Changes in serum sex hormone-binding globulin, free estradiol, and testosterone during gonadotropin treatment.

Sex hormone-binding globulin (SHBG), estradiol (E2), percent free E2, percent of E2 bound to SHBG, and testosterone (T) were evaluated in 28 ovulatory women during human menopausal gonadotropin-stimulated cycles for in vitro fertilization. Patients were divided into two categories: low responders, in whom serum E2 concentration reached levels less than 1000 pg/ml (mean, 638 +/- 93), and high responders, with serum E2 levels greater than 1000 pg/ml (mean, 2219 +/- 330). A significant increase in SHBG can occur within a short time in high responders (from 62.8 to 103.9 nmol/l) but not in low responders. This increase is accompanied by a significant decrease in the percent free (bioavailable) E2, but the distribution of E2 between the fraction bound to SHBG or albumin did not vary. Despite the increase in the levels of SHBG, the concentration of bioavailable (free) E2 in hyperstimulated women is higher than in normal cycles. The significant increase in T in high responders, by virtue of its higher affinity for SHBG, probably contributes to the increased levels of bioavailable E2.