Prenatal diagnosis of thyroid hormone resistance

Thyroid hormone resistance: Mechanisms and therapeutic development

As an essential primary hormone, thyroid hormone (TH) is indispensable for human growth, development and metabolism. Impairment of TH function in several aspects, including TH synthesis, activation, transportation and receptor-dependent transactivation, can eventually lead to thyroid hormone resistance syndrome (RTH). RTH is a rare syndrome that manifests as a reduced target cell response to TH signaling. The majority of RTH cases are related to thyroid hormone receptor β (TRβ) mutations, and only a few RTH cases are associated with thyroid hormone receptor α (TRα) mutations or other causes. Patients with RTH suffer from goiter, mental retardation, short stature and bradycardia or tachycardia. To date, approximately 170 mutated TRβ variants and more than 20 mutated TRα variants at the amino acid level have been reported in RTH patients. In addition to these mutated proteins, some TR isoforms can also reduce TH function by competing with primary TRs for TRE and RXR binding. Fortunately, different treatments for RTH have been explored with structure-activity relationship (SAR) studies and drug design, and among these treatments. With thyromimetic potency but biochemical properties that differ from those of primary TH (T3 and T4), these TH analogs can bypass specific defective transporters or reactive mutant TRs. However, these compounds must be carefully applied to avoid over activating TRα, which is associated with more severe heart impairment. The structural mechanisms of mutation-induced RTH in the TR ligand-binding domain are summarized in this review. Furthermore, strategies to overcome this resistance for therapeutic development are also discussed.

Mutational Landscape of Resistance to Thyroid Hormone Beta (RTHβ)

Resistance to thyroid hormone beta (RTHβ) is a syndrome characterized by reduced responsiveness of peripheral tissues to thyroid hormone (TH). In most cases, the disorder is associated with germline pathogenic variants in the thyroid hormone receptor beta (THRB) gene. This paper summarizes the clinical and biochemical presentation of the disease, providing a comprehensive overview on molecular genetic features. Particular care is given in reporting all identified THRB variants with an assessed or unknown clinical significance. Our aim is to offer a useful tool for clinical and genetic specialists in order to ease clinical diagnosis and genetic counseling.

Monocarboxylate Transporter 8 Deficiency: Delayed or Permanent Hypomyelination?

Monocarboxylate transporter 8 (MCT8) deficiency or the Allan-Herndon-Dudley Syndrome (AHDS) is an X-linked psychomotor disability syndrome with around 320 clinical cases described worldwide. SLC16A2 gene mutations, encoding the thyroid hormone (TH) transporter MCT8, result in intellectual disability due to impaired TH uptake in the developing brain. MCT8 deficiency is a multi-organ affecting disease with a predominant neuronal cell-based pathology, with the glial component inadequately investigated. However, deficiency in myelin, a key component of white matter (WM) enabling fast nerve conduction, is a TH-dependent hallmark of the disease. Nevertheless, analysis of the myelin status in AHDS patients has led to conflicting interpretations. The majority of individual case studies reported delayed myelination, that was restored later in life. In contrast, post-mortem studies and high-resolution MRIs detected WM (micro-) abnormalities throughout adolescence, suggesting permanent hypomyelination. Thus, interpretations vary depending on methodology to investigate WM microstructure. Further, it is unknown whether the mutation within the MCT8 is linked to the severity of the myelin deficiency. Consequently, terminology is inconsistent among reports, and AHDS is occasionally misdiagnosed as another WM disorder. The evolutionary conserved TH signaling pathway that promotes the generation of myelinating oligodendrocytes enabled deciphering how the lack of MCT8 might affect myelinogenesis. Linking patient findings on myelination to those obtained from models of MCT8 deficiency revealed underlying pathophysiological mechanisms, but knowledge gaps remain, notably how myelination progresses both spatially and temporally in MCT8 deficiency. This limits predicting how myelin integrity might benefit therapeutically, and when to initiate. A recurrent observation in clinical trials is the absence of neurological improvement. Testing MCT8-independent thyromimetics in models, and evaluating treatments used in other demyelinating diseases, despite different etiologies, is crucial to propose new therapeutic strategies combatting this devastating disease.

Thyroid hormone resistance from newborns to adults: a Spanish experience

OBJECTIVE

Thyroid hormone resistance (RTH β) is a rare genetic disorder characterized by an altered response of target tissue to the action of thyroid hormone. Few studies on RTH β have been carried out in southern European populations. We aimed to describe the clinical and genetic characteristics at the time of diagnosis in a Spanish cohort of patients with genetically confirmed RTH β, with ages ranging from newborns to adults.

METHODS

Retrospective multicenter study of 28 patients who were genetically confirmed as RTH β. Clinical and biochemical data were collected from the reference centers, and the studied variables included age, sex, anthropometric data, clinical characteristics and biochemical results. In the Basque country, a simultaneous analysis of TSH and T4 is carried out in the program for the screening of inborn errors of metabolism. A molecular analysis of the thyroid hormone beta (THRB) gene was performed.

RESULTS

The total cohort included 20 adults and eight pediatric patients (six newborns). Of the total, 5 (17.8%) were diagnosed by clinical characteristics (goiter, hypertension or tachycardia), 13 (46.4%) were analyzed in the context of a family study and 10 (35.7%) were diagnosed after obtaining an altered fT4 and/or TSH level in a biochemical analysis performed due to clinical symptoms unrelated to RTH β. Four of the newborns included in the series were diagnosed by the result of neonatal screening, which allows us to estimate a minimum local incidence of RTH β of 1/18,750 live newborns. The genetic analysis showed the presence of 12 different heterozygous mutations in the THRB gene.

CONCLUSIONS

We report the clinical and genetic characteristics of a Spanish RTH β cohort, from neonates to adults. We also describe one novel mutation in the THRB gene as the cause of the disease. The simultaneous analysis of TSH and T4 carried out in the program for the screening of inborn errors of metabolism facilitates the early diagnosis of RTH β in newborns and has allowed us to estimate a minimum local incidence of RTH of 1/18,750 live newborns.

From zebrafish to human: A comparative approach to elucidate the role of the thyroid hormone transporter MCT8 during brain development

Monocarboxylate transporter 8 (MCT8) facilitates transmembrane transport of thyroid hormones (THs) ensuring their action on gene expression during vertebrate neurodevelopment. A loss of MCT8 in humans results in severe psychomotor deficits associated with the Allan-Herndon-Dudley Syndrome (AHDS). However, where and when exactly a lack of MCT8 causes the neurological manifestations remains unclear because of the varying expression pattern of MCT8 between specific brain regions and cells. Here, we elaborate on the animal models that have been generated to elucidate the mechanisms underlying MCT8-deficient brain development. The absence of a clear neurological phenotype in Mct8 knockout mice made it clear that a single species would not suffice. The evolutionary conservation of TH action on neurodevelopment as well as the components regulating TH signalling however offers the opportunity to answer different aspects of MCT8 function in brain development using different vertebrate species. Moreover, the plethora of tools for genome editing available today facilitates gene silencing in these animals as well. Studies in the recently generated mct8-deficient zebrafish and Mct8/Oatp1c1 double knockout mice have put forward the current paradigm of impaired TH uptake at the level of the blood-brain barrier during peri- and postnatal development as being the main pathophysiological mechanism of AHDS. RNAi vector-based, cell-specific induction of MCT8 knockdown in the chicken embryo points to an additional function of MCT8 at the level of the neural progenitors during early brain development. Future studies including also additional in vivo models like Xenopus or in vitro approaches such as induced pluripotent stem cells will continue to help unravelling the exact role of MCT8 in developmental events. In the end, this multispecies approach will lead to a unifying thesis regarding the cellular and molecular mechanisms responsible for the neurological phenotype in AHDS patients.

Therapeutic applications of thyroid hormone analogues in resistance to thyroid hormone (RTH) syndromes

Thyroid hormone (TH) is crucial for normal development and metabolism of virtually all tissues. TH signaling is predominantly mediated through binding of the bioactive hormone 3,3',5-triiodothyronine (T3) to the nuclear T3-receptors (TRs). The intracellular TH levels are importantly regulated by transporter proteins that facilitate the transport of TH across the cell membrane and by the three deiodinating enzymes. Defects at the level of the TRs, deiodinases and transporter proteins result in resistance to thyroid hormone (RTH) syndromes. Compounds with thyromimetic potency but with different (bio)chemical properties compared to T3 may hold therapeutic potential in these syndromes by bypassing defective transporters or binding to mutant TRs. Such TH analogues have the potential to rescue TH signaling. This review describes the role of TH analogues in the treatment of RTH syndromes. In particular, the application of 3,3',5-triiodothyroacetic acid (Triac) in RTH due to defective TRβ and the role of 3,5-diiodothyropropionic acid (DITPA), 3,3',5,5'-tetraiodothyroacetic acid (Tetrac) and Triac in MCT8 deficiency will be highlighted.

Prenatal Diagnosis of Resistance to Thyroid Hormone and Its Clinical Implications

CONTEXT

Resistance to thyroid hormone-β (RTH-β) is an autosomal dominant disorder characterized by reduced sensitivity of target tissues to thyroid hormones (THs). Individuals with RTH-β have high TH levels usually due to mutations in the TH receptor-β (THRB) gene. The management of RTH-β during pregnancy is challenging, as wild-type (WT) fetuses born to RTH-β mothers have low birth weight and suppressed postnatal thyroid-stimulating hormone (TSH), due to intrauterine exposure to excess TH.

OBJECTIVE

To determine birth weight and postnatal TSH of WT fetuses carried by mothers with RTH-β whose fT4 levels were maintained below 20% of the upper limit of normal (ULN).

DESIGN

Retrospective chart review.

SETTING

Academic institution in collaboration with off-site hospitals and private practices.

PATIENTS

Thirteen women harboring THRB gene mutations were evaluated during 18 pregnancies.

INTERVENTION

Prenatal genetic diagnosis by amniocentesis. Women carrying WT fetuses were given the option of treatment with antithyroid medication by their treating physicians with the aim to avoid serum fT4 levels above 20% of the ULN.

RESULTS

No significant difference was found in birth weight corrected for gestational age and in serum TSH levels at birth between WT and RTH-β infants born to RTH-β mothers.

CONCLUSIONS

Prenatal diagnosis may play an important role in the management of RTH-β during pregnancy. Aiming for maternal fT4 levels not above 50% of the ULN in RTH-β mothers carrying WT fetuses seems to be a prudent approach that prevents the otherwise expected low birth weight and postnatal TSH suppression.

Triiodothyroacetic acid in health and disease

Thyroid hormone (TH) is crucial for development and metabolism of many tissues. The physiological relevance and therapeutic potential of TH analogs have gained attention in the field for many years. In particular, the relevance and use of 3,3',5-triiodothyroacetic acid (Triac, TA₃) has been explored over the last decades. Although TA₃ closely resembles the bioactive hormone T₃, differences in transmembrane transport and receptor isoform-specific transcriptional activation potency exist. For these reasons, the application of TA₃ as a treatment for resistance to TH (RTH) syndromes, especially MCT8 deficiency, is topic of ongoing research. This review is a summary of all currently available literature about the formation, metabolism, action and therapeutic applications of TA₃.

Conservative management of pregnancy in patients with resistance to thyroid hormone associated with Hashimoto's thyroiditis

BACKGROUND

Resistance to thyroid hormone (RTH) is a rare thyroid disorder characterized by elevated free thyroid hormones with non-suppressed thyrotropin (TSH). Guidelines for the management of pregnancy in patients with RTH are not well defined. Chorionic villus biopsy is sometimes proposed to manage treatment based on the genotype of the fetus.

PATIENT FINDINGS

A woman with RTH (c1243C>T, pR320C mutation in the thyroid hormone receptor β (THRB gene)) associated with Hashimoto's thyroiditis (HT) had three successful pregnancies. During the pregnancies, the mother was treated with levothyroxine (LT4). She never underwent chorionic villus sampling. The babies had normal birth weights. The first child harbored the THRB mutation.

SUMMARY AND CONCLUSIONS

The management of pregnancies in patients with RTH and the indication for chorionic villus sampling are discussed in these cases. In RTH patients, pregnancies must be planned and closely followed. There is no need for prenatal diagnosis of RTH if the patient, due to limited thyroidal reserve, cannot produce excess of thyroid hormones to harm a normal fetus. The more common challenge in RTH remains how to best manage high maternal thyroid hormone levels, and whether or not to lower thyroid hormone levels based on the genotype of the fetus.

Tetrac can replace thyroid hormone during brain development in mouse mutants deficient in the thyroid hormone transporter mct8

The monocarboxylate transporter 8 (MCT8) plays a critical role in mediating the uptake of thyroid hormones (THs) into the brain. In patients, inactivating mutations in the MCT8 gene are associated with a severe form of psychomotor retardation and abnormal serum TH levels. Here, we evaluate the therapeutic potential of the TH analog 3,5,3',5'-tetraiodothyroacetic acid (tetrac) as a replacement for T(4) in brain development. Using COS1 cells transfected with TH transporter and deiodinase constructs, we could show that tetrac, albeit not being transported by MCT8, can be metabolized to the TH receptor active compound 3,3',5-triiodothyroacetic acid (triac) by type 2 deiodinase and inactivated by type 3 deiodinase. Triac in turn is capable of replacing T(3) in primary murine cerebellar cultures where it potently stimulates Purkinje cell development. In vivo effects of tetrac were assessed in congenital hypothyroid Pax8-knockout (KO) and Mct8/Pax8 double-KO mice as well as in Mct8-KO and wild-type animals after daily injection of tetrac (400 ng/g body weight) during the first postnatal weeks. This treatment was sufficient to promote TH-dependent neuronal differentiation in the cerebellum, cerebral cortex, and striatum but was ineffective in suppressing hypothalamic TRH expression. In contrast, TSH transcript levels in the pituitary were strongly down-regulated in response to tetrac. Based on our findings we propose that tetrac administration offers the opportunity to provide neurons during the postnatal stage with a potent TH receptor agonist, thereby eventually reducing the neurological damage in patients with MCT8 mutations without deteriorating the thyrotoxic situation in peripheral tissues.

Perioperative management of a patient with thyroid hormone resistance who underwent total thyroidectomy for thyroid cancer

Resistance to thyroid hormone (RTH) is a rare, predominantly inherited syndrome that involves impaired tissue responsiveness to thyroid hormones. We describe the perioperative management of a patient with RTH who underwent total thyroidectomy. Although surgery performed under general anesthesia was uneventful, after the surgery the patient developed difficult-to-treat hypocalcemia that lasted until postoperative day 4. Moreover, thyroid function even after discharge remained unstable despite replacement therapy. We suggest that the parathyroid and thyroid function of patients with RTH be followed very closely and that nociceptive stimulus of the surgery and postoperative pain be reduced as much as possible.

Thyroid hormone resistance detected by routine neonatal screening

We report the clinical and laboratory findings, and molecular analysis of a Brazilian patient with resistance to thyroid hormone syndrome (RTH) detected by neonatal screening. The index case was born at term by normal delivery with 2,920 g and 45 cm. TSH of the neonatal screening test performed on the 5(th) day of life was of 13.1 µU/mL (cut-off = 10 µU/mL). In a confirmatory test, serum TSH level was 4.3 µU/mL, total T4 was 19 µg/dL (confirmed in another sample, Total T4 = > 24.0 µg/dL), free T4 was 3.7 ηg/dL, and free T3 was 6.7 pg/mL. Direct sequencing of the beta thyroid hormone receptor gene revealed mutation c.1357C>A (P453T), confirming the diagnosis of RHT. Family study demonstrated the presence of RTH in his 1-year-and-3-month-old sister, in his 35-year-old father, and in his 68-year-old paternal grandfather. All of them had goiter and only his father had received an erroneous diagnosis of hyperthyroidism. The present case shows that clinical evaluation and a judicious interpretation of total T4/free T4 concentrations in a newborn recalled due to slightly altered neonatal TSH can contribute to the diagnosis of RTH.

Approach to the patient with resistance to thyroid hormone and pregnancy

Resistance to thyroid hormone (RTH), a syndrome of reduced end-organ responsiveness to thyroid hormone (TH), is mostly caused by mutations in the TH receptor (TR) beta gene. Diagnosis is based on persistent elevations of serum free T(4) and often T(3) levels in the absence of TSH suppression, and confirmation in most cases is by way of genetic testing. The mainstay in the management of RTH patients who are asymptomatic is to recognize the correct diagnosis and avoid antithyroid treatment. Deciding whether to manage these patients with TH replacement is made even more challenging when an affected individual is pregnant. How one approaches such a patient with pregnancy and RTH would depend on the genotype of the fetus. This requires obtaining prenatal information on the genotype of the fetus and a thorough history of the outcome of previous pregnancies as well as a history of the course and outcome of other family members with RTH. If the TRbeta mutation is known in the mother, the fetus can be rapidly genotyped from DNA from amniocentesis for the same mutation, and then management decisions could be made regarding thyroid or antithyroid hormone treatment.

9 years follow-up of a patient with pituitary form of resistance to thyroid hormones (PRTH): comparison of two treatment periods of D-thyroxine and triiodothyroacetic acid (TRIAC)

Patients with pituitary resistance to thyroid hormones (PRTH) exhibit features of hyperthyroidism due to normal sensitivity to thyroid hormones in some of the peripheral tissues. There is a lack of information in the literature on the long-term follow-up and treatment of patients with PRTH. Here, we present long-term (9 years) clinical and biochemical follow-up of a patient with PRTH under the treatment of D-T4 initially (for 1.5 years) followed by TRIAC for 5.5 years. An 11.5 year-old girl was evaluated for goiter, palpitations, heat intolerance, sleep disorders, nervousness and frequent stools for 3 years. Her thyroid function tests were consistent with PRTH. Molecular analysis revealed a heterozygous missense mutation of the TRbeta gene at codon 243 in exon 7 (R243Q) and a silent mutation at codon 245 in the index patient and the mother who was later also diagnosed to have PRTH. The patient was started on D-T4 treatment since she exhibited clinical symptoms of hyperthyroidism. After 1.5 years, D-T4 treatment was switched to TRIAC which lasted 5.5 years. During the long course of both treatments, thyroid hormones, TSH, heart rate, thyroid size, and markers of peripheral thyroid status (SHBG and alkaline phosphatase) were monitored. It was concluded that compared to D-T4, TRIAC treatment is more effective in suppressing TSH and lowering thyroid hormone levels in PRTH. However, both treatments were unable to reduce thyroid size. The effects of treatment on symptomatology were also modest. Spontaneous improvement in symptoms was observed with age.

Augmentation index in resistance to thyroid hormone (RTH)

OBJECTIVE

Resistance to thyroid hormone (RTH) is associated with a varied clinical presentation. The cardiac effects of RTH have been described but vascular function has yet to be fully evaluated in this condition. We have measured the arterial function of those with RTH to assess any vascular changes.

DESIGN

An observational study.

PATIENTS

Twelve RTH patients were recruited from the thyroid clinic (mean value +/- SD), age 40.8 +/- 18.7 years; BMI 27.2 +/- 4.2 kg/m(2) and compared with 12 healthy, euthyroid, age-matched controls (age 41.4 +/- 19.3; BMI 24.8 +/- 4.4 kg/m(2)) with no history of cardiovascular disease. No interventional measures were instituted.

MEASUREMENTS

Arterial stiffness was measured using pulse wave analysis at the radial artery. Thyroid function, fasting lipids and glucose were also measured on the same occasion in both patients and controls. Results The corrected augmentation index, a surrogate marker of arterial stiffness was significantly higher in patients compared with controls (21.0% +/- 14.1%vs. 5.4% +/- 18.2%, P < 0.03). Low density lipoprotein cholesterol (LDL-cholesterol) levels were also significantly elevated in patients compared with controls (3.0 +/- 0.6 vs. 2.1 +/- 0.5 mmol/l; P < 0.002).

CONCLUSION

RTH patients show evidence in this study of increased augmentation index consistent with an increase in arterial stiffness compared with euthyroid controls. They also demonstrate elevated LDL-cholesterol levels. Both these measures may lead to increased cardiovascular risk.

[Therapeutic possibilities in patients with selective pituitary resistance to thyroid hormones]

Selective pituitary resistance to thyroid hormones (SPRTH) is a non-neoplastic form of inappropriate secretion of thyrotropin (TSH). The etiology of this hormonal resistance is linked to inactivating mutations in the thyroid hormone receptor beta (TR-beta) gene. These mutations affect critical portions of the receptor's triiodothyronine (T3)-binding domain. Clinically, SPRTH is characterized by hyperthyroidism with goiter and absence of pituitary mass in the morphologic study. Laboratory data show an elevation of free T3 and free thyroxine concentrations without suppression of TSH, with normal molar subunit alpha/TSH ratio. At this time, there is no specific therapy for SPRHT. Beta blockers, such as atenolol, and benzodiazepines have been used as a symptomatic therapy. Among the drugs with the capacity for reducing TSH secretion are TR agonists, such as triiodothyroacetic acid, D-thyroxine, triiodothyropropionic acid, and L-T3.

Design and synthesis of complementing ligands for mutant thyroid hormone receptor TRβ(R320H): a tailor-made approach toward the treatment of resistance to thyroid hormone

The thyroid hormone receptors (TR) are ligand-dependant transcription factors that regulate key genes involved in metabolic regulation, thermogenesis and development. Resistance to thyroid hormone (RTH) is a genetic disease associated with mutations to TRbeta that lack or show reduced responsiveness to thyroid hormone (triiodothyronine). Previously we reported that the neutral alcohol-based thyromimetic HY-1 can selectively restore activity to a functionally impaired form of TR associated with RTH without over-stimulating TRalpha, which has been associated with undesirable side effects. Two new series of tetrazole and thiazolidinedione based ligands were evaluated for their ability to recover potency and efficacy to three of the most common RTH-associated mutants, TRbeta(R320C), TRbeta(R320H), and TRbeta(R316H), in cell based assays. A new thiazolidinedione based ligand AH-9 was identified, which has near wild-type potency (EC(50)=0.54 nM) to TRbeta(R320C) and TRbeta(R320H). Significantly, AH-9 is equipotent toward TRalpha(wt), TRbeta(wt), TRbeta(R320C), and TRbeta(R320H), suggesting that AH-9 may have the potential to restore the normal homeostatic balance of thyroid hormone actions in patients or models harboring these mutations.

Mutant-Selective Thyromimetics for the Chemical Rescue of Thyroid Hormone Receptor Mutants Associated with Resistance to Thyroid Hormone†

The thyroid hormone receptors (TRs) are ligand-dependent transcription factors that control the expression of multiple genes involved in development and homeostasis in response to thyroid hormone (triiodothyronine, T3). Mutations to TRbeta that reduce or abolish ligand-dependent transactivation function are associated with resistance to thyroid hormone (RTH), an autosomal dominant human genetic disease. A series of neutral alcohol-based compounds, based on the halogen-free thyromimetic GC-1, have been designed, synthesized, and evaluated in cell-based assays for their ability to selectively rescue three of the most common RTH-associated mutations (i.e., Arg320 --> Cys, Arg320 --> His, and Arg316 --> His) that affect the basic carboxylate-binding arginine cluster of TRbeta. Several analogues show improved potency and activity in the mutant receptors relative to the parent compound GC-1. Most significantly, two of these mutant-complementing thyromimics show high potency and activity with a strong preference for the mutant receptors over wild-type TRalpha(wt), that is associated with the cardiotoxic actions of T3. The compounds were evaluated in reporter gene assays using the four common thyroid hormone response elements, DR4, PAL, F2 (LAP), and TSH, and show activities and selectivites consistent with their unique potential as agents to selectively rescue thyroid function to these RTH-associated mutants.

[Thyroid hormone resistance syndrome]

Resistance to thyroid hormone (RTH) is a syndrome characterized by elevated serum thyroid hormone (TH) levels and elevated or inappropriately normal thyrotropin levels. In general, patients exhibit TH resistance in the pituitary and peripheral tissues. The phenotype of RTH is variable; the affected individuals are clinically euthyroid or even hypothyroid depending on the severity of the mutation, the variable hyposensitivity to TH among individuals as well as in different tissues. In almost all cases the genetic basis of RTH lies in mutation of the carboxyl-terminus of the ss-thyroid hormone receptor. RTH is a dominant disorder, except in one family; most individuals are heterozygous for the mutant allele. New standard techniques and genetically engineered mouse model systems have increased our understanding on TH receptor action, in particular, how mutant thyroid receptors from RTH patients can block wild-type thyroid receptor function (dominant negative activity), and how the mutant receptors can differently affect various tissues and individuals.

Neonatal thyrotoxicosis and maternal infertility in thyroid hormone resistance due to a mutation in the TRbeta gene (M313T)

We report two unusual cases of resistance to thyroid hormone (RTH) in one family. The first case, a male infant, had clinical features of thyrotoxicosis in the neonatal period. In the fourth week of life weight gain was poor despite a daily intake of standard infant formula almost double the infant's estimated requirements. At this time serum free T4 (fT4) was 60.7 pmol/l (Normal range [NR] 11-25 pmol/l) and TSH was inappropriately normal at 1.8 mU/l (NR 0.3-4.0 mU/l). The infant responded clinically and biochemically to propylthiouracil (PTU) at a dose of 10 mg/kg/day. Following 27 days of treatment serum fT4 was 22.6 pmol/l and TSH had risen to 24.9 mU/l. As the infant was thriving treatment was discontinued. The infant, now aged 6 months old, remains clinically euthyroid and developmentally normal off treatment. The infant's mother, from whom he had inherited a mutation of the thyroid receptor beta (TRbeta) gene (M313T), presented earlier with secondary infertility and clinical features of thyrotoxicosis. Treatment with PTU restored her fertility and she spontaneously conceived. In the subsequent pregnancy, clinical and biochemical features of RTH improved, and she gave birth to a small but healthy female infant. In the next pregnancy, resulting in the birth of the affected male infant, clinical and biochemical features of RTH worsened, and high doses of PTU were required to maintain a clinically euthyroid state. To our knowledge, these are the first case reports of RTH associated with added features of a hypermetabolic state in infancy and secondary infertility.

Cardiac involvement in thyroid hormone resistance

To analyze the cardiovascular alterations thought to occur in resistance to thyroid hormone (RTH), cardiac involvement in 54 patients with RTH was investigated with the help of two-dimensional and Doppler echocardiography. Data from 41 of 54 adult subjects with RTH were also compared with those of 24 and 20 cases with hyperthyroidism (H) and hypothyroidism (h), respectively, as well as 22 healthy euthyroid controls (C). With respect to the type of mutations, no correlation was found between cardiovascular features and genotype. Compared with affected adults, children with RTH showed markedly higher serum free T3 (FT3), free T4 (FT4), and baseline TSH concentrations. Compared with healthy children of comparable age, RTH children had significantly higher heart rate and lower left ventricular (LV) ejection fraction (P = 0.006). Also, higher heart rate and FT4 as well as shorter diastolic relaxation of the myocardium (all P = 0.001) between RTH subjects with and without thyrotoxic cardiovascular features were found. Cardiac symptoms (palpitations, 32% vs. 71%) and signs (sinus tachycardia, 26% vs. 79%; atrial fibrillation, 6% vs. 17%) were significantly less frequent in RTH vs. H (all P = 0.001). Compared with C and h, heart rate, cardiac output, stroke volume, and systolic aortic flow velocity were strongly increased in RTH (all P = 0.0001) and H, although ejection (P = 0.0012) and shortening (P = 0.0001) fractions of the LV were markedly lower in RTH vs. H. Diastolic parameters, such as isovolumic relaxation (P = 0.0001) and deceleration time (P = 0.013), were shorter in RTH vs. h and C. In RTH, positive correlations between FT3 and heart rate, and between FT4 and LV ejection fraction were observed, whereas negative correlations between both FT3 and FT4 and isovolumic relaxation were noted. In conclusion, these findings indicate a modulated hyperthyroid effect on cardiac systolic and diastolic function of the myocardium in RTH, whereas other parameters, such as ejection and shortening fractions of the LV, systolic diameter, and LV wall thickness, were comparable to C. Differences in term of cardiovascular changes were smaller between the RTH and C groups than the RTH and the H or h groups. Thus, an incomplete cardiac response to thyroid hormone is present in RTH.

A novel mutation (M310L) in the thyroid hormone receptor beta causing resistance to thyroid hormone in a Brazilian kindred and a neonate

Resistance to thyroid hormone (RTH) is an inherited syndrome of reduced tissue responsiveness to thyroid hormone (T3) caused by mutations in the thyroid hormone receptor beta (TRbeta). The index patient of the family reported here, a 17-year-old woman, came to medical attention because of a diffuse goiter, short stature, and learning disabilities. Biochemical tests revealed an elevated free T4 of 5.2 ng/dl (0.8-2.1), a T3 of 270 ng/dl (80-220), and a nonsuppressed TSH of 1.79 mU/l (0.4-4). Administration of exogenous T4 or T3 did not result in the usual TSH suppression, prompting the clinical diagnosis of RTH. Her father and one of her brothers also had clinical and biochemical findings consistent with RTH. Direct sequence analysis of the TRbeta gene revealed a heterozygous transition 928A>G in exon 9 resulting in substitution of methionine 310 by leucine (M310L). This novel receptor mutant has a reduced affinity for T3 ( approximately 10% of normal) and dominant negative properties that are similar in comparison to other RTH mutations. The index patient had a normal pregnancy and delivery. At birth, the female neonate had no goiter, a significantly elevated T4, and increased TSH. The diagnosis of RTH was confirmed by sequencing the TRbeta gene. She was underweight at birth and her length was between the 5th and 10th percentile. At 26 months, her height remained at the 10th percentile but her bone age was 18 months, suggesting mild hypothyroidism at the level of the bone. In contrast, increased heart rate and restlessness are consistent with hyperthyroidism in other tissues, such as the heart and possibly the brain.

Resistance to thyroid hormone caused by a new mutation (V336M) in the thyroid hormone receptor beta gene

Resistance to thyroid hormone (RTH), usually caused by an inherited defect of the thyroid hormone receptor, (TRbeta), results in a reduced responsiveness of target tissues to thyroid hormone. Until now, more than 600 cases with RTH have been identified. Although usually linked to the TRbeta gene, located on chromosome 3, RTH may also occur in the absence of mutations in the coding region of this gene. We report a 10-month-old boy who had laboratory findings typical of RTH. He was born prematurely on the 34th week of gestation and his thyrotropin (TSH) during neonatal screening was 121 microU/mL, a value very high for RTH or prematurity. Direct sequencing of the TRbeta gene from the patient's genomic DNA revealed a heterozygous substitution of the normal valine with a mutant methionine in codon 336 (V336M) that has not been previously reported. In vitro expression studies showed that this mutant TRbeta has an impaired triiodothyronine (T3)-dependent transactivation that reduces the activity of the wild-type TRbeta (dominant negative effect). While the functional impairment of V336M is not unusual compared to other TRbeta gene mutations, the very high TSH value in this prematurely born infant suggests that fetuses with RTH have altered maturation of the hypothalamo-pituitary-thyroid axis or actually may suffer from hypothyroidism.