Seasonal changes in pituitary function: amplification of midfollicular luteinizing hormone secretion during the dark season

To analyze the effect of season on the pulsatility of gonadotropin secretion in women living in an area with a large annual variability in daylight length. A prospective study. Pulse studies were carried out in each subject during both the dark and light season. The gynecologic endocrine research unit of the University Central Hospital of Oulu. Eleven ovulatory, healthy women volunteering for the study. Serum samples were collected at 10-minute intervals for 6 hours on days 7 to 9 of the cycle. Serum LH and FSH concentrations were measured and the data were analyzed with an algorithm computer-based program. The mean area of LH pulses analyzed was significantly higher during the dark season than the light season (49.1 ±3.1 versus 38.5 ± 1.7 mIU/mL; conversion factor to SI unit, 1.00), while in the amplitude (1.9 ± 0.1 versus 1.8 ± 0.1 mIU/mL), number of pulses (5.2 ± 0.3 versus 4.4 ± 0.6), and the mean level (9.6 ± 0.5 versus 9.4 ± 0.9 mIU/mL) the difference did not reach statistical significance. The number (5.2 ± 0.5 versus 5.2 ± 0.4), amplitude (1.0 ± 0.05 versus 1.1 ± 0.07 mIU/mL; conversion factor to SI unit, 1.00), area (29.9 ± 2.4 versus 29.6 ± 3.1 mIU/mL), and the mean level of FSH (5.4 ± 0.6 versus 6.0 ± 0.8 mIU/mL) during the dark and light seasons were identical, showing no seasonal variability. The results indicate increased pituitary LH secretion in the midfollicular phase during the dark season that may be related to increased melatonin secretion and decreased ovarian activity at this time of the year.