The human thyrotropic adenoma: pathologic diagnosis in five cases and critical review of the literature

Five out of 400 surgically removed pituitary tumors (frequency: 1.2%) were identified as thyrotropic adenomas according to the following criteria: identification of tumoral thyrotropic cells by immunocytochemistry and ultrastructural study; elevated serum TSH levels with decrease after surgery; and elevated concentration of TSH in the tumor. Four patients presented with hyperthyroidism and one with euthyroidism. From these five cases and 11 similar observations extracted from a critical review of literature, the morphologic, immunocytochemical, and hormonal characteristics of thyrotropic adenoma are described. Thyrotropic adenomas are more often large tumors but may also be microadenomas. The diagnosis is asserted by immunoreactivity with anti-TSH antisera. The TSH positive tumor cells are numerous. In some tumors, rare cells of other types are also found (PRL, GH, FSH, or ACTH cells). Some morphologic characteristics strongly suggest the diagnosis. The cells are often large with thin processes. They show argyrophil granulations in a slightly basophil cytoplasm and signs of secretory activity. Their secretory granules are round and small without striking variations in size, shape, and electron density. Elevated concentration of TSH in the tumor confirms the diagnosis. The presence of high serum TSH levels and a molar ratio of alpha hTSH to the whole TSH molecule greater than one are other good criteria. Decrease of TSH after surgery may not be observed in invasive tumors. TSH adenoma is most often associated with hyperthyroidism but it can also be associated with hypothyroidism or euthyroidism.