Familial inappropriate TSH secretion: evidence suggesting a dissociated pituitary resistance to T3 and T4

The first case of association between postpartum thyroiditis and thyroid hormone resistance in an Italian patient showing a novel p.V283A THRB mutation

BACKGROUND

Postpartum thyroiditis (PPT) is characterized by the development of postpartum thyroid dysfunction, which may occur up to 12 months after delivery. The syndrome usually presents with transient thyrotoxicosis, followed by transient hypothyroidism. The association of this condition with resistance to thyroid hormones (RTH) has never been described.

PATIENT FINDINGS

In this report, we describe a 30-year-old patient affected by RTH due to a novel p.V283A thyroid hormone receptor-β (THRB) heterozygous mutation in exon 8, which affects the ligand-binding domain, never before described in literature. A simple polymorphism was excluded through screening of 100 healthy controls.

SUMMARY

The patient became pregnant twice (in 2008 and in 2009) and developed PPT after both deliveries. Two months after her first pregnancy and one month after her second pregnancy, she presented with severe endogenous thyrotoxicosis and concomitant suppressed thyrotropin (TSH) levels, which represents an unusual finding in patients affected by RTH. Other causes of hyperthyroidism were excluded. After the hyperthyroid phase, she became hypothyroid (TSH >75 mU/L and low free-thyroxine and free-tri-iodothyronine levels), and eventually returned to her usual euthyroid status. During the course of PPT, no specific treatment was required, except for β-blockers used to treat tachycardia during the hyperthyroid phase.

CONCLUSIONS

We report a unique case of a woman affected by RTH, due to a novel mutation V283A in THRB, who experienced PPT with a severe thyrotoxic phase after both her pregnancies. The association between RTH and PPT has never been reported in the literature. In particular, the marked suppression of TSH occurring when levels of TH are particularly elevated is not a frequent condition during RTH.

Resistance to thyroid hormone associated with a novel mutation of the thyroid β receptor gene in a four-year-old female

Resistance to thyroid hormone (RTH) is a rare syndrome of reduced responsiveness of target tissues to thyroid hormone and is caused mutation in the thyroid β receptor gene. We report a novel mutation, E445X, causing RTH in a 4-year old girl. The patient exhibited extreme signs and symptoms of RTH at an early age, and had a large compressive goiter. Following total extracapsular thyroidectomy, upper airway compression was relieved and symptoms of hyperthyroidism improved. This case appears to be the youngest child recorded to have undergone total thyroidectomy for RTH. Post-operative TSH elevations were managed with every-other-day triiodothyronine therapy.

[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.

Effects of thyroid hormones (T4,T3), bromocriptine and Triac on inappropriate TSH hypersecretion

Inappropriate TSH hypersecretion was diagnosed in a 38-year-old woman (case 1) and in a 38-year-old man (case 2). Both of them had earlier been treated by ablative therapy for hyperthyroidism. The present diagnosis was based on elevated basal serum TSH levels despite elevated serum free thyroid hormone levels. Both of them had exaggerated TSH responses to TRH (peak value 240 mU/l in case 1 and 408 mU/l in case 2). Their albumin and prealbumin levels were normal. The serum TBG level was normal in case 1 but was elevated in case 2. Serum levels of alpha-subunits of TSH, and pituitary CT scans were normal. Despite mild clinical hyperthyroidism, peripheral indices of thyroid hormone action were normal. They had also relatives with apparent resistance to thyroid hormones. In view of the possibility that prolonged pituitary thyrotrophic stimulation is detrimental, various therapeutic approaches to suppress TSH levels were tried. Both T3 and T4 treatments lowered serum TSH levels, but were poorly tolerated. Acute administration of L-dopa or bromocriptine reduced serum TSH levels, but this was not seen during long-term therapy. TRIAC treatment lowered serum TSH levels, and the drug was well tolerated. Serum TSH responses to TRH were not blunted during T3, T4 or TRIAC treatments. Somatostatin also reduced serum TSH levels, but did not potentiate the effect of low dose T3 therapy. Our results suggest that the patients had unbalanced pituitary and peripheral thyroid hormone resistance, predominantly at the pituitary level. Of the drugs studied, TRIAC seemed to be the most suitable therapy.

Non-adenomatous inappropriate TSH hypersecretion and euthyroidism requires no treatment

The syndrome of inappropriate TSH secretion is described in a euthyroid girl and her father. Based on nuclear T3 binding studies in fibroblasts, generalized tissue resistance was associated with a lower binding affinity for T3 in nuclear extracts suggestive of a structurally abnormal receptor for T3. Early recognition of the syndrome and observation of the short-term response to thyroid medication prevented unnecessary trials of antithyroid medication and later radical ablative thyroid treatment.