Total estradiol, free estradiol, sex hormone-binding globulin, and the fraction of estradiol bound to sex hormone-binding globulin in human follicular fluid

Sex hormone-binding globulin (SHBG), percent free estradiol (E2), the fraction of E2 bound to SHBG, and total E2 were measured in the serum and follicular fluid of 12 women (25 follicles) who had received gonadotropin stimulation in an in vitro fertilization program. The women were classified as high or low responders based on peak serum E2 levels (high responders: peak E2, greater than 1500 pg/ml; low responders: peak E2, less than 1000 pg/ml). During treatment, serum levels of SHBG increased in high responders from 55 +/- 8.8 (+/- SEM) to 96 +/- 16 nM (P less than 0.01), but did not change in low responders. SHBG was more concentrated in follicular fluids from high responders (142 +/- 12.5 nM) than in those from low responders (44.4 +/- 5.8 nM). A positive correlation was found between serum and follicular fluid levels of SHBG (r = 0.873; P less than 0.01). In follicular fluid, total E2 levels, which varied from 100-2650 ng/ml, correlated (r = 0.790; P less than 0.01) closely with SHBG levels. The percent free E2 averaged 5.9% (range, 4-10.6%) in follicular fluid compared to 1.8% (range, 1.5-2.1%) in serum. An inverse correlation (r = -0.661; P less than 0.01) was found between total E2 concentrations and percent free E2 in follicular fluid. The relationship between serum and follicular fluid levels of SHBG suggests that SHBG in follicles arises from the circulation. Although SHBG is present in follicular fluid in amounts similar to those in serum, the large quantities of E2 in preovulatory follicules exceed the binding capacity for SHBG, and the majority of E2 appears to be bound to albumin. Hence, it seems unlikely that SHBG in follicular fluid regulates estrogen action in ovarian target cells.