Circadian rhythm of plasma testosterone, cortisol and gonadotropins in normal male subjects

Cortisol, gonadotropins PSH-LH and testosterone were estimated in 13 normal men: in 10 subjects blood samples were collected at 8 a.m., 2 p.m. and 10 p.m. and in the others every 2h.The results obtained show significant diurnal variations of cortisol and testosterone with the highest values at 8 a.m. and the lowest at 10 p.m.On the contrary, the daily variations in the concentration of plasma FSH and LH were less constant: the two hormones when recorded on plasma samples collected every 2 h, did not show a gradual decrease during the day, but oscillations reproducing a jagged line. However the mean values of the evening seem to be lower than those of the morning.These data confirm the existence of a gonadal diurnal cycle for testosterone, which is probably controlled by the hypophysis, although a close correlation between the cyclicity of gonadotropins and testosterone secretion is not clearly demonstrated.