TSH secreting pituitary adenoma is a rare disorder and represents about 1% of all pituitary adenomas. About 80 cases have so far been reported. We experienced four cases of TSH secreting pituitary adenoma. Three of them were macroadenomas and 1 was microadenoma. Serum TSH levels varied between 4.1~185.6 uIU/ml. The serum -subunit levels of two patients tested were slightly elevated (0.87, 0.96 mIU/ml). Serum TSH and -subunit response to TSH, dexamethasone, T3, bromocriptine, and octreotide were variable when studied. Three patients underwent transsphenoidal microsurgery. One of them received radiotherapy. They were cured. Treatment with octreotide in 1 patient who did not received surgery resulted in normalization of serum TSH and thyroid hormone levels and reduction in tumor size. (J Kor Soc Endocrinol 7:331~342, 1992)