Assessment of Cardiometabolic Risk Factors and Insulin Sensitivity by Hyperinsulinemic-Euglycemic Clamp in Resistance to Thyroid Hormone ß Syndrome.

Background: Resistance to thyroid hormone beta (RTHß) is a rare disease resulting from mutations in the THRB gene, characterized by reduced T3 action in tissues with high thyroid hormone receptor ß expression. Thyroid hormones regulate body composition and metabolism in general, and increased or decreased hormone levels are associated with insulin resistance. This study evaluated the presence of cardiometabolic risk factors and insulin sensitivity in patients with RTHß. Methods: In all, 16 patients, 8 adults (52.3 ± 16.3 years of age) and 8 children (10.9 ± 3.9 years of age), were compared to 28 control individuals matched for age, sex, and body mass index (BMI). Anthropometry evaluation and blood samples were collected for glycemia, lipids, insulin, interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), leptin, adiponectin, ultrasensitive C-reactive protein (CRPus), free thyroxine, total triiodothyronine, thyrotropin, and anti-thyroid peroxidase measurements. Body composition was assessed using dual-emission X-ray absorptiometry and bioimpedance. Insulin sensitivity was evaluated in adult patients and controls using the hyperinsulinemic-euglycemic clamp (HEC), whereas homeostasis model assessment of insulin resistance (HOMA-IR) was calculated in all individuals studied. Results: Patients and controls presented similar weight, BMI, abdominal perimeter, and total fat body mass. Patients with RTHß demonstrated higher total cholesterol (TC), p = 0.04, and low-density lipoprotein cholesterol (LDL-C), p = 0.03, but no alteration was observed in other parameters associated with metabolic risk, such as leptin, TNF-α, and CRPus. Two adult patients met the criteria for metabolic syndrome. There was no evidence of insulin resistance assessed by HEC or HOMA-IR. Elevated IL-6 levels were observed in patients with RTHß. Conclusion: Using HEC as the gold standard method, no evidence of reduced insulin sensitivity in skeletal muscle was documented in RTHß adult patients; however, higher levels of TC and LDL-C were observed in these patients, which suggest the need for active monitoring of this abnormality to minimize cardiometabolic risk. In addition, we demonstrated, for the first time, that the increase in IL-6 levels in patients with RTHß is probably secondary to metabolic causes as they have normal levels of TNF-α and CRPus, which may contribute to an increase in cardiovascular risk. A larger number of patients must be studied to confirm these results.

Autoimmune thyroid disease and thyroid function test fluctuations in patients with resistance to thyroid hormone.

OBJECTIVE: Resistance to thyroid hormone beta (RTHß) is an inherited syndrome caused by mutations in the thyroid hormone receptor ß (THRB) gene. Patients with RTHß typically have elevated thyroid hormone levels with non-suppressed serum thyroid-stimulating hormone (TSH). We aimed to elucidate the clinical, laboratory, and imaging findings of RTHß patients and further to explore their association with THRB gene mutations. DESIGN AND METHODS: We retrospectively reviewed the clinical charts and compared the clinical findings of 68 RTHß patients (45 probands and 23 relatives) and 30 unaffected relatives in Kuma Hospital. RESULTS: Genetic testing revealed 35 heterozygous THRB gene mutations. Among all RTHß patients, autoimmune thyroid disease (AITD) was detected in 42.1% of men and 40.9% of women, showing that the prevalence of AITD in affected males was significantly higher than in unaffected relatives (P = 0.019). During the follow-up of 44 patients, 13 patients (29.5%; 8 (42.1%) with AITD and 5 (20%) without AITD) temporarily showed thyroid function test results inconsistent with RTHß. Two patients with the R383H mutation, which has little dominant-negative effect, temporarily showed normal thyroid hormone and TSH levels without AITD. CONCLUSIONS: The frequency of AITD in male RTHß patients was significantly higher compared to unaffected relatives. More than 20% of RTHß patients temporarily showed laboratory findings atypical of RTHß during their follow-up, and patients with AITD and specific THRB mutations were prone to display such findings. Therefore, genetic testing should be performed even for patients with fluctuations in thyroid function test results to avoid misdiagnosis and inappropriate treatment.

An overview of thyroid function tests in subjects with resistance to thyroid hormone and related disorders.

Confirmation of sustained syndrome of inappropriate secretion of thyrotropin (SITSH) is a milestone in diagnosis of ß type of resistance to thyroid hormone (RTHß). The differential diagnoses of RTHß include TSH-producing pituitary adenoma (TSHoma) and familial dysalbuminemic hyperthyroxinemia (FDH), which also present SITSH. Recently, patients with RTHα caused by a mutation in thyroid hormone receptor α were reported and they did not present SITSH but a decline in the serum T4/T3 ratio. This review was aimed to overview thyroid function tests in RTH and related disorders. First, the characteristics of the thyroid function in RTHß, TSHoma, and FDH obtained from a Japanese database are summarized. Second, the degrees of SITSH in patients with truncations and frameshifts were compared with those in patients with single amino acid deletions and single amino acid substitutions obtained from the literature. Third, the degrees of SITSH in homozygous patients were compared with those in heterozygous patients with cognate mutations. Finally, the FT3/FT4 ratios in RTHα are summarized. In principle, the TSH values in FDH were within the normal range and apparent FT4 values in FDH were much higher than in RTHß and TSHoma. The FT3/FT4 values in RTHß were significantly lower than in TSHoma. The degrees of SITSH in patients with truncations and frameshifts were more severe than those in patients with single amino acid deletions and single amino acid substitutions, and those in homozygous patients were more severe than those in heterozygous patients with cognate mutations. The FT3/FT4 ratios in RTHα were higher than 1.0.

Generation and Characterization of a New Resistance to Thyroid Hormone Mouse Model with Thyroid Hormone Receptor Alpha Gene Mutation.

Background: In humans, resistance to thyroid hormone (RTH) caused by mutations in the thyroid hormone receptor alpha (THRA) gene, RTHα, manifests as tissue-specific hypothyroidism and circulating thyroid hormone levels exhibit hypothyroid-like clinical features. Before the identification of patients with RTHα, several Thrα1 knock-in mouse models were generated to clarify the function of TRα1. However, the phenotypes of these mice were not consistent with the clinical presentation of RTHα in humans. For the present study, we generated an RTHα mouse model that carries the Thra1E403X mutation found in human RTHα patients. Here, we report the gross phenotypes of this mouse RTHα model. Methods: Traditional homologous recombination gene targeting techniques were used to introduce a mutation (Thra1E403X) in the mouse Thra gene. The phenotypes of the resulting mice were studied and compared with clinical features observed for RTHα with THRAE403X. Results: Thrα1E403X/E403X homozygous mice exhibited severe neurological phenotypes, such as spasticity and motor ataxia, which were similar to those observed in endemic cretinism. Thrα1E403X/+ heterozygous mice reproduced most clinical manifestations of patient with RTHα, such as a normal survival rate and male fertility, as well as delayed postnatal growth and development, neurological and motor coordination deficits, and anemia. The mice had typical thyroid function with a modest increase in serum triiodothyronine (T3) levels, a low thyroxine (T4)/T3 ratio, and low reverse T3 (rT3) levels. Conclusions: The Thrα1E403X/+ mice faithfully recapitulate the clinical features of human RTHα and thus can provide a useful tool to dissect the role of TRα1 in development and to determine the pathological mechanisms of RTHα.

Two Novel Cases of Resistance to Thyroid Hormone Due to THRA Mutation.

Resistance to thyroid hormone alpha (RTHα) is a rare and under-recognized genetic disease caused by mutations of THRA, the gene encoding thyroid hormone receptor α1 (TRα1). We report here two novel THRA missense mutations (M259T, T273A) in patients with RTHα. We combined biochemical and cellular assays with in silico modeling to assess the capacity of mutant TRα1 to bind triiodothyronine (T3), to heterodimerize with RXR, to interact with transcriptional coregulators, and to transduce a T3 transcriptional response. M259T, and to a lower extent T273A, reduces the affinity of TRα1 for T3. Their negative influence is only reverted by large excess of T3. The severity of the two novel RTHα cases originates from a reduction in the binding affinity of TRα1 mutants to T3 and thus correlates with the incapacity of corepressors to dissociate from TRα1 mutants in the presence of T3.

Hyperthyroxinemia with a non-suppressed TSH: how to confidently reach a diagnosis in this clinical conundrum.

Disorders of thyroid function are among the commonest referrals to endocrinology. While interpretation of thyroid function testing is usually straightforward, accurate interpretation becomes significantly more challenging when the parameters do not behave as would be expected in normal negative feedback. In such cases, uncertainty regarding further investigation and management arises. An important abnormal pattern encountered in clinical practice is that of high normal or raised free thyroxine (fT4) with inappropriately non-suppressed or elevated thyroid-stimulating hormone (TSH). In this short review using two clinical vignettes, we examine the diagnostic approach in such cases. A diagnostic algorithm is proposed to ensure that a definitive diagnosis is reached in these challenging cases.

Resistance to Thyroid Hormone Alpha in an 18-Month-Old Girl: Clinical, Therapeutic, and Molecular Characteristics.

BACKGROUND: Recently, the first patients with resistance to thyroid hormone alpha (RTHα) due to inactivating mutations in the thyroid hormone receptor alpha (TRα) were identified. These patients are characterized by growth retardation, variable motor and cognitive defects, macrocephaly, and abnormal thyroid function tests. The objective was to characterize a young girl (18 months old) with a mutation in both TRα1 and TRα2, and to study the effects of early levothyroxine (LT4) treatment. METHODS: The patient was assessed clinically and biochemically before and during 12 months of LT4 treatment. In addition, the consequences of the mutation for TRα1/2 receptor function were studied in vitro. RESULTS: At 18 months of age, the patient presented with axial hypotonia, delayed motor development, severe growth retardation, and abnormally elevated triiodothyronine (T3)/thyroxine (T4) ratios. RTHα was suspected, and concomitantly a c.632A>G/p.D211G missense mutation was identified, affecting both the TRα1 and TRα2 proteins. This mutation was also found in the girl's father. LT4 treatment was started, resulting in a marked improvement of her hypotonia, motor skills, and growth. Functionally, the missense mutation led to decreased transcriptional activity of TRα1, which could be overcome by higher T3 levels in vitro. The mutant TRα1 showed a moderate dominant negative activity on wild type (WT) TRα1. In contrast, WT TRα2 and mutant TRα2 had negligible transcriptional activity and showed no dominant-negative effect over TRα1. CONCLUSIONS: This report describes the phenotype of a young RTHα patient with a mild TRα mutation before and during early LT4 treatment. Treatment had beneficial effects on her muscle tone, motor development, and growth.

The different requirement of L-T4 therapy in congenital athyreosis compared with adult-acquired hypothyroidism suggests a persisting thyroid hormone resistance at the hypothalamic-pituitary level.

BACKGROUND: Levothyroxine (l-T4) is commonly employed to correct hormone deficiency in children with congenital hypothyroidism (CH) and in adult patients with iatrogenic hypothyroidism. OBJECTIVE: To compare the daily weight-based dosage of the replacement therapy with l-T4 in athyreotic adult patients affected by CH and adult patients with thyroid nodular or cancer diseases treated by total thyroidectomy. DESIGN AND METHODS: A total of 36 adult patients (27 females and nine males) aged 18-29 years were studied; 13 patients (age: 21.5±2.1, group CH) had athyreotic CH treated with l-T4 since the first days of life. The remaining 23 patients (age: 24±2.7, group AH) had hypothyroidism after total thyroidectomy (14 patients previously affected by nodular disease and nine by thyroid carcinoma with clinical and biochemical remission). Patient weight, serum free thyroid hormones, TSH, thyroglobulin (Tg), anti-Tg, and anti-thyroperoxidase antibodies were measured. Required l-T4 dosage was evaluated. At the time of the observations, all patients presented free thyroid hormones within the normal range and TSH between 0.8 and 2 µIU/ml. RESULTS: Patients had undetectable Tg and anti-thyroid antibodies. The daily weight-based dosage of the replacement therapy with l-T4 to reach euthyroidism in patients of group CH was significantly higher than that in those of group AH (2.16±0.36 vs 1.73±0.24 µg/kg, P<0.005). Patients of group CH treated with l-T4 had significantly higher serum TSH levels than patients of group AH (P=0.05) as well as higher FT4 concentrations. CONCLUSIONS: To correct hypothyroidism, patients of group CH required a daily l-T4 dose/kg higher than group AH patients, despite higher levels of TSH. The different requirement of replacement therapy between adult patients with congenital and those with surgical athyroidism could be explained by a lack of thyroid hormones since fetal life in CH, which could determine a different set point of the hypothalamus-pituitary-thyroid axis.

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 ß (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.

Serum copper as a novel biomarker for resistance to thyroid hormone.

Thyroid hormone action is mediated by the thyroid hormone receptors TRα1 and TRß. Defects in TRß lead to RTH (resistance to thyroid hormone) ß, a syndrome characterized by high levels of thyroid hormone and non-suppressed TSH (thyroid-stimulating hormone). However, a correct diagnosis of RTHß patients is difficult as the clinical picture varies. A biochemical serum marker indicative of defects in TRß signalling is needed and could simplify the diagnosis of RTHß, in particular the differentiation to TSH-secreting pituitary adenomas, which present with clinically similar symptoms. In the present paper we show that serum copper levels are regulated by thyroid hormone, which stimulates the synthesis and the export of the hepatic copper-transport protein ceruloplasmin into the serum. This is accompanied by a concerted reduction in the mRNA levels of other copper-containing proteins such as metallothioneins 1 and 2 or superoxide dismutase 1. The induction of serum copper is abolished in genetically hyperthyroid mice lacking TRß and human RTHß patients, demonstrating an important role of TRß for this process. Together with a previously reported TRα1 specific regulation of serum selenium, we show that the ratio of serum copper and selenium, which is largely independent of thyroid hormone levels, volume changes or sample degradation, can constitute a valuable novel biomarker for RTHß. Moreover, it could also provide a suitable large-scale screening parameter to identify RTHα patients, which have not been identified to date.

Differentiated thyroid cancer in two patients with resistance to thyroid hormone.

BACKGROUND: Resistance to thyroid hormone (RTH) is a genetic disease characterized by a reduced responsiveness of the pituitary and peripheral target tissues to TH. We describe two patients with RTH in whom differentiated thyroid cancer was diagnosed. PATIENT FINDINGS: In both patients, RTH was unequivocally diagnosed and both underwent thyroidectomy for multinodular goiter. In Patient 1, histology showed a papillary thyroid carcinoma pT2. Because serum thyrotropin (TSH) levels were elevated even while the patient was taking 150 µg daily of levothyroxine (LT(4)), the patient was treated with (131)I 100 mCi for ablation of the thyroid remnant without discontinuing his LT(4) therapy. We obtained a clinically adequate response by administering LT(4) 175 µg/day (2.18 µg/kg), but the serum TSH was persistently elevated on this dose. The patient was considered "free of disease" after 8 years of follow-up. In Patient 2, histology revealed a papillary microcarcinoma (0.6 cm). Diagnostic whole-body scan was performed while the patient was taking 100 µg/day LT(4), a time that his serum TSH was 38 µU/mL. Only a small remnant was revealed, so (131)I remnant ablation was not performed. While taking LT(4) at a dose of 175 µg/day (3 µg/kg), the serum TSH was persistently high, serum TH levels were in the normal-high range, and he appeared to be clinically euthyroid. There has been no evidence of persistent or recurrent thyroid carcinoma in ultrasonography and thyroglobulin measurements that have been performed on a yearly basis for 3 years. CONCLUSION: Patients with thyroid carcinoma and RTH are a unique model of thyroid cancer in which follow-up likely occurs in the setting of constantly elevated serum TSH concentrations. The concern in these patients is that their persistent elevation of serum TSH may have an adverse effect on their thyroid cancer, and management choices in terms of the dose of LT(4) that provides the optimum lowering of serum TSH without toxicity are difficult, particularly in the situation wherein, as was the case with one of our patients, there is cardiac disease.

[A 46-year-old patient with atrial fibrillation, elevated thyroid hormones and normal thyrotropine]. / Vorhofflimmern, erhöhte Schilddrüsenhormone und nicht supprimiertes TSH bei einem 46-jährigen Patienten.

We report on a 46 year old patient with a history of paroxysmal atrial fibrillation who presented to our emergency room. Diagnostic evaluation showed elevated free peripheral thyroid hormone levels and thyrotropine (TSH) hormone within normal limits. Ultrasound of the thyroid was normal, and thyroid autoantibodies were found in the normal range. There was a positive family history for thyroid dysfunction. TSH-producing adenoma (TSHoma) of the pituitary gland - the main differential diagnosis - was excluded by cranial MRI and laboratory tests. Familial thyroid hormone resistance (Refetoff syndrome) was suspected and could be confirmed by detection of a pathogenic mutation within the beta-thyroidhormone receptor gene. After spontaneous conversion to sinusrhythm the patient was treated with a beta(1)-selective betareceptor blocker. Up to now, no specific treatment is available to correct the defective beta-thyroidhormone receptor.

Identification of TSH receptor mutations in three families with resistance to TSH.

OBJECTIVE: Genetic analysis of the TSH receptor gene in seven subjects with subclinical hypothyroidism (SH), in whom the diagnosis of autoimmune thyroid disease had been excluded by laboratory and instrumental techniques currently available. PATIENTS: Three families where different members (2 children and 5 adults) affected by SH were studied. GENETIC ANALYSIS: Genomic DNA was extracted from peripheral lymphocytes and the entire coding sequence of the TSHr gene was sequenced. pSVL-TSHr construct harbouring a Q8fsX62 insertion was obtained by site-directed mutagenesis. COS-7 cells transfected with wild-type and mutant receptor were used for binding studies, flow cytometry, and cyclic AMP (cAMP) determination. RESULTS: A four base pair (bp) duplication in position 41 (41TGCAins), leading to a premature stop of translation at codon 62 (Q8fsX62), was found to be heterozygous in the proband, the father and the sister in Family 1. In Family 2 the proband and the sister were heterozygous for the mutation D410N. In Family 3 the proband and the father were heterozygous for the mutation P162A. After transfection in COS-7 cells, the mutant receptor Q8fsX62 displayed a low expression at the cell surface, and a reduced response to bovine TSH (bTSH) in terms of cAMP production. CONCLUSIONS: We identified TSH receptor mutations in seven members of three families with subclinical hypothyroidism.

Thyroid hormone resistance.

Resistance to thyroid hormone (RTH) is a rare autosomal dominant inherited syndrome of reduced end-organ responsiveness to thyroid hormone. Patients with RTH have elevated serum free thyroxine (FT4) and free triiodothyronine (FT3) concentrations and normal or slightly elevated serum thyroid stimulating hormone (TSH) level. Despite a variable clinical presentation, the common characteristic clinical features are goitre but an absence of the usual symptoms and metabolic consequences of thyroid hormone excess. Patients with RTH can be classified on clinical grounds alone into either generalized resistance (GRTH), pituitary resistance (PRTH) or combined. Mutations in the thyroid hormone receptor (TR) beta gene are responsible for RTH and 122 different mutations have now been identified belonging to 300 families. With the exception of one family found to have complete deletion of the TRbeta gene, all others have been demonstrated to have minor alterations at the DNA level. The differential diagnosis includes a TSH-secreting pituitary adenoma and the presence of endogenous antibodies directed against thyroxine (T4) and triiodothyronine (T3). Failure to differentiate RTH from primary thyrotoxicosis has resulted in the inappropriate treatment of nearly one-third of patients. Although occasionally desirable, no specific treatment is available for RTH; however, the diagnosis allows appropriate genetic counselling.

Fetal loss associated with excess thyroid hormone exposure.

CONTEXT: Maternal hypothyroidism and hyperthyroidism have deleterious effects on the outcome of pregnancy. While the effects of thyroid hormone (TH) deprivation on the fetus, independently from that on the mother, can be studied in infants with congenital hypothyroidism, this is not the case in those with fetal thyrotoxicosis. OBJECTIVE: To study the effects of TH excess on fetuses carried by mothers with resistance to TH (RTH) who are euthyroid despite high TH levels but who may carry normal fetuses that are exposed to high maternal hormone levels. DESIGN, SETTING, AND PARTICIPANTS: Retrospective study of 167 members of an Azorean family with RTH. Affected individuals had the RTH phenotype (high serum concentration of free thyroxine and triiodothyronine without suppressed thyrotropin) confirmed by genotyping to identify the Arg243-->Gln mutation in the TH receptor beta gene. MAIN OUTCOME MEASURES: Pregnancy outcome of affected mothers vs that of unaffected mothers carrying fetuses conceived by affected fathers, as well as that of unaffected first-degree relatives and outcomes from the general island population. Comparison of birth weights and blood concentrations of thyrotropin (TSH) obtained during routine neonatal screening of infants born to these 3 groups. RESULTS: Thirty-six couples with complete information belonged to 1 of 3 groups: affected mothers (n = 9), affected fathers (n = 9), and unaffected relatives (n = 18). Mean miscarriage rates were 22.9%, 2.0%, and 4.4%, respectively (chi2 = 8.66, P =.01). Affected mothers had an increased rate of miscarriage (z = 3.10, P =.002, by Wilcoxon rank-sum test). They had marginally higher than expected numbers of affected offspring, ie, 20 affected and 11 unaffected children (P =.07), while affected fathers had 15 affected and 12 unaffected children (P =.35). Unaffected infants born to affected mothers were significantly smaller than affected infants, having a mean SD score for gestational age of -1.79 (SD, 0.86) vs -0.06 (SD, 1.11) to -0.22 (SD, 0.70) for all other groups (P<.001). Only unaffected infants born to affected mothers had undetectable blood levels of TSH. CONCLUSION: There was a higher rate of miscarriage in mothers affected by RTH that may have involved predominantly unaffected fetuses. The lower birth weight and suppressed levels of TSH in unaffected infants born to affected mothers indicates that the high maternal TH levels produce fetal thyrotoxicosis. These data indicate a direct toxic effect of TH excess on the fetus.

Thyroid hormone resistance and enlargement of the sella turcica during pregnancy.

CASE REPORT: A 26-year-old pregnant woman was admitted to our institution running her 30 weeks of gestation. The patient had a past history of total thyroidectomy cause of a thyroid papillary carcinoma and presented with increased supraphysiological TSH levels under 250 microg T4, while slightly hyperthyroid, from the clinical point of view. Partial resistance to thyroid replacement therapy or TSH-secreting tumour was evoked. Pituitary MRI revealed a pituitary enlargement without excluding a pituitary adenoma. To avoid further stress on pituitary a caesarean section was performed at 38 weeks of gestation. MRI 7 months later was normal, while the patient remained under high doses of T4 replacement therapy and TSH was found at the upper limits of normalcy, while T3, T4 and FTI were above normalcy. CONCLUSION: We conclude that, in the absence of thyroid gland, high TSH levels due to thyroid hormone resistance could be erroneously attributed to a pituitary TSH secreting tumour, when associated with a pregnancy-related pituitary enlargement.

Atrial fibrillation and mitral prolapse in a subject affected by Refetoff syndrome.

Inappropriate secretion of TSH (IST) refers to a heterogeneous group of syndromes in which patients show unsuppressed TSH levels in spite of high serum free thyroid hormone concentrations. It has been recognised that IST can be due to both thyroid hormone resistance (RTH) and pituitary TSH-secreting tumours. The former can be generalised (GRTH) or pituitary (PRTH) if the resistance is more severe in the pituitary than in the remaining tissues. This case report describes a peculiar coexistence of atrial fibrillation and mitral valve prolapse in a patient affected by generalized resistance to thyroid hormone. This finding is suggestive for a major and almost physiological sensitivity of the myocardium to the thyroid hormones activity which in the course of years may determine the modifications responsible for the pathologies described.

Quantitative assessment of pituitary resistance to thyroid hormone from plots of the logarithm of thyrotropin versus serum free thyroxine index.

Previous studies have shown that, in patients with primary alterations in thyroid hormone secretion, the level of the natural logarithm of serum TSH (lnTSH) is negatively related to the level of free T4. Because such patients can generally be assumed to exhibit normal tissue responsivity to thyroid hormone, we were interested in determining whether the lnTSH/free T4 index (FTI) relationship in patients with established thyroid hormone resistance (THR) exhibit a lower slope than patients with normal tissue sensitivity to thyroid hormone. We have therefore analyzed the relationship between the lnTSH and the FTI in members of three families with documented THR. In these patients, a given dose of T4 was maintained for a 1- to 2-month period, to achieve hormonal equilibration. Two of the families, though not related, exhibited the same mutation, E460K. The third was identified as A317T. As anticipated, the slope of the lnTSH/FTI ratio was significantly lower in the patients with THR than in T4-treated patients who were presumed to have normal sensitivity to thyroid hormone. The slope of the lnTSH/FTI relationship seemed to be characteristic of the specific mutation involved in the three genotypes (wild-type and two mutations) examined. Further, the in vivo slope of the lnTSH/FTI relationship seemed to be linearly related to the T3 association constant of the in vitro translated receptor. These findings support the potential usefulness of measuring the slope of lnTSH, as a function of the FTI, in quantitating pituitary THR.

Adverse effects of thyroid hormones.

The adverse health effects of thyrotoxicosis have been carefully documented and most practitioners are familiar with the clinical consequences for the patient. Until recently, many patients experienced the adverse effects of excessive thyroxine dosages. Which can now be avoided by the application of highly sensitive immunometric assays for monitoring serum thyrotrophin (thyroid-stimulating hormone; TSH) levels. However, sensitive monitoring of serum thyrotrophin levels has led to the frequent recognition of biochemical subclinical hyperthyroidism (isolated suppression of thyrotrophin). Because of the increased recognition of this condition, the adverse effects of thyroxine therapy can be divided into those associated with subclinical hyperthyroidism and those associated with the euthyroid state. Investigation of the potential clinical consequences of thyrotrophin-suppressing dosages of thyroxine has dominated studies over the last decade, with less attention being given to euthyroid patients. It appears that the adverse effects of thyroxine are considerably more common when serum thyrotrophin has been suppressed. They are usually manifested in older patients as increased bone mineral loss in postmenopausal women and as cardiac effects in patients with intrinsic heart disease. These patients may have subtle behavioural alterations and other clinically silent organ effects that occur infrequently. Children who are euthyroid while taking thyroxine occasionally develop pseudotumour cerebri shortly after starting hormone replacement for hypothyroidism. Otherwise, thyroxine dosages that render patients euthyroid, as evidenced by thyrotrophin values that are within the normal range, rarely cause adverse effects. Thus, avoidance of dosages that cause thyrotrophin suppression, when not clinically indicated, is the primary approach to the management of these adverse effects.