Thyrotropin (TSH)-secreting pituitary adenomas are a rare cause of hyperthyroidism. They account for less than 1 percent of all functioning pituitary tumors [1,2], and much less than 1 percent of all cases of hyperthyroidism. Nevertheless, the diagnosis should be considered in all hyperthyroid patients, especially those with a diffuse goiter and no extrathyroidal manifestations of Graves' disease.
This topic will review the clinical presentation, diagnosis, and treatment of TSH-secreting pituitary tumors. Other causes of hyperthyroidism are reviewed separately. (See "Disorders that cause hyperthyroidism" and "Diagnosis of hyperthyroidism".)
TSH-secreting adenomas secrete biologically active thyrotropin (TSH) in a more or less autonomous fashion. Thus, TSH secretion usually does not increase much in response to thyrotropin-releasing hormone (TRH) and does not decrease much in response to exogenous thyroid hormone administration. The biological activity of the TSH that is secreted varies considerably; as a result, serum immunoreactive TSH concentrations range from normal (albeit inappropriately high in the presence of hyperthyroidism) to markedly elevated (>500 mU/L) .
Approximately 25 percent of the adenomas secrete one or more other pituitary hormones; about 15 percent secrete growth hormone, 10 percent secrete prolactin, and rare tumors secrete gonadotropins . There have been no reported instances of cosecretion of corticotropin (ACTH) and TSH.
Adenomas secreting TSH and growth hormone are equally common in men and women, whereas cosecretion of TSH and prolactin is about five times more common in women than in men. Hyperprolactinemia is not always due to tumor secretion of prolactin; in some patients, it is caused by compression of the pituitary stalk and interruption of tonic hypothalamic inhibition of prolactin secretion. (See "Causes of hyperprolactinemia", section on 'Decreased dopaminergic inhibition of prolactin secretion'.)