Hyperthyroidism due to a Thyrotropin-Secreting Pituitary Adenoma: Studies of Thyrotropin and Subunit Secretion

A 58-year-old man had symptoms of hyperthyroidism and congestive heart failure. While hyperthyroid, his serum thyrotropin (TSH) level was inappropriately elevated at 6.1 μU/mL. The molar ratio of alpha subunit to TSH was 2.5, suggesting the presence of a TSH-secreting pituitary tumor. Further evaluation disclosed an enlarged sella turcica with posterior erosion, and an intrasellar mass was visualized on computed tomographic scan. Neither serum TSH nor alpha subunit levels became elevated after administration of thyrotropin-releasing hormone, nor were they suppressed by a dopamine infusion. Serum TSH but not alpha subunit levels rose during antithyroid drug therapy. Estrogens produced a partial reduction in serum alpha subunit concentration (presumably reflecting the nontumorous gonadotroph contribution to circulating alpha subunit). Dexamethasone completely suppressed serum TSH level but had no effect on the alpha subunit level, suggesting a differential feedback of glucocorticoids on TSH and alpha secretion. The patient was treated with pituitary irradiation rather than surgery because of his underlying heart disease.