Pituitary Resistance To Thyroid Hormone Research Articles

Selective Pituitary Resistance to Thyroid Hormone: Clinical Hyperthyroidism with High TSH on Levothyroxine Administration in I-131 Ablated “Graves Disease”

Resistance to Thyroid Hormone (RTH) is a rare form of hormone resistance secondary to changes in the genes encoding thyroid hormone receptors. The two subtypes, Pituitary RTH (PRTH) and Generalized RTH (GRTH), cause clinically distinguishable patient presentations. In PRTH, typically only the pituitary gland is resistant to thyroid hormone (TH) while the rest of the body maintains sensitivity. Selective pituitary resistance to thyroid hormone results in dysregulation of thyroid hormone homeostasis with clinical presentation as either euthyroid or hyperthyroidism. PRTH is characterized by elevated thyroid hormone levels with an elevated or inappropriately normal TSH concentration. Herein we describe a case report of a 70-year-old woman who complained of weight loss of over 35 lbs., palpitations, jitters, hair loss, diarrhea, fatigue, muscle weakness, etc. over 6 months, thus, indicating the presence of iatrogenic hyperthyroidism while receiving levothyroxine 175 ug daily prescribed by her primary care provider because of a reported history of “Graves disease” treated by radioactive iodine ablation of the thyroid several years ago. The daily dose of levothyroxine had been increased gradually at an interval of 3 months over a year because of persistent elevation of serum TSH level. Laboratory tests revealed markedly elevated Free T4, Free T3 and TSH levels, along with low concentrations of all lipid fractions, serum creatinine and urea nitrogen levels, indicating TSH induced hyperthyroidism or PRTH. Further testing documented a mutation of thyroid hormone receptor beta gene 2 confirming presence of PRTH. We believe that the initial diagnosis of Graves Disease was erroneous and I-131 ablation further confounded and missed the diagnosis of PRTH. Thus, the purpose of this report is to report a patient with PRTH and describe potential pitfalls in diagnosis and management of this rare disorder.

Variable Clinical Characteristics and Molecular Spectrum of Patients with Syndromes of Reduced Sensitivity to Thyroid Hormone: Genetic Defects in the THRB and SLC16A2 Genes

Background/Aims: Syndromes of reduced sensitivity to thyroid hormone can be caused by innate resistance to thyroid hormone (RTH), thyroid hormone cell transporter defects, or thyroid hormone metabolism defects. This study was performed to describe clinical, endocrinological, and molecular characteristics of patients with disorders associated with impaired sensitivity to thyroid hormone due to THRB or SLC16A2 mutations. Methods: This study included 5 probands (1 male and 4 females) with RTH and 6 patients with Allan-Herndon-Dudley syndrome (AHDS). Clinical features and endocrine findings were reviewed retrospectively. Molecular analysis of two candidate genes, THRB or SLC16A2, confirmed the diagnosis. Results: Among RTH patients, median age at diagnosis was 5.6 years. Three patients were classified as having generalized RTH, whereas the other 2 patients were regarded as having isolated pituitary RTH. Three novel heterozygous mutations and 2 known mutations in THRB were identified from 5 independent pedigrees. All mutations were located in the major ligand-binding domain. In AHDS patients, delayed development was apparent between 3 and 6 months of age. Direct sequencing of SLC16A2 identified 6 hemizygous missense mutations in each patient: p.I188N, p.G221R, p.A224V, p.G276R, p.W398R, and p.G401R. Conclusions: This study identified 3 novel mutations in THRB in RTH patients and 1 novel mutation in SLC16A2 in AHDS patients. Routine neonatal screening based on the TSH assay has a limited role in detecting RTH or AHDS. Therefore, genetic testing of the candidate genes THRB and SLC16A2 should be performed for diagnosis of RTH and AHDS in patients with the suggestive clinical phenotype.

Evaluation and Management of Children with Thyrotoxicosis.

Uncontrolled thyrotoxicosis, especially in early infancy, may cause irreversible damage to the central nervous system as well as profound effects on the function of many organs. Thyrotoxicosis has multiple etiologies and treatment depends on the underlying etiology. An accurate diagnosis is essential so that appropriate treatment can be initiated without undue delay. To review in depth the evaluation, diagnosis, and treatment of children with thyrotoxicosis. A PubMed search was completed in Clinical Queries using the key terms "thyrotoxicosis" and "hyperthyroidism". The search strategy included meta-analysis, randomized controlled trials, clinical trials, observational studies, and reviews. Patents were searched using the key terms "thyrotoxicosis" and "hyperthyroidism" from www.freepatentsonline.com and www.google.com/patents. Graves' disease accounts for approximately 96% of pediatric cases of thyrotoxicosis. Other causes include Hashitoxicosis, toxic adenoma, toxic multinodular goiter, subacute granulomatous thyroiditis, acute suppurative thyroiditis, pituitary thyroid-stimulating hormone-secreting adenoma, pituitary thyroid hormone resistance, iodine-induced thyrotoxicosis, and drug-induced thyrotoxicosis. Familiarity of the clinical features would allow prompt diagnosis and institution of treatment. The underlying cause of thyrotoxicosis should be treated if possible. Treatment options for Graves' disease include antithyroid medications, radioiodine therapy, and surgery. Recent patents related to the management of thyrotoxicosis are discussed. Currently, antithyroid medications are considered to be the initial treatment of choice for Graves' disease in the pediatric age group. Radioactive iodine treatment is generally used for children with poor compliance with antithyroid medications, children not in remission after 1 to 2 years of antithyroid medications, and children with a major adverse effect while receiving an antithyroid medication. Total or near-total thyroidectomy should be considered in children younger than 5 years of age who do not respond to or experience a major adverse effect to antithyroid medications. Surgery should also be considered in those with very large goiter, severe ophthalmopathy, pregnancy, persistent hyperthyroidism in spite of treatment with antithyroid medications and radioactive iodine, and personal preference.

Low Thyroid Hormone Levels Disrupt Thyrotrope Development

Low thyroid hormone (TH) conditions caused by a variety of prenatal and perinatal problems have been shown to alter postnatal regulatory thyrotropin (TSH) responsiveness to TH in humans and rodents. The mechanisms underlying this pituitary TH resistance remain unknown. Here we use the evolutionarily conserved zebrafish model to examine the effects of low TH on thyrotrope development and function. Zebrafish were exposed to the goitrogen 6-propyl-2-thiouracil (PTU) to block TH synthesis, and this led to an approximately 50% increase in thyrotrope numbers and an 8- to 10-fold increase in tshb mRNA abundance in 2-week-old larvae and 1-month-old juveniles. Thyrotrope numbers returned to normal 3 weeks after cessation of PTU treatment, demonstrating that these effects were reversible and revealing substantial plasticity in pituitary-thyroid axis regulation. Using a T4 challenge assay, we found that development under low-TH conditions did not affect the ability of T4 to suppress tshb mRNA levels despite the thyrotrope hyperplasia that resulted from temporary low-TH conditions. Together, these studies show that low developmental TH levels can lead to changes in thyrotrope number and function, providing a possible cellular mechanism underlying elevated TSH levels seen in neonates with either permanent or transient congenital hypothyroidism.

Pedigree analysis of a pituitary-resistance to thyroid hormone syndrome family caused by heterozygous A317T mutation in thyroid hormone receptor β gene

To study thyroid hormone receptor β(THRβ)gene mutation in a pituitary-resistance to thyroid hormone syndrome family. The peripheral blood samples of the patient, his sister, parents, and 4 maternal relatives were collected. Then serum was isolated for detecting thyroid hormone levels with chemiluminescence immunoassay, and DNA was extracted for PCR, and 10 exons of THRβ gene were sequenced. The patient and his mother had the hyperthyroid symptom for many years and his mother with atrial fibrillation. The G→A heterozygous transition mutation was confirmed by exon sequencing at nucleotide 949 within exon 9 of THRβ gene in the patient and his mother, which was a missense mutation causing a substitution of Alanine to Threonine(A317T). No mutation was found in THRβ gene in other family members. This is the first Chinese family reported with pituitary thyroid hormone resistance syndrome caused by a A317T mutation in the thyroid hormone receptor β gene. (Chin J Endocrinol Metab, 2017, 33: 215-219) Key words: Thyroid hormone resistance syndrome; Pituitary-resistance to thyroid hormone syndrome; Thyroid hormone receptor β; Mutation; Chinese

Thyroid Profiles in a Patient with Resistance to Thyroid Hormone and Episodes of Thyrotoxicosis, Including Repeated Painless Thyroiditis

Thyrotoxic disease can be difficult to recognize in patients with resistance to thyroid hormone (RTH) because the clinical symptoms of thyrotoxicosis cannot be observed, and thyrotropin (TSH) may not be suppressed because of hormone resistance. Painless thyroiditis is a relatively common cause of thyrotoxicosis, but its occurrence in RTH has not been reported. We assessed the thyroid profile in a patient with RTH and episodes of thyrotoxicosis who experienced repeated painless thyroiditis. A 44-year-old Japanese woman with RTH, which was confirmed by the presence of a P453A mutation in the thyroid hormone receptor β (TRβ) gene, showed a slight elevation of the basal levels of thyroid hormones, which indicated that her pituitary RTH was mild. She experienced a slight exacerbation of hyperthyroxinemia concomitant with TSH suppression. A diagnosis of painless thyroiditis was made because of the absence of TSH receptor antibodies, low Tc-99m pertechnetate uptake by the thyroid gland, and transient suppression followed by a slight elevation of TSH following the elevation of thyroid hormones. The patient's complaints of general malaise and occasional palpitations did not change throughout the course of painless thyroiditis. Three years later, painless thyroiditis occurred again without any deterioration of the clinical manifestations. Mild pituitary RTH can be overcome by slight exacerbation of hyperthyroxinemia during mild thyrotoxicosis. When pituitary resistance is severe and TSH is not suppressed, thyrotoxicosis may be overlooked.

Pituitary resistance to thyroid hormones: pathophysiology and therapeutic options

Thyroid hormone secretion suppresses the expression of thyroid stimulating hormone (TSH), both of which are strictly controlled by a negative feedback loop between the hypothalamus-pituitary and thyroid. Pituitary resistance to thyroid hormone (PRTH) is defined as resistance to the action of thyroid hormone that is more severe in the pituitary than at the peripheral tissue level. Although the molecular basis of PRTH is not well understood, the clinical issue mainly involves imbalance between the hypothalamus-pituitary and peripheral thyroid hormone responsivity, which may induce peripheral thyrotoxic phenomena. Here, we review the pathogenesis and molecular aspects of PRTH, present a single case with inappropriate TSH secretion suffering from thyrotoxicosis treated with PTU, and discuss the possible choice of therapeutic options to correct the imbalance of thyroid hormone responsivity in both the hypothalamus-pituitary and peripheral tissues.

A Mechanism for Pituitary-Resistance to Thyroid Hormone (PRTH) Syndrome: a Loss in Cooperative Coactivator Contacts by Thyroid Hormone Receptor (TR)β2

Thyroid hormone receptors (TR) are hormone-modulated transcription factors that regulate overall metabolic rate, lipid utilization, heart rate, and development. TR are expressed as a mix of interrelated receptor isoforms. The TRβ2 isoform is expressed in the hypothalamus and pituitary, where it plays an important role in the feedback regulation of thyroid hormone levels. TRβ2 exhibits unique transcriptional properties that parallel the ability of this isoform to bind to certain coactivators cooperatively through multiple contact surfaces. The more peripherally expressed TRβ1 isoform, in contrast, appears to recruit these coactivators through a single contact mechanism. We report here that clusters of charged amino acids in the TR hormone-binding domain are required for this enhanced mode of coactivator recruitment and that mutations in these charge clusters, by disrupting TRβ2 coactivator binding, are a molecular basis for pituitary resistance to thyroid hormone, a disease characterized by inappropriate thyroid hormone feedback regulation. We propose that the charge clusters allow wild-type TRβ2 to assume a conformation compatible with its mode of multiple contact coactivator recruitment, whereas disruption of these charge clusters disrupts normal T(3) homeostasis by reducing TRβ2 to a TRβ1-like, single contact mode of coactivator binding.

Resistance to Thyroid Hormone in a Patient with Coexisting Graves' Disease

Resistance to thyroid hormone (RTH) is a rare disorder in which an abnormality in the binding domain of the thyroid hormone receptor results in end-organ insensitivity to the actions of thyroid hormones. This disorder is sometimes misdiagnosed as Graves' disease due to the presence of a goiter with high Iodine-123 thyroid uptake and high levels of total and free thyroid hormones. Additionally, some patients with RTH may experience palpitations or tachycardia. There is a previous report of a patient with pituitary RTH and concurrent Graves' disease. We describe a patient who was found to have a goiter with signs and symptoms of hyperthyroidism by his primary care physician. Work-up revealed a low thyroid stimulating hormone (TSH), high thyroid-stimulating immunoglobulins, high Iodine-123 thyroid uptake, as well as increased homogenous activity in both thyroid glands on nuclear scan and a suppressed TSH. Diagnosis of Graves' disease was made, and the patient underwent radioactive ablation of the thyroid gland. The patient subsequently developed hypothyroidism and required a much higher dose of levothyroxine than his weight-based estimate of a full replacement dose to maintain euthyroidism. On 325 micog of daily levothyroxine the patient was clinically euthyroid and had a normal TSH although his levels of T3 and T4 were high. Work-up revealed a mutation in the thyroid hormone beta-receptor gene, consistent with diagnosis of RTH. RTH is often misdiagnosed as Graves' disease. However, these disorders can coexist, and the concurrent presence of both disorders in a patient can present diagnostic challenges. A previous report of a patient with Graves' disease associated with RTH was published before gene sequencing could be used to confirm diagnosis of RTH. We present a patient with Graves' disease and concurrent RTH that was confirmed by gene sequencing, showing a mutation in the thyroid hormone receptor beta gene.

A thyroid hormone receptor mutation that dissociates thyroid hormone regulation of gene expression in vivo

Resistance to thyroid hormone (RTH) is most often due to point mutations in the beta-isoform of the thyroid hormone (TH) receptor (TR-beta). The majority of mutations involve the ligand-binding domain, where they block TH binding and receptor function on both stimulatory and inhibitory TH response elements. In contrast, a few mutations in the ligand-binding domain are reported to maintain TH binding and yet cause RTH in certain tissues. We introduced one such naturally occurring human RTH mutation (R429Q) into the germline of mice at the TR-beta locus. R429Q knock-in (KI) mice demonstrated elevated serum TH and inappropriately normal thyroid-stimulating hormone (TSH) levels, consistent with hypothalamic-pituitary RTH. In contrast, 3 hepatic genes positively regulated by TH (Dio1, Gpd1, and Thrsp) were increased in R429Q KI animals. Mice were then rendered hypothyroid, followed by graded T(3) replacement. Hypothyroid R429Q KI mice displayed elevated TSH subunit mRNA levels, and T(3) treatment failed to normally suppress these levels. T(3) treatment, however, stimulated pituitary Gh levels to a greater degree in R429Q KI than in control mice. Gsta, a hepatic gene negatively regulated by TH, was not suppressed in R429Q KI mice after T(3) treatment, but hepatic Dio1 and Thrsp mRNA levels increased in response to TH. Cardiac myosin heavy chain isoform gene expression also showed a specific defect in TH inhibition. In summary, the R429Q mutation is associated with selective impairment of TH-mediated gene repression, suggesting that the affected domain, necessary for TR homodimerization and corepressor binding, has a critical role in negative gene regulation by TH.

Evaluation of Thyroid Hormone Action in a Case of Generalized Resistance to Thyroid Hormone with Chronic Thyroiditis: Discovery of a Novel Heterozygous Missense Mutation (G347A)

Resistance to thyroid hormone (RTH) is a dominantly inherited syndrome of variable tissue hyporesponsiveness to thyroid hormone (TH). Its characteristics are a high level of TH and inappropriate lack of TSH suppression. RTH is mainly categorized as generalized RTH (GRTH), pituitary RTH (PRTH), and peripheral tissue RTH (PTRTH). Untreated subjects with GRTH usually achieve a normal metabolic state. We describe a 21-year-old Japanese woman with GRTH and coincidental chronic thyroiditis. Physical examination revealed palpable goiter, congenital alopecia on top of the head, and short stature. She showed elevated levels of free triiodothyronine (FT3) and free thyroxine (FT4), and an inappropriately normal level of TSH. Anti-thyroglobulin and anti-thyroid peroxidase antibodies were positive. A TRH stimulation test showed a normal TSH response. The patient received the standardized diagnostic protocol, administration of incremental doses of liothyronine (L-T3). The peak TSH level after the TRH stimulation test gradually decreased. The patient showed low sensitivity to TH in terms of bone metabolism, protein catabolism, lipid metabolism, and urine magnesium metabolism. Sequence analysis of the TR beta gene was performed with informed consent, and this revealed a novel heterozygous mutation at codon 347 resulting in a GGG (glycine) to GCG (alanine) substitution (G347A). The patient was diagnosed as having GRTH with chronic thyroiditis, and carrying a novel mutation, G347A, of the TR beta gene.

A Novel Thyroid Hormone Receptor-β Mutation That Fails to Bind Nuclear Receptor Corepressor in a Patient as an Apparent Cause of Severe, Predominantly Pituitary Resistance to Thyroid Hormone

Resistance to thyroid hormone (RTH) is a dominantly inherited syndrome of variable tissue hyporesponsiveness to thyroid hormone (TH). We report a newborn who presented with severe RTH (Mkar) with serum TSH 1500 mU/liter and free T(3) greater than 50 pm (normal 3.1-9.4) and free T(4) 25.3 pm (normal 12-22). We hypothesized that the RTH was due to reduced ligand binding and/or abnormal interaction with nuclear cofactors. These were prospective in vivo and in vitro studies. The study was conducted at a tertiary care university hospital. Patients included a newborn child and two other subjects with RTH. The effect of various TH-lowering agents in the subject with RTH was studied. In vitro studies including EMSA and mammalian two-hybrid assay as well as in vitro transfection studies were conducted. Sequencing of the TH receptor (TR)beta and in vitro measurements of receptor-cofactor interaction were measured. Sequencing of the TRbeta demonstrated a de novo heterozygous mutation, 1590_1591insT, resulting in a frameshift producing a mutant TRbeta (mutTR)-beta with a 28-amino acid (aa) nonsense sequence and 2-amino acid carboxyl-terminal extension. The Mkar mutation was evaluated in comparison to three other TRbeta frameshift mutations in the carboxyl terminus. EMSA demonstrated that the Mkar mutTRbeta1 had impaired ability to recruit nuclear receptor corepressor but intact association with silencing mediator of retinoid and thyroid receptor (SMRT). Our data suggest that alterations in codons 436-453 in helix 11 result in significantly diminished association with nuclear receptor corepressor but not SMRT. This novel mutTRbeta demonstrates nuclear corepressor specificity that results in severe predominantly pituitary RTH due to impaired release of SMRT.

Pituitary Resistance to Thyroid Hormone Syndrome Is Associated with T3Receptor Mutants that Selectively Impair β2 Isoform Function

Resistance to thyroid hormone (RTH) syndrome is an inherited inability to respond appropriately to T3 hormone. In generalized RTH, the T3 response of both the pituitary and periphery is disrupted. In pituitary (or central) RTH, the ability of the pituitary to sense (and down-regulate) elevated T3 is selectively impaired, whereas the periphery remains relatively T3 responsive, resulting in peripheral thyrotoxicity. Both forms of disease are linked to mutations in thyroid hormone receptor (TR)-beta. TRbeta is expressed by alternate mRNA splicing as two isoforms: TRbeta2, found primarily in the pituitary/hypothalamus, and TRbeta1, expressed broadly in many tissues. We report here that the wild-type TRbeta2 isoform displays an enhanced T3 response relative to the TRbeta1 isoform. Mutations associated with generalized RTH (P453S, G345S) impair both TRbeta2 and TRbeta1 function proportionally, whereas mutations associated with pituitary-specific RTH (R338L, R338W, R429Q) disproportionately disrupt TRbeta2 function. We propose that in the normal organism, and in generalized RTH, TRbeta2 in the pituitary can sense rising T3 levels in advance of TRbeta1 in the periphery, preventing thyrotoxicity. In contrast, the TRbeta mutations associated with pituitary RTH disproportionately disrupt the pituitary's ability to sense and suppress elevated T3 levels in advance of the periphery, producing symptoms of thyrotoxicity.

Eficacia de la quinagolida en el tratamiento de un paciente con resistencia hipofisaria a hormonas tiroideas

The pituitary resistance to thyroid hormones (PRTH) is not very frequent and well-known entity, their treatment it continues being topic of controversy. In this work we have evaluated the quinagolida effectiveness in the treatment of it unites patient with (PRTH). The relationship among thyroid stimulating hormone (TSH) and free triiodothyronine (FT3) it was used as marker of the thyroid resistance and of the response to the treatment. The concentrations of TSH and FT3 were normalized after adding quinagolida to methimazole. These results suggest that the quinagolida could be an useful drug in the treatment of this pathology, next to the classic treatments.

Introducing a point mutation identified in a patient with pituitary resistance to thyroid hormone (Arg 338 to Trp) into other mutant thyroid hormone receptors weakens their dominant negative activities.

Clinical resistance to thyroid hormone (RTH) has been classified into generalized resistance to thyroid hormone (GRTH) and pituitary resistance to thyroid hormone (PRTH) types. Since similar mutations have been identified in tri-iodothyronine (T3) receptor (TR) beta gene in GRTH and PRTH, and since considerable overlap has been seen in the clinical manifestations in patients with GRTH and PRTH, two subtypes of RTH are now considered to be a continuous spectrum with the same genetic defect. A point mutation at amino acid Arg 338 to Trp (R338W) which we identified in a patient with PRTH is very interesting, since R338W has been found in several other patients with PRTH, raising the possibility that this mutation may tend to associate with a phenotype of PRTH. In our previous study, we found that R338W had relatively less impaired transcriptional potency, weaker dominant negative activity on various T3 response elements and poor homodimer formation, as compared with another GRTH mutant. In this study, to investigate the functional properties of R338W further, especially in terms of the relation between transcriptional activity and dimer formations, we introduced the R338W mutation into the mutant receptors, K443E and F451X, constructing the double mutants, R338W/K443E and R338W/ F451X. Both R338W/K443E and R338W/F451X showed negligible T3 binding and transcriptional activities. The dominant negative activities of K443E and F451X were, however, significantly weakened by introducing the R338W mutation. As a control, a double mutant G345R/K443E was constructed by introducing a point mutation, G345R, located in the same exon 9 as R338W, into the K443E mutant. Dominant negative activity did not differ between G345R/K443E and K443E. Homodimer formation was significantly reduced in the double mutants containing R338W, but not G345R. In summary, introducing the R338W mutation, but not G345R, into the mutant TR significantly weakened the dominant negative activity, despite further impairment of the T3 binding and transcriptional activities.

Functional properties of a mutant T 3 receptor β (R338W) identified in a subject with pituitary resistance to thyroid hormone

Previously, we identified a point mutation of the T 3 receptor (TR) β gene (R338W) in a patient with pituitary resistance to thyroid hormone (PRTH). The mutation existed in one of two hot spot areas in TR β gene where clusters of mutations have been found in subjects with generalized resistance to thyroid hormone (GRTH). Interestingly, R338W has been identified independently in several different subjects with ‘PRTH’. This raises the question how R338W induces the phenotypical features responsible for PRTH. In the present study, we examined the functional properties of R338W in comparison with those of a GRTH-mutant, K443E. The levels of thyroid hormones and inappropriately elevated TSH (STTSH) were similar between subjects with K443E and R338W. Transcriptional activities and dominant negative potencies were measured by CAT assay in CV1 cells transfected with each mutant TRβ1 or along with wild-type TR. When a reporter gene containing T 3-responsive elements (TRE), TRE-pal2, DR4 or myosin heavy chain α subunit, was used, transcriptional activation induced by R338W was higher than that by K443E on any TREs. The dominant negative effects on wild-type TRβ1 or wild-type TRα1 were milder in R338W than in K443E. At 50 nM T 3, K443E decreased the transcriptional activity of wild-type TRβ1 on TRE-pal2 by 31.5%, while R338W reduced by 13.6% ( n = 15, P < 0.05). Co-expression of retinoid X receptor (RXR) α increased transcriptional activity of R338W and K443E, but not of wild-type TRβ1. Dominant negative activity on TRE-TSHα subunit of R338W was milder than that of K443E. When T 3-binding activities of mutant TRβ1s expressed in the cells were assayed under the same cell conditions for CAT assay, both mutant TRβ1 showed remarkably reduced activity with no difference between the two. Gel mobility shift assay using TRE-DR4 showed poor homodimer formation of R338W. Heterodimerization with RXRα was similar between R338W, K443E and wild-type TRβ1. The result of the present study suggested that R338W had relatively mild transcriptional and dominant negative activities on several TREs including TRE-TSHα subunit. We also showed poor homodimerization of R338W, which might be related to its weak dominant negative potency.

Resistance to thyroid hormone in children

The genetic etiology of thyroid hormone resistance syndromes is now well established. Two clinical variants, generalized resistance to thyroid hormone (GRTH) and selective pituitary resistance to thyroid hormone (PRTH), are, in most cases, caused by heterozygous mutations in the ligand-binding domain of the c-erbA beta thyroid hormone receptor gene. No human mutations have yet been described in the other related receptor gene, c-erbA alpha. In resistant patients, the mutant beta receptors act as dominant negative proteins and inhibit function of the normal beta receptor (expressed from one allele) and the normal alpha receptor (expressed from two alleles). Patients homozygous for a dominant negative allele (the Bercu patient) and without any beta receptor (the Refetoff patient) have been described. Patients with GRTH and PRTH both present with elevated free thyroxine and triiodothyronine and inappropriately normal thyroid-stimulating hormone, but the former patients are clinically euthyroid, whereas the latter patients have symptoms and signs of hyperthyroidism. However, in some cases, different patients who have been classified as having GRTH and PRTH have been found to have identical beta mutations. A recent study of the level of pituitary resistance in a large kindred with GRTH (ARG-320-HIS mutation) indicated a contributory gene in the regulation of thyroid hormone action. Relative overexpression of the mutant PRO-453-HIS receptor at the level of messenger RNA in patient fibroblasts (kindred A) was associated with short stature. Finally, an ARG-316-HIS mutation (kindred G-H) was associated with relatively weak dominant negative activity and perturbed DNA-binding properties.(ABSTRACT TRUNCATED AT 250 WORDS)

An arginine to histidine mutation in codon 311 of the C-erbA beta gene results in a mutant thyroid hormone receptor that does not mediate a dominant negative phenotype.

We have examined the c-erbA beta thyroid hormone receptor gene in a kindred, G.H., with a member, patient G.H., who had a severe form of selective pituitary resistance to thyroid hormones (PRTH). This patient manifested inappropriately normal thyrotropin-stimulating hormone, markedly elevated serum free thyroxine (T4) and total triiodothyronine (T3), and clinical hyperthyroidism. The complete c-erbA beta 1 coding sequence was examined by a combination of genomic and cDNA cloning for patient G.H. and her unaffected father. A single mutation, a guanine to adenine transition at nucleotide 1,232, was found in one allele of both these members, altering codon 311 from arginine to histidine. In addition, a half-sister of patient G.H. also harbored this mutant allele and, like the father, was clinically normal. The G.H. receptor, synthesized with reticulocyte lysate, had significantly defective T3-binding activity with a Ka of approximately 5 x 10(8) M-1. RNA phenotyping using leukocytes and fibroblasts demonstrated an equal level of expression of wild-type and mutant alleles in patient G.H. and her unaffected father. Finally, the G.H. receptor had no detectable dominant negative activity in a transfection assay. Thus, in contrast to the many other beta-receptor mutants responsible for the generalized form of thyroid hormone resistance, the G.H. receptor appeared unable to antagonize normal receptor function. These results suggest that the arginine at codon 311 in c-erbA beta is crucial for the structural integrity required for dominant negative function. The ARG-311-HIS mutation may contribute to PRTH in patient G.H. by inactivating a beta-receptor allele, but it cannot be the sole cause of the disease.

Pituitary resistance to thyroid hormones.

Pituitary thyroid hormone resistance (PRTH) refers to a particular form of thyroid hormone refractoriness that is accompanied by peripheral hyperthyroidism, as only the TSH-secreting pituitary cells appear to be resistant to the effects of thyroid hormones. The presence of PRTH is suspected and diagnosed on the basis of the finding of high free thyroid hormone levels along with unsuppressed TSH, clinical signs and symptoms of hyperthyroidism and values of at least one of the parameters evaluating peripheral thyroid hormone action in the hyperthyroid range. However, most patients with PRTH present with clinical signs and symptoms of thyroid dysfunction, particularly goiter and tachycardia, overlapping those recorded in patients with generalized thyroid hormone resistance (GRTH), i.e. refractoriness to thyroid hormones at both pituitary and peripheral tissue level. Moreover, most of them display normal values of other parameters evaluating the peripheral effects of thyroid hormones and bear mutations in the gene encoding for T3 nuclear receptors similar to those found in patients with GRTH. These findings are questioning the existence of PRTH as a separate clinical entity and support the view that the various forms of thyroid hormone resistance may be part of a spectrum of disease with variable expression in different issues.

Sex Hormone-Binding Globulin Measurement in Patients with Inappropriate Secretion of Thyrotropin (IST): Evidence against Selective Pituitary Thyroid Hormone Resistance in Nonneoplastic IST*

The differential diagnosis of the various forms of inappropriate secretion of TSH (IST), i.e. generalized thyroid hormone resistance (GRTH), selective pituitary resistance [non-neoplastic IST (nnIST)], and tumoral pituitary TSH hypersecretion [neoplastic IST (nIST)], mainly rests on clinical observation, skull imaging, and measurement of several parameters assessing peripheral thyroid hormone effects. Clinically, patients with GRTH usually display compensated hypothyroidism, while those with nnIST or nIST are hyperthyroid. Since sex hormone-binding globulin (SHBG) measurement has been shown to be a reliable parameter in distinguishing between euthyroid and hyperthyroid states, we evaluated serum SHBG levels in 39 patients with IST (7 with GRTH, 15 with nnIST, and 17 with nIST). The results were compared to those in 68 normal subjects, 76 hyperthyroid patients, and 31 hypothyroid patients. SHBG levels in patients with either GRTH or nnIST were similar to those in controls or hypothyroid patients [GRTH, 40.5 +/- 11.8 (+/- SD) nmol/L (range, 26.4-57.5); nnIST, 29.7 +/- 12.8 nmol/L (range, 6.8-46.8); controls, 36.7 +/- 21.7 nmol/L (range, 5.4-96.5); hypothyroid, 30.8 +/- 14.4 nmol/L (range, 10.4-63.3)]. On the contrary, SHBG levels in patients with either overt hyperthyroidism or nIST were significantly higher than those in the above groups [hyperthyroid, 149 +/- 111 nmol/L (range, 48-557); nIST, 99.5 +/- 54.7 nmol/L (range, 21.6-259)]. The apparent overlap of SHBG values between hyperthyroid patients and controls almost completely disappeared when comparisons were made with control groups matched for age and sex. Additional indices of peripheral thyroid hormone action (basal metabolic rate, cardiac systolic time intervals, and Achilles' reflex time) were normal in patients with GRTH, while they were in the hyperthyroid range in patients with nnIST and nIST. After successful treatment of hyperthyroidism, SHBG levels normalized in patients with nIST, but they did not change in patients with nnIST. In conclusion, the measurement of SHBG in patients with IST is useful in differentiating the neoplastic form from that due to thyroid hormone resistance, but it fails to distinguish between generalized and pituitary resistance to thyroid hormone action. Moreover, the present data suggest that the resistance to thyroid hormone action in patients with nnIST is not selective at the thyrotroph cell level, but also involves the hepatic SHBG-synthesizing cells, thus supporting the view that the various forms of thyroid hormone resistance could represent a continuum of the same defect with variable expression in different tissues.