The Apparent Free Testosterone Concentration, An Index of Androgenicity

As free non-protein bound testosterone is generally considered to represent the active androgen fraction in plasma, we developed a method, based on equilibrium dialysis of diluted plasma, by which the “apparent free testosterone concentration” (AFTC) in undiluted plasma is obtained. This AFTC was determined in normal adult, (pre)adolescent and hypogonadal males, in male diabetics, as well as in hirsute and pregnant women and in hyperthyroid patients of both sexes. The values were compared with those obtained for total plasma testosterone in the same subjects. The AFTC in normal males age 20–50 yr varied between 5.5 and 20 ng/100 ml and decreased with age; this decrease was already significant in the age group 50–70 yr, whereas total testosterone concentration in these subjects was not different from younger adults. In females, the AFTC varied between 0.2 and 0.75 ng/100 ml. In (pre)adolescent males plasma testosterone levels and AFTC increased in function of sexual maturity, but whereas sometimes adult plasma testosterone levels were already observed before full sexual development was achieved, adult AFTC levels were only obtained in subjects with full sexual maturity. In male hypogonadism the decrease in AFTC was more pronounced than the decrease in total testosterone. In male diabetics, the total testosterone concentration was generally low normal, but, due to a slight decrease in TeBG capacity, the AFTC was entirely within normal limits. In female hirsutism, plasma testosterone levels and AFTC were variable, but the AFTC was increased more frequently than total plasma testosterone. In pregnancy, whereas plasma testosterone levels were increased, the AFTC was normal or moderately decreased. In hyperthyroidism finally, plasma testosterone levels were often increased, but the AFTC was normal. It appears to the authors that the AFTC is a better index of androgenicity than total testosterone.