I. Introduction The term “TSH-secreting pituitary tumors” includes two opposite clinical conditions: true thyrotroph neoplasia that results in secondary hyperthyroidism, also called “central hyperthyroidism,” and pituitary hyperplasia resulting from longstanding primary hypothyroidism. The latter condition was first recognized 145 yr ago (1), while the former was not clearly identified until the RIA era (2–8). However, in the 1950s and 1960s, while it became clear that Graves' disease was not caused by hyperpituitarism (9, 10), scattered reports suggested a possible association between pituitary tumors and hyperthyroidism (11–15), although no measurement of TSH levels was available during this time. The first case of TSH-secreting pituitary adenoma (TSH-oma) was documented in 1960 by measuring serum TSH levels with a bioassay (16). In 1970, Hamilton et al. (17) reported the first case of TSH-oma proved by a RIA that was much more sensitive and specific than the previously used bioassays. Classically, TSH-...