SEASONAL-CHANGES IN PITUITARY-FUNCTION - AMPLIFICATION OF MIDFOLLICULAR LUTEINIZING-HORMONE SECRETION DURING THE DARK SEASON

Objective: To analyze the effect of season on the pulsatility of gonad otropin secretion in women Living in an area with a large annual varia bility in daylight length. Design: A prospective study. Pulse studies were carried out in each subject during both the dark and light season . Setting: The gynecologic endocrine research unit of the University C entral Hospital of Oulu. Participants: Eleven ovulatory, healthy women volunteering for the study. Interventions: Serum samples were collect ed at 10-minute intervals for 6 hours on days 7 to 9 of the cycle. Mai n Outcome Measures: Serum LH and FSH concentrations were measured and the data were analyzed with an algorithm computer-based program. Resul ts: The mean area of LH pulses analyzed was significantly higher durin g 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 amplitud e (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. Th e number (5.2 +/- 0.5 versus 5.2 +/- 0.4), amplitude (1.0 +/- 0.05 ver sus 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. Conclusions: The resu lts indicate increased pituitary LH secretion in the midfollicular pha se during the dark season that may be related to increased melatonin s ecretion and decreased ovarian activity at this time of the year.