| Resistance to thyroid hormone is characterized by a decreased pituitary and tissue responsiveness to thyroid hormones. More than 500 cases have been reported. This disease is usually identified by the combination of persistent elevation of serum free T4 and T3 and inappropriately normal serum TSH in the absence of assay interferences. Also, administration of supraphysiologic doses of thyroid hormone fail to suppress TSH secretion. Resistance to thyroid hormone is associated with mutations in the thyroid hormone receptor b-gene. It is familial in 75 % of the cases. Inheritance is autosomal dominant. Two types of resistance have been described: general and, more rarely, pituitary. In the general resistance type, high levels of thyroid hormone compensate for the relative insensitivity of the tissues to the hormone so that patients are euthyroid. However, tissues are not always resistant to thyroid hormones at the same degree so that features of hypo and hyperthyroidism may coexist in the same patient. Usually, pituitary, brain and bone are more insensitive to thyroid hormone than liver, heart and basal metabolism. In the selective pituitary resistance syndrome, patients present with a TSH-dependent type hyperthyroidism. Other features may be observed in patients with resistance to thyroid hormone: goiter is nearly constant and attention deficit/hyperactivity disorder, decreased IQ, delayed growth are of variable intensity. |
