The effect of TRH administration on TSH and PRL release was investigated in 11 obese women and 16 normal weight women. There were no differences in basal serum levels of estradiol, T3, T4, TSH, or PRL between the 2 groups. The increment of TSH levels in the obese group [mean maximum change (delta max), 19.3 +/- 3.0 (+/-SEM) mIU/liter] was significantly higher (P less than 0.025) than that in the control group (delta max, 11.3 +/- 1.3 mIU/liter), whereas PRL levels rose significantly less (P less than 0.025) in these obese women than in the control group (delta max, 738 +/- 132 and 1311 +/- 133 mIU/liter, respectively). Since serotonin is known to stimulate PRL and inhibit TSH release, deficiency of serotonin has been hypothesized as the cause of this disparity between TSH and PRL levels in obesity.