Determination of Bioavailable Testosterone [Non–Sex Hormone–Binding Globulin (SHBG)-Bound Testosterone] in a Population of Healthy French Men: Influence of Androstenediol on Testosterone Binding to SHBG

Background: Bioavailable testosterone (BT) is measured [assayed BT (aBT)] or calculated (cBT) in the diagnosis of hypogonadism in men. The cBT depends, however, on the values of the association constants of total testosterone (TT) for sex hormone–binding globulin (SHBG; K s) and albumin ( K a), and its use therefore remains controversial. Methods: In 503 selected, untreated healthy men, 20–74 years old, we measured TT, dihydrotestosterone (DHT), and androstenediol (5-diol) by GC-MS, SHBG by RIA, and BT after ammonium sulfate precipitation or by calculation according to the law of mass action. Results: A slight decrease in TT, significant decreases in BT and 5-diol, no variation in DHT, and an increase in SHBG were observed with age. In young males (≤39 years), the lower normal limits were between 2.30 and 2.72 nmol/L for aBT and 8.50 nmol/L for TT. For K s = 1 × 109 L/mol and K a = 3.6 × 104 L/mol, the lower cBT limit was found to be 2-fold higher than for aBT. With optimized K s = 1.9 × 109 L/mol and K a = 2.45 × 104 L/mol, cBT values close to aBT were obtained. When 5-diol was included in the model as a competitive SHBG inhibitor, the correlation between cBT and aBT was better and the cBT:aBT ratios vs 5-diol were less biased. Conclusion: Lower normal serum aBT concentration in normal men appears to be between 2.30 and 2.72 nmol/L. Much higher serum cBT concentrations are associated with use of different association constants that may be inappropriate. When using the optimized binding constants, taking age-related 5-diol values into consideration slightly improves prediction of cBT.