Serum forms of testosterone in men after an hCG stimulation: relative increase in non-protein bound forms

A postulated function of steroid binding proteins in serum is to smooth changes in steroid levels. To test this, testosterone levels in six normal men were increased by injecting 6000 IU hCG i.m., and changes in serum forms of testosterone were measured. Blood was collected every 10 min for 2.5 h and then once a day for 4 days. By day 4 the mean serum testosterone level had risen to 178% +/- 13% (SEM) of a mean basal level (first five samples). This rise was less than that in free testosterone (221% +/- 18%), which was in turn less than the rise in the non-SHBG-bound fraction (255% +/- 19%). The concentrations of SHBG and albumin were constant. Thus, two putative bioactive fractions of testosterone, the free and non-SHBG-bound, increased to a greater extent than did total testosterone. Because the binding protein concentrations were constant, this implies they may act not as a buffer, but as an enhancer of active testosterone over this time interval.