Objective : To explore seasonal variations in the patterns of circulating hormones including the relation of pituitary tropic hormones to their target organs.
Methods : Two hundred elderly subjects (77±8 years of age) were studied over one to four 24 hour spans distributed over all four seasons. During each 24-hour sampling session six blood samples were collected at 4-hour intervals. Seventeen hormonal parameters (ACTH, aldosterone, cortisol, C-peptide, DHEA-S, FSH, growth hormone, insulin, LH, prolactin, 17-OH progesterone, testosterone, TT4, TT3, TSH, and estradiol and progesterone (women only)) were determined in plasma by. Circadian rhythms were analyzed by the single and population mean cosinor procedures. Seasonal patterns of the circadian mean of these variables were determined by cluster and principal component analysis and their relationship to each other by Pearson correlation.
Results : Statistically significant seasonal differences in endocrine patterns of circulating hormones were detected. There were seasonal differences in the relationship of tropic hormones of the pituitary and the circulating hormone concentration of their target organs in the gonadotropic and adrenotropic axis.
Conclusions : The different patterns in hormonal relationships indicate seasonal variations in endocrine function and suggest seasonally changing interactions of tropic hormones with their target organs. It becomes apparent that in endocrinology not only the levels of single hormones but wider hormonal patterns of related variables may determine the functional state of individuals and that these patterns change not only as a function of the circadian stage but also as a function of the season.