Integrated concentration and circadian variation of plasma testosterone in normal men

The hourly integrated concentrations of testosterone and LH in the plasma of 6 young adult, normal men were determined during 24 hr of normal activity and sleep. A portable non-thrombogenic pump system was used for the constant blood withdrawal. Although no distinct rhythm of testosterone was discernible when the various curves were examined separately, an analysis of variance of the results obtained on all 6 subjects revealed a significant (p < 0.0005) circadian variation, the peak of the mean levels being at 6-7 AM (675 ng/100 ml) and the lowest values being reached at 7-8 PM (464 ng/100 ml). However, only 20% of the variation in plasma testosterone levels were time-related, while 80% of the variation was random in nature. The small amplitude of the changes and the individual variations render difficult the physiological interpretation of the circadian rhythm of plasma testosterone. No significant circadian rhythm of plasma LH could be demonstrated and there was no significant correlation between testosterone and LH levels.