Hyperthyroidism caused by inappropriate thyrotropin hypersecretion: studies in patients with selective pituitary resistance to thyroid hormone

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Abstract
The condition of TSH-induced hyperthyroidism secondary to selective pituitary insensitivity to thyroid hormone was identified in 3 patients. Each patient was clinically hyperthyroid, with elevated serum levels of thyroxine (T4) and triiodothyronine (T3) and detectable levels of serum TSH before therapy. After therapy, each patient had notably elevated TSH levels at a time that peripheral levels of thyroid hormones were in the hyperthyroid range. Before and after therapy, serum levels of TSH were suppressed by therapy with liothyronine sodium and were stimulated by protirelin (TRH) both before and after liothyronine and dexamethasone treatment. Dexamethasone therapy decreased the levels of TSH, protirelin-stimulated TSH, and circulating T4 and T3. Serum levels of glycoprotein .alpha.-subunit were 0.6 to 2.4 ng/ml, values considerably lower than found in patients with TSH-secreting pituitary tumors. The frequency of TSH-induced hyperthyroidism secondary to pituitary insensitivity to thyroid hormone may be higher than presently indicated in the medical literature.