Detection of a new de novo mutation at codon 251 of exon 8 of thyroid hormone receptor beta gene in an Italian kindred with resistance to thyroid hormone

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.

A Novel Thyroid Hormone Receptor Beta Gene Mutation (G251V) in a Thai Patient with Resistance to Thyroid Hormone Coexisting with Pituitary Incidentaloma

A patient is reported with resistance to thyroid hormone beta caused by a novel THRB gene mutation and coexisting pituitary microadenoma. A 41-year-old Thai woman presented with elevated serum thyroid hormone levels and non-suppressed thyrotropin (TSH). Magnetic resonance imaging showed a 4 mm × 2 mm pituitary adenoma. Five of her relatives had similar thyroid tests abnormalities, but a sister had Graves' disease. Thyroperoxidase and thyroglobulin antibodies were positive in all affected family members, except for the proband's 4.5-year-old niece. Lack of thyrotoxic symptoms and TSH suppression by triiodothyronine indicated incidentaloma rather than a TSH-secreting pituitary adenoma. Genetic analysis revealed a THRB gene mutation (c.1037G>T), resulting in p.G251V.

Clinical and genetic characteristics of a large monocentric series of patients affected by thyroid hormone (Th) resistance and suggestions for differential diagnosis in patients without mutation of Th receptor β

OBJECTIVE

The syndrome of resistance to thyroid hormone (RTH) is caused by a mutation of TH receptor β (TRβ) in 80% of cases. Patients without mutation (non-TR-RTH) may have a biochemical pattern that is difficult to differentiate from that of pituitary TSH-secreting adenoma (TSHoma). Herein, we report a large monocentric series of RTH focusing on patients with non-TR-RTH, to evaluate possible clinical or biochemical parameters able to distinguish them from TSHoma.

DESIGN AND PATIENTS

We retrospectively reviewed the data of 99 consecutive patients with inappropriate TSH secretion (IST) syndrome referred to our Department between 1983 and 2011, identifying 68 patients with RTH and 31 patients with TSHomas.

MEASUREMENTS

Patient records were reviewed for the main clinical, biochemical and imaging characteristics.

RESULTS

Of our 68 patients with RTH, 16 (23·5%) did not show a TRβ mutation and did not have affected family members. Of these 16 patients, three developed a TSHoma, during follow-up. To distinguish non-TR-RTH from TSHoma, we identified appropriate cut-off values for the main biochemical parameters that demonstrated the greatest sensitivity and specificity (T3 suppression test, α-subunit/TSH molar ratio, α-subunit assay and TRH test) and we calculated the probability for each patient to develop a TSHoma.

CONCLUSIONS

The application of the identified cut-offs could become a very useful tool in the challenging differential diagnosis between sporadic non-TR-RTH and TSHoma. It would then be possible to select the patients at higher risk of developing a TSHoma and therefore needing a closer follow-up.

Resistance to thyroid hormone accompanied by Graves' disease

Resistance to thyroid hormone (RTH) is characterized by elevated serum levels of thyroid hormones and normal or slightly increased serum thyrotropin (TSH) levels. Recently it has been suggested that chronic TSH stimulation in RTH activates intrathyroidal lymphocytes, leading to thyroid damage and autoimmune thyroid disease (AITD). Therefore, individuals with RTH have an increased likelihood of AITD compared to unaffected relatives. We here report a 33-year-old woman in whom we diagnosed Graves' disease and treated her with thiamazole (MMI). For two years, her TSH levels were suppressed when thyroid hormones were elevated and conversely they were increased when thyroid hormones levels were decreased. These findings were common for a clinical course during treatment for Graves' disease with anti-thyroid drug. However, three years after the initiation of MMI therapy, she had a normal or gradually elevated serum TSH level even though the level of thyroid hormones never decreased, indicating inappropriate secretion of TSH. We concluded she had RTH clinically, and we demonstrated by direct sequence analysis a mutation of the TRβ gene, causing replacement of a glycine (G) with arginine (R) at codon 251. The finding of an elevated TSH level without decreased thyroid hormones should suggest the presence of RTH during therapy of Graves' disease.

Thyroid color flow Doppler sonography: an adjunctive tool for differentiating patients with inappropriate thyrotropin (TSH) secretion due to TSH-secreting pituitary adenoma or resistance to thyroid hormone

BACKGROUND

Thyrotropin (TSH)-secreting pituitary adenoma (TSHoma) and resistance to thyroid hormone (RTH) are two forms of inappropriate TSH secretion. Thyroid blood flow is largely TSH dependent.

OBJECTIVE

To assess whether thyroid blood flow may help to differentiate TSHoma and RTH.

DESIGN

Intrathyroidal color flow Doppler sonography (CFDS) pattern and peak systolic velocity (PSV) were assessed at baseline and during T(3) suppression test on eight consecutive patients with TSHoma and 10 with RTH.

MAIN OUTCOME

All controls had CFDS pattern 0. Three RTH patients had pattern I and seven had pattern II. Two TSHoma patients had pattern I, five had pattern II, and one had pattern III. PSV at baseline was 3.8 +/- 1.3 cm/s in controls, 8.8 +/- 2.5 cm/s in RTH, 11.1 +/- 2.7 cm/s in TSHoma (p < 0.0003 vs. controls, p = 0.087 RTH vs. TSHoma). After T3 suppression test, PSV values were lower in RTH than in TSHoma (4.6 +/- 1.8 vs. 7.7 +/- 2.6 cm/s, p = 0.008). PSV values and CFDS pattern normalized in nine and eight RTH patients, respectively, after T(3) suppression test; conversely, only one TSHoma patient had a normalization of PSV values, and none had a normalization of CFDS pattern (p < 0.003 vs. RTH).

CONCLUSIONS

Both RTH and TSHoma have increased CFDS pattern and PSV values; however, after T(3) both parameters normalized in most patients with RTH but not in those with TSHoma. Accordingly, CFDS pattern and PSV are adjunctive tools to differentiate these two forms of inappropriate TSH secretion.