Serotonergic control of TSH and PRL secretion in obese men

Abstract

To evaluate whether the inhibitory control of TSH and the stimulatory control of prolactin (PRL) secretion exerted by endogenous serotonin was altered in obesity, 22 obese men and 10 normal controls were tested with TRH (200 μg IV bolus) in the presence (experimental test) and absence (control test) of the serotonergic agonist fenfluramine (60 mg PO 90 min before TRH). Control and experimental tests were also performed in seven male patients with subclinical hypothyroidism and were repeated in the same obese subjects after substantial weight loss. Basal TSH levels were similar in control and obese men. Normal TSH responses to TRH (peak ≤ 14 mU/L) were observed in all normal controls (mean peak ± SE 9.8 ± 0.6 mU/L). In contrast, obese men were divided into two groups: nine in whom the TRH-induced TSH rise was higher than normal (group I: mean peak = 16.5 ± 0.5 mU/L) and 13 in whom it was normal (group II: mean peak = 10.6 ± 0.7 mU/L). The hypothyroid men all had elevated basal and TRH-stimulated TSH levels. Basal PRL concentrations were similar in the normal controls and both groups of obese subjects. The PRL response to TRH was lower in both group I and group II obese men than in normal controls and was similar between group I and group II. Following fenfluramine, basal TSH and PRL levels of the normal weight and obese men were similar to those observed in the control test, and there was no effect on the TRH-induced TSH rise in the normal controls, in subjects with subclinical hypothyroidism, or in the group II obese men. In contrast, fenfluramine significantly reduced the TSH response to TRH in the group I obese subjects. Fenfluramine produced a similar increment (10%) in the PRL response to TRH in normal controls and both groups of obese men. After substantial weight loss, all previously obese subjects had normal basal and TRH-stimulated TSH and PRL responses, both in the presence and in the absence of fenfluramine. These data suggest that in some obese men there is a serotonergic disorder affecting TSH, but not PRL, secretion.