Thyrotropin-secreting pituitary tumors in hyper- and hypothyroidism.

Thyrotropin (TSH)-secreting pituitary tumors may be found in two opposite clinical situations: the hyperthyroidism secondary to thyrotroph adenomas, also called central hyperthyroidism, and the long-standing primary hypothyroidism which can be accompanied by a compensatory pituitary enlargement. TSH-secreting pituitary adenomas belong to the syndromes of "inappropriate secretion of TSH" (IST). The adjective "inappropriate" indicates the lack of the expected suppression of TSH secretion when free thyroid hormone levels are actually elevated, as in the other forms of thyrotoxicosis. Moreover, TSH-omas have to be differentiated from the non-neoplastic form of IST which is due to resistance to thyroid hormone. Differently, pituitary hyperplasia, which is reversible on thyroid hormone replacement, is the more frequent cause of a pituitary mass occurring in the context of untreated primary hypothyroidism. Failure or delay in the recognition of the above clinical situations may cause dramatic consequences, such as unnecessary pituitary surgery in hypothyroid patients or improper thyroid ablation in those with central hyperthyroidism. In contrast, early diagnosis and proper treatment of TSH-secreting pituitary tumors prevents the appearance of signs and symptoms of mechanical compression of the adjacent structures by the expanding tumor mass (visual field defects, headache and hypopituitarism).