Thyroid function, basal serum thyroid-stimulating hormone (TSH) and prolactin concentrations, and the effects of 200 micrograms TSH-releasing hormone (TRH) given intravenously on TSH (delta TSH) and prolactin (delta prolactin) were investigated in 25 euthyroid obese subjects and 20 lean controls. No significant differences in serum thyroid hormone concentrations, glucose metabolism parameters, or basal TSH and prolactin concentrations were detected between groups, but a significant (P less than 0.01) increase in delta TSH and a significant (P less than 0.01) decrease in delta prolactin were observed in obese subjects. No significant differences in basal TSH and prolactin were observed in obese men and women compared with corresponding lean controls, but delta prolactin decreased significantly (P less than 0.01) in obese men and women compared with lean controls and TSH increased significantly (P less than 0.01) in obese men and women. No correlation was found between delta TSH or delta prolactin and body mass index. The study suggests that hyper-responsiveness of TSH and hyporesponsiveness of prolactin to TRH in obese subjects can be related to changes in the central serotoninergic system.