Evening administration of melatonin enhances the pulsatile secretion of prolactin but not of LH and TSH in normally cycling women.

Objective: The aim of the study was to evaluate the effects of exogenous melatonin on the spontaneous pulsatile release of PRL, TSH and LH in normal women.

Design: A double blind placebo-controlled protocol was designed to study seven subjects in the mid follicular phase of two non-consecutive cycles. Two mg of exogenous melatonin or placebo were given at 1600 and 2000 h, and blood samples were collected every 10 minutes from 1800 to 2400 h for hormone determination.

Results: Melatonin treatment caused a significant upward resetting of the pulsatile pattern of PRL in six out of seven subjects. Average maximal peak height was significantly increased (median 716 mIU/l (range 198-1433) on melatonin vs 324 mIU/l (212-688) on placebo, P < 0.001), nadir value (572 mIU/l (148-1084) vs 216 mIU/l (54-580), P < 0.001) and area under the peak (26352 mIU/l min (5904-93672) vs 12096 mIU/l min (2340-33552), P < 0.001), whereas peak number, amplitude and interpeak interval did not change significantly. TSH pulsatility was unaffected by melatonin administration in four out of six subjects. Distribution of LH patterns after melatonin was inhomogeneous: level of pulsatility was higher in two cases and reduced in three; group analysis did not therefore show significant variation of pulsatility parameters.

Conclusions: Exogenous melatonin has a stimulatory effect on PRL release without affecting the temporal pattern of its pulsatile secretion in normal women. Melatonin has minor, if any, effect on TSH secretion whereas the effect on LH may depend on individual sensitivity.