To understand the expression of thyroid hormone receptors (TRs) in various types of pituitary tumor cells, we performed the reverse transcription coupled to polymerase chain reaction (RT-PCR). Specimens of pituitary adenomas were obtained by transsphenoidal adenomectomies from 2 patients with acromegaly, 6 patients with prolactinoma, 3 patients with Cushing's disease, 2 patients with gonadotropin-secreting tumors, one patient with a TSH-secreting tumor, and 3 patients with non-functioning adenoma with no clinical evidence of pituitary hormone overproduction. Human thyroid hormone receptors beta 1 and alpha 1, alpha 2 (h-TR beta 1, alpha 1, alpha 2) are expressed in nonfunctioning adenomas; h-TR alpha 1 and beta 1 are specific for prolactinomas; h-TR beta 1 is involved in acromegalies and FSH-secreting tumors, while h-TR alpha 1, alpha 2, and beta 1 are not expressed in TSH-secreting and ACTH-secreting tumors. The results suggest that human thyroid hormone receptors are differentially expressed in human pituitary tumor cells, and our studies have shed new light on the understanding of the role of TRs in pituitary tumors.