THYROTROPIN-SECRETING PITUITARY-TUMORS IN HYPERTHYROIDISM AND HYPOTHYROIDISM

Thyrotropin (TSH)-secreting pituitary tumors may be found in two oppos ite clinical situations: the hyperthyroidism secondary to thyrotroph a denomas, also called central hyperthyroidism, and the long-standing pr imary hypothyroidism which can be accompanied by a compensatory pituit ary enlargement. TSH-secreting pituitary adenomas belong to the syndro mes of ''inappropriate secretion of TSH'' (IST). The adjective ''inapp ropriate'' indicates the lack of the expected suppression of TSH secre tion when free thyroid hormone levels are actually elevated, as in the other forms of thyrotoxicosis. Moreover, TSH-omas have to be differen tiated from the non-neoplastic form of IST which is due to resistance to thyroid hormone. Differently, pituitary hyperplasia, which is rever sible on thyroid hormone replacement, is the more frequent cause of a pituitary mass occurring in the context of untreated primary hypothyro idism. Failure or delay in the recognition of the above clinical situa tions may cause dramatic consequences, such as unnecessary pituitary s urgery in hypothyroid patients or improper thyroid ablation in those w ith central hyperthyroidism. In contrast, early diagnosis and proper t reatment of TSH-secreting pituitary tumors prevents the appearance of signs and symptoms of mechanical compression of the adjacent structure s by the expanding tumor mass (visual field defects, headache and hypo pituitarism).