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 (< or = 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 x 10(9) L/mol and K(a) = 3.6 x 10(4) L/mol, the lower cBT limit was found to be 2-fold higher than for aBT. With optimized K(s) = 1.9 x 10(9) L/mol and K(a) = 2.45 x 10(4) 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.

Serum bioavailable testosterone: assayed or calculated?

BACKGROUND: Bioavailable testosterone (BT), circulating testosterone not bound to sex hormone-binding globulin (SHBG), is thought to easily penetrate cells. We compared BT measurements obtained by assays with those obtained by calculation with different testosterone association constants. METHODS: We obtained sera from 2 groups of hypogonadal men [group 1 (G1), 1421 samples; group 2 (G2), 170 samples] and a group of healthy men [group 3 (G3), 109 samples]. We added minute doses of [3H]testosterone to the sera, precipitated the SHBG-bound fraction of testosterone with ammonium sulfate (50% saturation), and then assayed serum BT (ABT) as %BT x total. Calculated BT (CBT) was determined with theoretical association constants of testosterone for SHBG (Ks = 1 x 10(9) L/mol) and albumin (Ka = 3.6 x 10(4) L/mol) and paired optimal Ks and Ka values obtained by use of Microsoft Excel software. RESULTS: CBT calculated with theoretical constants differed from ABT by >30% in 85.7% (G1), 84.1% (G2), and 77.9% (G3) of samples, and the mean CBT/ABT ratios were 1.57 (G1), 1.85 (G2), and 1.50 (G3) in spite of fairly good correlations. CBT calculated with paired optimal K(s) and K(a) differed from ABT by <30% in 87.4% (G1), 87.5% (G2), and 97.5% (G3) of samples, and mean CBT/ABT ratios were 0.95-1.04. CONCLUSIONS: To obtain CBT values as close as possible to ABT, optimal paired association constants determined for each studied population must be used instead of the theoretical association constants. Considering the uncertainty of calculating BT, however, use of the ammonium sulfate precipitation method for determining BT is advisable.