The effect of obesity and drastic caloric restriction on serum prolactin and thyroid stimulating hormone

Subnormal prolactin (PRL) and thyroid stimulating hormone (TSH) responses to thyrotropin releasing hormone (TRH) have been reported in massive obesity, while fasting and very-low-calorie diets have been shown to impair the reaction of PRL to TRH. The mechanism of these changes is poorly understood. We studied a group of moderately obese women on weight maintenance diets to determine whether PRL and TSH responses are reduced in moderate obesity, and repeated the TRH tests during treatment with a 320 kcal diet, with and without the addition of triiodothyronine (T3). Basal and maximal PRL levels after stimulation were not significantly different in the obese patients before dietary restriction from levels in non-obese controls. During the use of the diet the PRL response to TRH was significantly lower than the initial value (P less than 0.025) while basal PRL levels remained unchanged. The percentage reduction in PRL response showed a significant positive correlation with the percentage decrease in serum T3 (r = 0.80, P less than 0.05). Administration of 75 micrograms of T3 daily to the patients, however, was associated with a further significant decrease in the PRL response to TRH. Plasma oestradiol levels did not change with use of the diet. TSH response to TRH was normal in these patients and did not change with use of the diet alone. T3 treatment resulted in an almost complete suppression of the TSH response. In conclusion the reaction of prolactin to stimuli is normal in the moderately obese patient, but reduced during drastic caloric restriction. This decreased responsiveness is probably not due to changes in T3 or oestrogen levels.