Received on April 7, 2005
Accepted on December 8, 2005

Endocrinology and Metabolism

Serum Bioavailable Testosterone: Assayed or Calculated?

¹ Emi INSERM 03-37, Centre de Recherches Chirurgicales, CHU Henri Mondor, Faculté de Médecine, Créteil, France
² Emi INSERM 03-37, Centre de Recherches Chirurgicales, CHU Henri Mondor, Faculté de Médecine, Créteil, France, and Laboratoire de Biochimie, Faculté de Pharmacie, Paris, France
³ Laboratoire d'hormonologie, Hôpital St. Antoine, Paris, France
⁴ Laboratoire d'hormonologie, Hôpital St. Louis, Paris, France
⁵ IUT de Cachan, Cachan, France
⁶ Université Pierre et Marie, Paris, France

^* To whom correspondence should be addressed. E-mail: fiet{at}univ-paris12.fr.

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 (K_s = 1 x 10⁹ L/mol) and albumin (K_a = 3.6 x 10⁴ L/mol) and paired optimal K_s and K_a 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 79.2% (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 85.4% (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.