serum prolactin can be elevated in patients withhypothyroidism and is increased in some patients withRTHβ, particularly those who previously were treatedwith ablative therapy. However, most RTHβ patients hadnormal basal prolactin levels (i.e., without TRH stimula-tion). [36, 40] Among these markers, serum SHBG ap-pears to be the one that is most reliably affected bydecreased TH action. Serum SHBG levels in RTH pa-tients are similar to those found in euthyroid patientsbut is significantly decreased when compared to thyro-toxic patients. Thus, a normal SHBG level in conjunc-tion with elevated TH levels and unsuppressed TSHwould be suggestive of RTH. Ferritin and osteocalcinlevels are typically elevated in hyperthyroidism; thus,normal levels also would be supportive of RTH. SGOT,SGPT, cholesterol, and triglyceride levels are responsiveto TH but are nonspecific for hyperthyroidism, and thusmay have limited utility. Additionally, TH effects on theneuromuscular system also can be assessed by measur-ing serum CPK concentration (elevated in RTH), andperforming careful neurological examination looking forsigns of hypothyroidism. Finally, if clinical and labora-tory evidence support the diagnosis of RTHβ, a directsequencing of the THRB gene exons, particularly thosesequences that encode the LBD should be considered(see below). Identification of the mutation may be usefulfor future prenatal diagnosis of RTHβ. Specific TRβ mu-tation testing is available commercially from Quest Diag-nostics (Madison, NJ) and several companies offer wholeexome sequencing (e.g., Macrogen (Rockville, MD), Oto-genetics (Atlanta, GA), and GATC (Constance,Germany)). Finally, a letter with a clear explanation ofthe diagnosis should be provided to the patient and bepresented to any physican taking care of the patient inorder to prevent inappropriate treatment for elevatedserum T3 or T4.

TRβ Mutations and mechanismIn both familial and sporadic cases of RTHβ, TRβ pointmutations cluster in the 3 major “hot spots” of the LBD[13, 17, 38]. In familial RTHβ, affected members haveone normal and one abnormal THRB allele, consistentwith the autosomal dominant pattern of inheritance seenin these families. In sporadic RTHβ mutations, similarfindings in the THRB alleles also are observed. SinceTRβ mutations occur in the LBD, they often lead to de-creased T3-binding affinity. So far, no germ line

mutations have been identified in the DBD or N-terminal regions of TRβ. In patients with RTHβ, mostmutations are nucleotide substitutions that result in sin-gle amino acid changes. However, nucleotide deletionsor insertions that cause single amino acid deletions andframeshift mutations, and premature stop codons alsohave been reported. Interestingly, the first described caseof RTHβ occurred was inherited in a recessive pattern,and later shown to be due to complete from the exon-coding region resulting in absence of TRβ [42]. SinceDNA-binding is required for the autosomal dominantinheritance in RTH, it is possible that mutations in theamino-terminus or DNA-binding may have a recessivephenotype. The inability to find mutations in these re-gions, suggests that if they exist, they may have little orno distinctive phenotype suggesting TH dysfunction. Itis noteworthy that so far, no LBD mutations have beenfound that increase T3-binding affinity or its transcrip-tional activity.At the molecular level, mutant TRβs have decreased

transcriptional activity due to their reduced ligand-binding affinity. They also can competitively block nor-mal TRs from binding to TREs since they generally re-tain their DNA-binding capability [54]. This interferenceof normal TR function by the mutant TRβ (so called“dominant negative effect”) leads to decreased overalltranscriptional activity in target genes (Fig. 4) [30, 52].Further support for this model comes from studiesshowing loss of dominant negative activity by mutantTRβs in which a second mutation was introduced intothe DBD to abrogate DNA binding [27]. Moreover, it islikely that unliganded TR/co-repressor complex needs toleave the TREs in the presence of TH before ligandedTR/co-activator complex can bind to the TREs and acti-vate transcription. In this connection, constitutive bind-ing of mutant TRs to TREs, combined with decreasedcorepressor dissociation and coactivator recruitmentprevent normal T3-bound TRs from binding to TREsand activate transcription of target genes [26, 39]. To-gether, these effects likely are the main contributors forthe dominant negative inhibition on transcription of tar-get genes by mutant TRs. In general, the severity of T3binding impairment by mutant TRs correlates with theseverity of clinical phenotype, although there are someexceptions [7].

TreatmentIn most patients, RTHβ appears to be adequately com-pensated by the increased endogenous supply of THreflected by the increased serum fT3 and fT4 levels andnormal or near normal TSH levels. These patients ap-pear clinically euthyroid and eumetabolic [50]. Specialcare must be made not to misdiagnose and inappropri-ately treat these RTHβ patients for “hyperthyroidism”

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because of the high serum TH levels [49]. Unfortunately,some patients with RTHβ have undergone unnecessaryradioactive iodine ablation, thyroid surgery, or anti-thyroidal medical treatment (e.g., propylthiouracil, carbi-mazole) based upon the presumption of hyperthyroid-ism. These inappropriate treatments led to worsenedsymptoms, as patients were rendered more hypothyroidin resistant tissues despite normalization of serum TSHand T4 levels. Likewise, patients with compensatedRTHβ do not require additional thyroxine treatmentdespite their RTHβ. Such treatment should only be con-sidered in uncompensated RTHβ patients that haveundergone thyroid ablation or surgery and have limitedor no thyroid reserve or have decreased thyroid functiondue to autoimmune disease. Previous thyroid functiontest results when the patient was in the “compensated”baseline state before surgery or thyroid injury can be ex-tremely useful for determining the optimal replacementdose in these RTHβ patients level, and can be used tofollow patients’ responses to treatment.The possibility of uncompensated RTHβ in de novo

cases should be suspected if patients have TSH levelshigher than normal levels, together with elevatedserum fT3 and fT4 levels. Thus, elevated TSH levelswithout any signs or symptoms of hypothyroidismshould raise the suspicion of possible RTHβ, andserum fT3 and fT4 levels obtained if they were notmeasured during an initial screening. In patients withprevious thyroid surgery or Hashimoto’s thyroiditis,uncompensated RTHβ might be suspected if un-usually high replacement doses of levothyroxine arenecessary to reduce the elevated TSH levels. Finally,in some cases of uncompensated RTHβ, patients maybe asymptomatic or have complaints suggestive ofhypothyroidism while exhibiting paradoxically high

serum fT3 and fT4 levels and a TSH level that isabove the normal range.The assessment of uncompensated RTHβ also is made

clinically, in conjunction with laboratory tests that sug-gest peripheral resistance (e.g., decreased serum SHBG).However, RTHβ may manifest itself in children by de-creased and/or delayed growth or failure to gain weight.When RTHβ is not compensated, thyroxine can be givenin incremental doses with simultaneous monitoring ofparameters linked to TH action such as liver functiontests, CPK, SHBG, PRL, and TSH until normal levels areachieved. In children, levothyroxine has been used underclose supervision to improve growth and school per-formance; however, the results, have been variable. Thepresence of tachycardia should not be a reason to with-hold treatment for uncompensated RTHβ as it can bemanaged by concomitant administration of a β-adrenergic blocker such as atenolol.Recently, TRβ-specific analogs that have higher affinity

for TRβ than TRα have been developed [6]. These drugsprimarily are aimed as potential therapies for hyperchol-esterolemia, obesity, and diabetes. However, it is possiblethat these drugs may also be useful in patients withRTHβ. In this connection triiodothyroacetic acid(TRIAC) has been used to treat patients with RTHβ;however, there have been no studies thus far comparingthe effectiveness of TRIAC vs. levothyroxine for thetreatment of uncompensated RTHβ [32].

Resistance to thyroid hormone α (RTHα)Clinical featuresPrevious studies in genetic models of RTHα such asTRα knockout mice that do not express TRα and mu-tant TRα knock-in mice that express a TRα mutation inthe THRA gene locus, suggested that lack of TRα or

Fig. 4 Model for resistance to thyroid hormone in RTHβ patients. a In both normal and RTH patients, wild-type TRβ and TRα isoforms derivedfrom normal THRB and THRA alleles bind as TR/RXR heterodimers to the TRE and are able to activate transcription. b The mutant TRβ encoded bythe abnormal THRB allele in RTH patients bind to the TRE constitutively in both the presence and absence of T3. Since it has decreased ligand-bindingaffinity, its ability to recruit co-activators and activate transcription is impaired. The unliganded mutant TR/RXR heterodimer thus competeswith T3-bound wild type TR/RXR heterodimer for binding to the TRE

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expression of an inactive TRα were not lethal [16]. Sur-prisingly, these genetic perturbations caused only rela-tively mild hypothyroid-like symptoms, particularly inthe heart and bone. The prevalence of RTHα in man isnot known but it is possible that this disorder has notbeen adequately recognized clinically since it lacks a dis-tinctive phenotype and also may be associated with un-usual phenotypes such as autism spectrum disorder. Anexamination of large databases showed approximately100 non-synonymous variants in THRA in 60,000exomes; however, only a small number of these variantswere mutated at homologous TRα sites and would beexpected to give a distinct phenotype [24].RTHα in man was first described in a 6-year-old girl

with skeletal dysplasia, bradycardia, growth retardation,neurodevelopmental delay, and constipation. Interest-ingly, the patient harbored a TRα mutation (Glu403X)that led to a frameshift mutation as well as loss of helix12 in the LBD due to the introduction of a prematurestop codon. This mutation decreased both its ligandbinding affinity for TH and its transcriptional activitysimilar to the TRβ mutations found in RTHβ [10, 24].This individual had borderline low or normal T4, border-line high or normal T3, and normal TSH concentrationsin her serum. Shortly afterwards, several adult male andfemale individuals were identified that harbored frame-shift/premature stop mutations within the TRα1 LBD[14, 45]. These patients also had additional features intheir phenotypes such as macrocephaly, anemia, anddysmorphic facies. Based upon the reports of nearly 30patients with RTHα [14, 23, 24, 43], clinical features thatare most commonly found among RTHα patients in-clude bradycardia, constipation, reduced and delayedbone growth, delayed psychomotor development, de-creased metabolic rate, as well as skeletal abnormalitiesmanifested by delayed fusion of epiphyses and reducedbone growth (Table 1). Additionally, dysmorphic featureshave been reported in some affected individuals fromseveral kindreds, and they include: macrocephaly, latefontanelle closure, dysmorphic and broad facies, flat-tened nose, enlarged tongue, and thickened lips. Of note,many of these features can resemble those found in con-genital and primary hypothyroidism. Additionally, thetissues associated with these features contain mostlyTRα, and thus would be expected to be “hypothyroid”with respect to TH action due to the dominant negativeeffect by mutant TRα. Interestingly, several cases ofRTHα also were identified after screening for abnormalthyroid function in patients with dysmorphic features[24]. On the other hand, there can be large variation inthe severity of the phenotypes in RTHα as some patientscan have mild phenotypes with minimal symptoms [14].When RTHα patients are compared with RTHβ, it ap-pears that they can present with a wider repertoire of

phenotypes than RTHβ as well as exhibit phenotypesthat are distinct from RTHβ.RTHα patients typically have increased/high-normal

T3 and decreased/low-normal serum T4 levels, resultingin a markedly reduced T4/T3 ratio (Fig. 4a). Low serumrT3 levels also have been reported in some cases. Ofnote, serum TSH levels are usually normal. The reasonfor the low T4/T3 ratio in affected individuals is notknown; however, it is noteworthy that increased hepaticDIO1 expression was observed in a dominant negativeTRα knockin mouse model [55] so it is possible that in-creased conversion of T4 to T3 may be involved in gen-erating this serum TH profile. Additionally, TRα ishighly expressed in the skin so RTHα in that tissuecould lead to decreased DIO3 expression and activity,and thus lead to accumulation of serum T3 [11].

Differential diagnosisAlthough rare, RTHα should be considered in the differ-ential for children with decreasted growth rate, dys-morphic features, and delayed psychomotor development.It also should be considered in adults with a similar previ-ous history as well as in patients with unexplained consti-pation, megacolon, and bradycardia [24, 46]. The lowserum T4/T3 level is a distinctive and consistent feature inRTHα that can help identify potential cases. Of note, thisbiochemical abnormality also can be seen in disorders in-volving decreased TH synthesis since T4 is the major formof TH that is synthesized and released by the thyroidgland. Thus, congenital hypothyroidism or environmentalcauses of hypothyroidism (e.g., iodine deficiency) can ex-hibit this serum TH profile. Additionally, patients withAllan–Herndon–Dudley syndrome, a condition in whichpatients harbor a mutation in one of the major TH trans-porters, MCT8, can present with a similar TH profile [15].However, these patients have severe mental retardationand progressive spastic paralysis as well as an x-linked in-heritance pattern so it is relatively easy to distinguish themfrom patients with RTHα based upon their clinicalfeatures.

MechanismIn patients with RTHα, TRα mutations in the LBDdue to nucleotide substitutions that cause missenseamino acid changes, deletions, or insertions, as wellas frameshift/premature stop mutations have been de-scribed [14, 24, 43]. Of note, none of the THRA mu-tations described so far involve the exon regions orthe expression of the REV-ERBα, a gene that is tran-scribed from the opposite strand of the THRA locus.Heterozygous THRA mutations are found in bothsporadic and familial RTHα. Thus, the molecularmechanism for RTHα is similar to RTHα, by virtue ofthe expression of the mutant TRα from one THRA

Singh and Yen Clinical Diabetes and Endocrinology (2017) 3:8 Page 8 of 11

allele and a normal TRα from the other THRA allele,and normal TRβs from the two THRB alleles [24].The mutant TRα has “dominant negative activity” onnormal TRs expressed within the cell. The degree ofdominant activity depends upon the relative amountof mutant TRα expressed within a particular cell aswell as the residual ligand-binding and DNA-bindingaffinities of the mutant TRα.Mutant TRαs bind to T3 with decreased affinity or fail

to bind ligand; and thus lead to decreased or no tran-scriptional activity, respectively. Similar to TRβ muta-tions in RTHβ, TRα1 mutants inhibit the function ofnormal TRs in a dominant negative manner when theyare co-expressed in transfected cells. In support of thismechanism in affected individuals, expression of TH-responsive target genes are blunted in peripheral bloodmononuclear cells of a patient with RTHα [24], suggest-ing that mutant TRαs can exert dominant negative activ-ity in vivo (Fig. 4). Additionally, studies have shown thatmany of the naturally occurring TRα mutations have de-creased release of NCoR due to lower T3 binding affinityby mutant TRαs.

TreatmentIn adults with RTHα, titrating the appropriate levothyr-oxine dose is difficult. Heart rate and cardiac contractil-ity can remain blunted despite thyroxine therapy.Excessive thyroxine treatment to correct cardiac param-eters also may lead to undesirable toxicities in tissuesthat express predominantly TRβ such as the liver. Inter-estingly, thyroxine therapy does not ameliorate the an-aemia observed in RTHα patients. In children, thetreatment of RTHβ is challenging [24, 46]. TH inducesthe expression of insulin-like growth factor 1 (IGF1) andsex hormone binding globulin (SHBG) and decreases theproduction of cholesterol and triglycerides. Thus, thy-roxine therapy can improve overall height and bonegrowth in RTHα [23, 24]. Of note, growth hormone incombination with thyroxine to increase IGF1 has not ledto significant improvement in height and growth [24].Thyroxine therapy also can improve the constipationsymptoms commonly found in children with RTHα.Thyroxine therapy suppresses serum TSH levels and

increases fT3 above normal levels. Serum SHBG, whichis induced by TH in the liver, as well as bone turnovermarkers also can increase above normal levels, mostlikely due to increased TH activity in tissues and celltypes that express mostly TRβ Just as in the case forRTHβ, development of TRα1-selective thyromimeticsmay be helpful to selectively activate normal TRα1 and/or mutant TRα1 with weak binding affinity for T3 toovercome TH resistance in tissues that express predom-inantly TRα. Another potential therapeutic strategy is todevelop drugs that enable nuclear receptor co-repressor

(NCoR) to dissociate from unliganded TR or to abrogatethe activity of histone deacetylases recruited by NCoR.In this connection, an inhibitor of histone deacetylase,suberoylanilide hydroxamic acid improved some of thephenotypic abnormalities of RTHα such as delayed anddecreased growth and bone development in a mousemodel of RTHα [20].

RTH in patients without TRβ mutationsSeveral patients with RTH have been identified who donot have TRβ or TRα mutations [31, 37]. Additionally, nomutations in various candidate co-factors involved in TR-mediated transcription were found. It is likely that epigen-etic effects that alter the expression of various genes in-volved in transcription may be involved, although it hasnot been investigated in these patients so far.

Somatic TR mutationsSomatic TRα and TRβ mutations have been identified inhuman hepatic, thyroid, and renal cell cancers [19, 22]in addition to TSH-secreting pituitary adenomas [2, 3].These findings suggest that TR mutations likely contrib-ute to RTH in these tumors; however, they are not suffi-cient to cause oncogenesis since RTH patients withgermline TRβ mutations do not appear to have an in-creased risk for cancer.

ConclusionAlthough RTHβ and RTHα are rare genetic disordersthat cause RTH, they need to be considered when pa-tients present with enigmatic thyroid function tests. Inparticular, when patients present with high free T3 andT4 with non-suppressed TSH levels (RTHβ) or reducedfree T4/ free T3 ratio with normal TSH level in theserum (RTHα). Associated with each condition are somecharacteristic features in their phenotype that also high-light the isoform-specific expression and particular rolesof TRβ and TRα. The clinical spectrum for both RTHβand RTHα is wide and heterogenous; moreover, therecan be variable phenotypes in patients with the samemutations. These observations suggest that genetic andepigenetic modifiers likely play important roles in thephenotypes of affected individuals. The identification ofTR mutations as causes for the two forms RTH, elucida-tion of their mechanism for causing resistance, correl-ation of genotype with phenotype, and the developmentof criteria for clinical diagnosis and treatment of RTHprovide elegant examples of the convergence of basic,translational, and clinical research to improve the under-standing and management of a genetic endocrinedisorder.

AbbbreviationsChIP-Seq: chromatin immunoprecipitation sequencing; FDH: familialdysalbuminemic hyperthyroxinemia; fT3: serum free T3 concentration;

Singh and Yen Clinical Diabetes and Endocrinology (2017) 3:8 Page 9 of 11

fT4: serum free T4 concentration; HAS: human serum albumin; HAT: histoneacetyltransferase HREs hormone response elements; HPT: hypothalamic,pituitary, and thyroid; IGF1: insulin-like growth factor 1; NCoR: nuclearreceptor co-repressor; NR: nuclear receptor; rT3: serum reversetriiodothyronine concentration; RTH: resistance to thyroid hormone;RTHα: resistance to thyroid hormone receptor α; RTHβ: resistance to thyroidhormone β; RXRs: retinoid X receptors; SHBG: sex hormone binding globulin;TBG: thyroxine-binding globulin; TH: thyroid hormone; THRA: thyroidhormone receptor β gene; THRB: thyroid hormone receptor β gene;TR: thyroid hormone receptor; TREs: thyroid hormone response elements;TRH: thyrotropin releasing hormone; TRIAC: triiodothyroacetic acid;TRα: thyroid hormone receptor α; TRβ: thyroid hormone receptor β;TSH: thyrotropin/thyroid stimulating hormone; TSHoma: TSH-secretingpituitary adenoma; TSHβ: thyroid stimulating hormone β subunit;TTR: transthyretin; α-GSU: common glycoprotein hormone α-subunit protein

Acknowledgementsn/a

FundingNMRC Singapore.

Availability of data and materialsn/a

Authors’ contributionsWriting PMY Figures and writing BKS. Both authors read and approved thefinal manuscript.

Ethics approval and consent to participateManuscripts reporting studies involving human participants, human data orhuman tissue must: n/a.

Consent for publicationn/a

Competing interestsAll authors read and approved the final manuscript.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Received: 31 March 2017 Accepted: 6 September 2017

References1. Albright F, Burnett CH, Smith PH, Parson W. Pseudohypopara-thyroidism —

an example of Seabright's bantam syndrome. Endocrinology. 1942;30:922.2. Ando S, Sarlis NJ, Krishnan J, Feng X, Refetoff S, Zhang MQ, Oldfield EH, Yen

PM. Aberrant alternative splicing of thyroid hormone receptor in a TSH-secreting pituitary tumor is a mechanism for hormone resistance. MolEndocrinol. 2001a;15:1529–38.

3. Ando S, Sarlis NJ, Oldfield EH, Yen PM. Somatic mutation of TRbeta cancause a defect in negative regulation of TSH in a TSH-secreting pituitarytumor. J Clin Endocrinol Metab. 2001b;86:5572–6.

4. Ayers S, Switnicki MP, Angajala A, Lammel J, Arumanayagam AS, Webb P.Genome-wide binding patterns of thyroid hormone receptor beta. PLoSOne. 2014;9:e81186.

5. Bassett JH, Harvey CB, Williams GR. Mechanisms of thyroid hormonereceptor-specific nuclear and extra nuclear actions. Mol Cell Endocrinol.2003;213:1–11.

6. Baxter JD, Webb P. Thyroid hormone mimetics: potential applications inatherosclerosis, obesity and type 2 diabetes. Nat Rev Drug Discov. 2009;8:308–20.

7. Beck-Peccoz P, Chatterjee VK. The variable clinical phenotype in thyroidhormone resistance syndrome. Thyroid. 1994;4:225–32.

8. Bernal, J. (2000). Thyroid hormones in brain development and function. InEndotext, L.J. De Groot, P. Beck-Peccoz, G. Chrousos, K. Dungan, A.Grossman, J.M. Hershman, C. Koch, R. McLachlan, M. New, R. Rebar, et al.,eds. (South Dartmouth (MA)).

9. Bernal J, Guadano-Ferraz A, Morte B. Thyroid hormone transporters–functions and clinical implications. Nat Rev Endocrinol. 2015;11:406–17.

10. Bochukova E, Schoenmakers N, Agostini M, Schoenmakers E, RajanayagamO, Keogh JM, Henning E, Reinemund J, Gevers E, Sarri M, et al. A mutationin the thyroid hormone receptor alpha gene. N Engl J Med. 2012;366:243–9.

11. Cheng SY. Isoform-dependent actions of thyroid hormone nuclearreceptors: lessons from knockin mutant mice. Steroids. 2005;70:450–4.

12. Cheng SY, Leonard JL, Davis PJ. Molecular aspects of thyroid hormoneactions. Endocr Rev. 2010;31:139–70.

13. Collingwood TN, Wagner R, Matthews CH, Clifton-Bligh RJ, Gurnell M,Rajanayagam O, Agostini M, Fletterick RJ, Beck-Peccoz P, Reinhardt W, et al.A role for helix 3 of the TRbeta ligand-binding domain in coactivatorrecruitment identified by characterization of a third cluster of mutations inresistance to thyroid hormone. EMBO J. 1998;17:4760–70.

14. Demir K, van Gucht AL, Buyukinan M, Catli G, Ayhan Y, Bas VN, Dundar B,Ozkan B, Meima ME, Visser WE, et al. Diverse genotypes and phenotypes ofthree novel thyroid hormone receptor-alpha mutations. J Clin EndocrinolMetab. 2016;101:2945–54.

15. Dumitrescu, A.M., and Refetoff, S. (2000). Impaired sensitivity to thyroidhormone: defects of transport, metabolism and action. In Endotext, L.J.De Groot, G. Chrousos, K. Dungan, K.R. Feingold, A. Grossman, J.M.Hershman, C. Koch, M. Korbonits, R. McLachlan, M. New, et al., eds.(South Dartmouth (MA)).

16. Flamant F, Samarut J. Thyroid hormone receptors: lessons from knockoutand knock-in mutant mice. Trends Endocrinol Metab. 2003;14:85–90.

17. Hayashi Y, Sunthornthepvarakul T, Refetoff S. Mutations of CpGdinucleotides located in the triiodothyronine (T3)-binding domain of thethyroid hormone receptor (TR) beta gene that appears to be devoid ofnatural mutations may not be detected because they are unlikely toproduce the clinical phenotype of resistance to thyroid hormone. J ClinInvest. 1994;94:607–15.

18. Jameson, J.L. (2004). Molecular mechanisms of end-organ resistance.Growth Horm IGF res 14 Suppl A, S45-50.

19. Kamiya Y, Puzianowska-Kuznicka M, McPhie P, Nauman J, Cheng SY,Nauman A. Expression of mutant thyroid hormone nuclear receptors isassociated with human renal clear cell carcinoma. Carcinogenesis.2002;23:25–33.

20. Kim DW, Park JW, Willingham MC, Cheng SY. A histone deacetylase inhibitorimproves hypothyroidism caused by a TRalpha1 mutant. Hum Mol Genet.2014;23:2651–64.

21. Lin KH, Fukuda T, Cheng SY. Hormone and DNA binding activity of apurified human thyroid hormone nuclear receptor expressed in EscherichiaColi. J Biol Chem. 1990;265:5161–5.

22. Lin KH, Shieh HY, Chen SL, Hsu HC. Expression of mutant thyroid hormone nuclearreceptors in human hepatocellular carcinoma cells. Mol Carcinog. 1999;26:53–61.

23. Moran C, Agostini M, Visser WE, Schoenmakers E, Schoenmakers N, OffiahAC, Poole K, Rajanayagam O, Lyons G, Halsall D, et al. Resistance to thyroidhormone caused by a mutation in thyroid hormone receptor (TR)alpha1and TRalpha2: clinical, biochemical, and genetic analyses of three relatedpatients. Lancet Diabetes Endocrinol. 2014;2:619–26.

24. Moran C, Chatterjee K. Resistance to thyroid hormone due to defective thyroidreceptor alpha. Best Pract Res Clin Endocrinol Metab. 2015;29:647–57.

25. Mukku VR, Kirkland JL, Hardy M, Stancel GM. Evidence for thyroid hormonereceptors in uterine nuclei. Metabolism. 1983;32:142–5.

26. Nagaya T, Jameson JL. Thyroid hormone receptor dimerization is requiredfor dominant negative inhibition by mutations that cause thyroid hormoneresistance. J Biol Chem. 1993;268:15766–71.

27. Nagaya T, Madison LD, Jameson JL. Thyroid hormone receptor mutants thatcause resistance to thyroid hormone. Evidence for receptor competition forDNA sequences in target genes J Biol Chem. 1992;267:13014–9.

28. Oetting A, Yen PM. New insights into thyroid hormone action. Best PractRes Clin Endocrinol Metab. 2007;21:193–208.

29. Ohba K, Leow MK, Singh BK, Sinha RA, Lesmana R, Liao XH, Ghosh S,Refetoff S, Sng JC, Yen PM. Desensitization and incomplete recovery ofhepatic target genes after chronic thyroid hormone treatment andwithdrawal in male adult mice. Endocrinology. 2016;157:1660–72.

30. Ortiga-Carvalho TM, Sidhaye AR, Wondisford FE. Thyroid hormone receptors andresistance to thyroid hormone disorders. Nat Rev Endocrinol. 2014;10:582–91.

31. Parikh S, Ando S, Schneider A, Skarulis MC, Sarlis NJ, Yen PM. Resistance tothyroid hormone in a patient without thyroid hormone receptor mutations.Thyroid. 2002;12:81–6.

Singh and Yen Clinical Diabetes and Endocrinology (2017) 3:8 Page 10 of 11

32. Radetti G, Persani L, Molinaro G, Mannavola D, Cortelazzi D, Chatterjee VK,Beck-Peccoz P. Clinical and hormonal outcome after two years oftriiodothyroacetic acid treatment in a child with thyroid hormoneresistance. Thyroid. 1997;7:775–8.

33. Ramadoss P, Abraham BJ, Tsai L, Zhou Y, Costa-e-Sousa RH, Ye F, Bilban M,Zhao K, Hollenberg AN. Novel mechanism of positive versus negativeregulation by thyroid hormone receptor beta1 (TRbeta1) identified bygenome-wide profiling of binding sites in mouse liver. J Biol Chem.2014;289:1313–28.

34. Refetoff S, Bassett JH, Beck-Peccoz P, Bernal J, Brent G, Chatterjee K, DeGroot LJ, Dumitrescu AM, Jameson JL, Kopp PA, et al. Classification andproposed nomenclature for inherited defects of thyroid hormone action,cell transport, and metabolism. J Clin Endocrinol Metab. 2014;99:768–70.

35. Refetoff S, DeWind LT, DeGroot LJ. Familial syndrome combining deaf-mutism, stuppled epiphyses, goiter and abnormally high PBI: possible targetorgan refractoriness to thyroid hormone. J Clin Endocrinol Metab.1967;27:279–94.

36. Refetoff S, Weiss RE, Usala SJ. The syndromes of resistance to thyroidhormone. Endocr Rev. 1993;14:348–99.

37. Reutrakul S, Sadow PM, Pannain S, Pohlenz J, Carvalho GA, Macchia PE,Weiss RE, Refetoff S. Search for abnormalities of nuclear corepressors,coactivators, and a coregulator in families with resistance to thyroidhormone without mutations in thyroid hormone receptor beta or alphagenes. J Clin Endocrinol Metab. 2000;85:3609–17.

38. Ribeiro RC, Apriletti JW, Wagner RL, West BL, Feng W, Huber R, Kushner PJ,Nilsson S, Scanlan T, Fletterick RJ, et al. Mechanisms of thyroid hormoneaction: insights from X-ray crystallographic and functional studies. RecentProg Horm Res. 1998;53:351–92. discussion 392-354

39. Safer JD, Cohen RN, Hollenberg AN, Wondisford FE. Defective release ofcorepressor by hinge mutants of the thyroid hormone receptor found inpatients with resistance to thyroid hormone. J Biol Chem. 1998;273:30175–82.

40. Sarne, D.H., Refetoff, S,, Rosenfield, R.L, Farriaux, J.P. (1988) Sex hormone-binding globulin in the diagnosis of peripheral tissue resistance to thyroidhormone: the value of changes after short term triiodothyronineadministration. J Clin Endocrinol Metab 66:740–746.

41. Sarne DH, Sobieszczyk S, Ain KB, Refetoff S. Serum thyrotropin and prolactinin the syndrome of generalized resistance to thyroid hormone: responses tothyrotropin-releasing hormone stimulation and short term triiodothyroninesuppression. J Clin Endocrinol Metab. 1990;70:1305–11.

42. Takeda K, Sakurai A, DeGroot LJ, Refetoff S. Recessive inheritance of thyroidhormone resistance caused by complete deletion of the protein-codingregion of the thyroid hormone receptor-beta gene. J Clin Endocrinol Metab.1992;74:49–55.

43. Tang Y, Yu M, Lian X. Resistance to thyroid hormone alpha, revelation ofbasic study to clinical consequences. J Pediatr Endocrinol Metab. 2016;29:511–22.

44. Taylor PN, Peeters R, Dayan CM. Genetic abnormalities in thyroid hormonedeiodinases. Curr Opin Endocrinol Diabetes Obes. 2015;22:402–6.

45. van Mullem A, van Heerebeek R, Chrysis D, Visser E, Medici M, Andrikoula M,Tsatsoulis A, Peeters R, Visser TJ. Clinical phenotype and mutant TRalpha1. NEngl J Med. 2012;366:1451–3.

46. van Mullem AA, Visser TJ, Peeters RP. Clinical consequences of mutations inthyroid hormone receptor-alpha1. Eur Thyroid J. 2014;3:17–24.

47. Visser TJ. Thyroid hormone transporters and resistance. Endocr Dev. 2013;24:1–10.48. Weintraub BD, Menezes-Ferreira MM, Petrick PA. Inappropriate secretion of

TSH. Endocr Res. 1989;15:601–17.49. Weiss RE, Refetoff S. Treatment of resistance to thyroid hormone–primum

non nocere. J Clin Endocrinol Metab. 1999;84:401–4.50. Weiss RE, Refetoff S. Resistance to thyroid hormone. Rev Endocr Metab

Disord. 2000;1:97–108.51. Wondisford FE. Thyroid hormone action: insight from transgenic mouse

models. J Investig Med. 2003;51:215–20.52. Yen PM. Molecular basis of resistance to thyroid hormone. Trends

Endocrinol Metab. 2003;14:327–33.53. Yen PM. Classical nuclear hormone receptor activity as a mediator of

complex biological responses: a look at health and disease. Best Pract ResClin Endocrinol Metab. 2015;29:517–28.

54. Yen PM, Sugawara A, Refetoff S, Chin WW. New insights on themechanism(s) of the dominant negative effect of mutant thyroid hormonereceptor in generalized resistance to thyroid hormone. J Clin Invest.1992;90:1825–31.

55. Zavacki AM, Ying H, Christoffolete MA, Aerts G, So E, Harney JW, Cheng SY,Larsen PR, Bianco AC. Type 1 iodothyronine deiodinase is a sensitive markerof peripheral thyroid status in the mouse. Endocrinology. 2005;146:1568–75.

56. Zhang XY, Kaneshige M, Kamiya Y, Kaneshige K, McPhie P, Cheng SY.Differential expression of thyroid hormone receptor isoforms dictates thedominant negative activity of mutant Beta receptor. Mol Endocrinol.2002;16:2077–92.

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Singh and Yen Clinical Diabetes and Endocrinology (2017) 3:8 Page 11 of 11

AbstractBackgroundThyroid hormone actionThyroid hormone receptorsResistance to thyroid hormone βClinical featuresDifferential diagnosisTRβ Mutations and mechanismTreatment

Resistance to thyroid hormone α (RTHα)Clinical featuresDifferential diagnosisMechanismTreatment

RTH in patients without TRβ mutationsSomatic TR mutationsConclusionAbbbreviationsFundingAvailability of data and materialsAuthors’ contributionsEthics approval and consent to participateConsent for publicationCompeting interestsPublisher’s NoteReferences