title: Seasonal variation of estradiol, follicle stimulating hormone, and dehydroepiandrosterone sulfate in women and men.
abstract: BACKGROUND
  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.
  
  
  METHODS
  Circulating total and free estradiol, FSH, and DHEAS were measured.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  Seasonal variations should be considered while designing studies and interpreting results of estradiol measurements to avoid bias in comparative studies.
authors.affiliation: Institute of Community Medicine, University of Tromsø, N-9037 Tromsø, Norway. ashild.bjornerem@ism.uit.no
authors.forename: Ashild
authors.initials: A
authors.lastname: Bjørnerem
authors.forename: Bjørn
authors.initials: B
authors.lastname: Straume
authors.forename: Pål
authors.initials: P
authors.lastname: Oian
authors.forename: Gro K R
authors.initials: GK
authors.lastname: Berntsen
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003430:Cross-Sectional Studies; D019314:Dehydroepiandrosterone Sulfate / Q000097:blood*; D004958:Estradiol / Q000097:blood*; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012621:Seasons
chemical_list: D004958:Estradiol; D019314:Dehydroepiandrosterone Sulfate; D005640:Follicle Stimulating Hormone