Abstract
Seasonal variation in daylight regulates reproduction in animals living at higher latitude, but the influence of season on the sex hormones in humans remains unclear. OBJECTIVE, DESIGN, AND PARTICIPANTS: A cross-sectional population-based study in Tromsø, Norway (70 degrees N) included 1651 women and 1540 men aged 25-84 yr. Circulating total estradiol (and calculated free levels), FSH, and dehydroepiandrosterone sulfate (DHEAS) were measured between September 1994 and September 1995 and provided a unique opportunity to study effects of extreme seasonal variations in the daylight on hormone levels in an arctic population. Circulating total and free estradiol, FSH, and DHEAS were measured. Total and free estradiol showed differences between monthly means, with peak in June in postmenopausal women (P < 0.001), and in May in men (P = 0.002 and P < 0.001) by analysis of covariance. By cosinor analysis, a seasonal variation in total and free estradiol was evident in women (P = 0.02 and P = 0.03) and men (P = 0.004 and P = 0.001), but only 0.2-0.9% of the variation in total and free estradiol was explained by season. FSH and DHEAS showed no obvious seasonal variation in either sex. Seasonal variations should be considered while designing studies and interpreting results of estradiol measurements to avoid bias in comparative studies.
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