Objective: A diurnal variation in serum testosterone in adult men is well recognized, but whether this occurs during exogenous testosterone administration and the degree to which it is endogenous to the testis is unclear.
Design: A clinical research centre investigation of testicular function in normal men.
Patients: Twenty normal men were recruited, 10 of whom were investigated during administration of testosterone with etonogestrel to suppress gonadotrophin secretion.
Measurements: Hourly blood samples were taken over 24 h for measurement of testosterone, inhibin B, LH, FSH and cortisol. Urinary excretion of testosterone and the testicular steroid epitestosterone was also measured.
Results: In the controls, a diurnal variation in serum testosterone and LH but not FSH was detected. The treated group had similar testosterone concentrations but showed no diurnal variation. Periodicity was also detected in inhibin B concentrations in 5 of the controls and in 9 of the treated group, who also showed synchrony not seen in the controls. Both groups showed diurnal variation in cortisol. Urinary testosterone excretion did not show a diurnal variation in either group, but this was apparent for epitestosterone with a morning peak in both groups despite the markedly lower excretion in the treated men.
Conclusions: The diurnal variation of testosterone in normal men is due to a change in secretion rather than in clearance and is largely LH driven. An endogenous rhythm in both testicular steroidogenesis (epitestosterone) and Sertoli cell function (inhibin B) is also present.