Time-related behaviour of endocrine secretion: circannual variations of FT3, cortisol, HGH and serum basal insulin in healthy subjects.

Four healthy young male volunteers were submitted to the study of circadian and circannual bioperiodicities of several hormones: FT3, FT4, cortisol, HGH, prolactin, PTh and plasma insulin levels. They were observed for a whole year and their blood samples were collected six times a day, every other month. The results were analyzed by two-way ANOVA macroscopic analysis and Student t-test. Our data registered a circannual variation in the mean circadian plasma levels of the following hormones: cortisol (peak in December), HGH (peak in April), FT3 (peak in April), insulin (peak in February). FT4, prolactin and PTH showed no cyclic variation during the period of observation.