Syndromes of thyroid hormone resistance.

Resistance to the action of thyroid hormone can involve both peripheral tissues and the pituitary (global resistance), the pituitary only or peripheral tissues alone. Global resistance is of variable severity and has been observed in more than 60 individuals, the majority occurring in 17 families. Affected subjects are commonly eumetabolic and have goiters, elevated plasma levels of total and free thyroxine and triiodothyronine, normal thyroid hormone metabolism, and normal serum TSH levels (albeit high for the corresponding levels of thyroid hormone). A variable degree of delayed bone maturation, mental retardation, learning disabilities, and hearing defects have been reported; and a variety of treatment regimens, most of which are aimed at reducing the level of plasma hormones and/or goiter, have been attempted before the correct diagnosis has been reached. The clinical disorder is equally common in males and females and appears to be due to one or more autosomal gene mutations. The causes for the hormone resistance may be heterogeneous, either influencing the receptor for thyroid hormones or some unidentified steps in hormone action. At present, the diagnosis is one of exclusion; no effective therapy is available, and all measures designed to lower serum thyroid hormone levels should be avoided.