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

The syndromes of reduced sensitivity to thyroid hormone

BACKGROUND

Six known steps are required for the circulating thyroid hormone (TH) to exert its action on target tissues. For three of these steps, human mutations and distinct phenotypes have been identified.

SCOPE OF REVIEW

The clinical, laboratory, genetic and molecular characteristics of these three defects of TH action are the subject of this review. The first defect, recognized 45years ago, produces resistance to TH and carries the acronym, RTH. In the majority of cases it is caused by TH receptor β gene mutations. It has been found in over 3000 individuals belonging to approximately 1000 families. Two relatively novel syndromes presenting reduced sensitivity to TH involve membrane transport and metabolism of TH. One of them, caused by mutations in the TH cell-membrane transporter MCT8, produces severe psychomotor defects. It has been identified in more than 170 males from 90 families. A defect of the intracellular metabolism of TH in 10 individuals from 8 families is caused by mutations in the SECISBP2 gene required for the synthesis of selenoproteins, including TH deiodinases.

MAJOR CONCLUSIONS

Defects at different steps along the pathway leading to TH action at cellular level can manifest as reduced sensitivity to TH.

GENERAL SIGNIFICANCE

Knowledge of the molecular mechanisms involved in TH action allows the recognition of the phenotypes caused by defects of TH action. Once previously known defects have been ruled out, new molecular defects could be sought, thus opening the avenue for novel insights in thyroid physiology. This article is part of a Special Issue entitled Thyroid hormone signaling.

[Clinical and molecular study of five families with resistance to thyroid hormones]

BACKGROUND AND OBJECTIVE

Resistance to thyroid hormone (RTH) is a syndrome mostly caused by mutations in thyroid hormone receptor beta gen (THRB). We present five families with RTH phenotype.

PATIENTS AND METHODS

THRB gene sequencing. In vitro studies to evaluate the mutants response to thyroid hormones and their dominant negative effect. Mechanism of resistance in patients with RTH without THRB mutations quantifying expression of regulator of calcineurin 2 (ZAKI4) and Kruppel-like factor 9 (BTEB) genes in patients fibroblast cultures.

RESULTS

THRB mutations were found in three cases: R243Q, R320C, R429Q. Mutants showed a decreased response to T3. R243Q and R320C had a strong dominant negative effect. One subject without THRB mutation showed changes in ZAKI4 and BTEB expression similar to R320C and the other showed expression levels higher than normal controls.

CONCLUSIONS

Three cases of RTH were caused by THRB heterozygous mutations but in two cases mutations were not found. THRB mutation carriers and one of the patients without mutations share a similar mechanism of resistance and in the other subject RTH is TRβ independent.

[Selective pituitary resistance to thyroid hormone in a patient treated with amiodarone]

Selective pituitary resistance to thyroid hormones is a syndrome that involves inadequate response of thyroid-stimulating hormone to changes in thyroid hormones. Unlike generalized resistance syndromes, this entity produces central hyperthyroidism and clinical thyrotoxicosis. Sometimes the disease may not be properly diagnosed and is treated with drugs with harmful effects on the thyroid, such as amiodarone, hampering diagnosis and possibly exacerbating the disorder. The treatment of this condition can be symptomatic, based on control of tachycardia and anxiety, or etiological, acting on the pituitary to regulate thyrotropin secretion or on the thyroid gland to control thyroid hormone production. We report the case of a patient with pituitary resistance to thyroid hormone, who was treated with amiodarone to control paroxysmal atrial fibrillation.