A novel spreadsheet method for calculating the free serum concentrations of testosterone, dihydrotestosterone, estradiol, estrone and cortisol: with illustrative examples from male and female populations.

In humans, testosterone (T), dihydrotestosterone (DHT), estradiol (E2), estrone (E1) and cortisol (C) bind to the serum proteins sex hormone-binding globulin (SHBG), albumin (Alb) and corticosteroid-binding globulin (CBG). Equilibrium dialysis is considered to be the "gold standard" for measuring the free concentrations of these steroids but is technically difficult and not widely available. Based on a mathematical model of the 5-ligand/3-protein binding equilibria, we developed a novel spreadsheet method for calculating the free and bioavailable (free+Alb-bound) concentrations of each steroid in terms of the total steroid and protein concentrations. The model uses 15 association constants K(SHBG-X), K(Alb-X), and K(CBG-X) (X=T, DHT, E2, E1 and C) that have been estimated from a systematic review of published binding studies. The computation of the free and bioavailable concentrations uses an iterative numerical method that can be readily programmed on a spreadsheet. The method is illustrated with six examples corresponding to young men (YM), old men (OM), obese men (Ob M), young women (YM), pregnant women in the 3rd trimester (Preg T3) and oophorectomized women on oral conjugated equine estrogens (CEE). The resulting free hormone concentrations for YM and YW fall within the normal references ranges obtained by equilibrium dialysis for all five hormones. The model also accounts for the competitive binding effects of high estrogen levels on the free T levels in Preg T3. This novel spreadsheet method provides a "user-friendly" approach for estimating the free concentrations of circulating sex hormones and cortisol in men and women.