Large annual variation in photoperiodicity does not affect testicular endocrine function in man.

The purpose of the present study was to elucidate the possible influence of large differences in circannual photoperiodicity on human testicular activity. Serum testosterone was measured once a month in a group of 24 healthy young adult men for a period of 13 months in northern Finland, where the day length is 22 h in mid-summer and 3.5 h in mid-winter. The monthly levels of serum testosterone varied between 11.3 +/- 0.93 and 13.0 +/- 1.2 nmol/l (se, n = 24), with no significant differences between individual months. The amplitude of variations in serum testosterone was +/- 7% of the annual mean, which was less than in studies reported earlier from central Europe. Our negative findings suggest that the extreme circannual changes in the amount of day-light do not play an obvious role in the regulation of human testicular androgen production.