Plasma testosterone free index: A better indicator of plasma androgen activity? *

Testosterone, quantitatively the most important androgen in the circulation, must enter cells to exert an androgenic effect. It is accepted that it is that fraction of the hormone not bound to the specific, binding, beta-globulin which is available for entry into the cells. By equilibrium dialysis we obtained an in vitro measurement of the unbound fraction. The product of this fraction and the total testosterone level gave us a testosterone free index (TFI). In normal men and women, our ranges were 24.6 to 93.6 and 0.8 to 3.8 ng/100 ml, respectively. Of 54 hirsute women, 24 had an elevated TFI; 4 of these had normal plasma testosterone levels. Thirteen men with hypogonadism had a decreased TFI. Nine of ten women and two men with hyperthyroidism had a normal TFI, although there was an elevated plasma testosterone level in most instances. The calculated TFI, which reflects both the extent of binding and the total testosterone level, may therefore be a better indicator of circulating androgen activity.