Luteinizing hormone-releasing hormone and the human menstrual cycle.

Plasma concentrations of LHRH were measured by radioimmunoassay in daily samples obtained from 10 normally ovulating women. The normalcy of each menstrual cycle was determined by measuring luteinizing hormone (LH), follicle-stimulating hormone (FSH), total estrogen, and progesterone concentrations. Six women had consistently measurable immunoreactive LH-releasing hormone (LHRH) in every blood sample. In four, LHRH could not be detected in some of the samples, more frequently during the follicular phase. LHRH levels varied between 10 and 35 pg/ml during the menstrual cycle. At midcycle, coincident with the LH surge, the mean LHRH level (17.6 +/- 4.4 pg/ml) was not significantly different from the mean follicular or luteal phase values. The lowest LHRH level, 11.4 +/- 4.6 pg/ml, was observed on the day of the estrogen surge. A detailed evaluation was made of the specific days of the cycle. Blood samples obtained from an indwelling venous catheter every 20 minutes over a 3 or a 5 hour period during the days of the estrogen surge and the LH surge and on the following day showed no correlation between OHRH, LH, and FSH values.