Nocturnal Slowing of Pulsatile Luteinizing Hormone Secretion in Women during the Follicular Phase of the Menstrual Cycle*

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Abstract
The gonadotropin secretion pattern in normal reproductive age women (n = 5) was evaluated for the presence of a circadian rhythm. The women volunteered for a series of 24-h admissions in different phases of their menstrual cycles (early follicular, late follicular, and midluteal). Plasma LH and FSH levels were determined by RIA in blood samples drawn through indwelling venous catheters at 20-min intervals throughout a normal 24-h sleep-wake cycle. The gonadotropin secretory pattern was subjected to cosine analysis for identifying rhythmicity. The LH interpulse interval fluctuated with a significant 24-h rhythm during the early follicular phase in four of the five women. The maximum interpulse intervals occurred during the early morning between 0100 and 0500 h (mean, 0250 h), with a corresponding increase in LH pulse amplitude occurring within the same time interval (mean, 0320 h). We found no consistent 24-h rhythms in overall mean plasma LH levels during any phase of the menstrual cycle, nor did we find a significant rhythmicity in either LH interpulse interval or LH pulse amplitude during the late follicular or luteal phase. These results demonstrate that the LH pulse-generating system is frequency modulated on a circadian basis during the early follicular phase of the human menstrual cycle.