Daily and circadian variations in serum free testosterone levels are not clinically significant

Serum free testosterone levels were measured daily during an ovulatory, endocrinologically normal cycle in five women and at 6-hour intervals on six consecutive days near midcycle in four of these women. During ovulatory cycles, serum free and total testosterone, unlike the gonadotropins, estradiol, and progesterone, did not fluctuate in a cyclic pattern. The 6-hour assays demonstrated close parallelism of free and total testosterone levels and no circadian pattern similar to the diurnal variation seen in serum cortisol values. During both study intervals, the serum free testosterone levels rarely exceeded the upper limit of the normal range defined for our laboratory. Thus, serum free testosterone values measured in blood samples obtained at any day are equally reliable for clinical use.