Hurthle cell neoplasms are though to arise from the follicular cells of the thyroid gland, although some studies suggest that they originate from the parafollicular cells. We studied tissue from five patients (three men and two women, aged 33-72 yr) with Hurthle cell neoplasms (four adenomas and one carcinoma) to determine whether Hurthle cell neoplasms have an intact TSH receptor-adenylate cyclase (AC) system and, if so, whether it differs in benign and malignant Hurthle cell and neoplasms. Binding of [125I]bovine (b) TSH and an AC response to TSH occurred in all four Hurthle cell adenomas. In three of these tumors, there was good binding and a relatively good correlation between the concentration of bTSH producing half-maximal inhibition of [125I]bTSH binding (4.8 mU/ml) and the TSH concentration causing half-maximal stimulation of AC (2.2 mU/ml). There was also a 2- to 5-fold increase in AC activity in response to bTSH (300 mU/ml), a value comparable to that which occurs in follicular thyroid neoplasms and differentiated thyroid cancers. In the fourth Hurthle cell adenoma, however, binding was low, the apparent Kd (170 mU/ml) and Km (20 mU/ml) were high, and there was only a 1.4-fold increase in AC activity in response to bTSH (300 mU/ml). In the one metastatic Hurthle cell carcinoma, there was no high affinity TSH binding or AC response to TSH. Thus, Hurthle cell neoplasms are of follicular cell origin, since benign Hurthle cell tumors have an intact TSH receptor-AC system. Malignant Hurthle cell neoplasms, like undifferentiated and medullary thyroid cancer, lack a functional TSH receptor.