OBJECTIVE: Sex hormone-binding globulin (SHBG) binds testosterone (T) to a greater degree than it does estradiol (E 2), acting as an amplifier of E 2 action. However, it is not known whether the relative capacity of SHBG for E 2 vs. T is altered by the hormonal milieu. We hypothesized that an increase in circulating E 2 levels results in a compensatory increase in the relative binding capacity of SHBG for these hormones, dampening the E 2 amplification effect in hyperestrogenic conditions. STUDY DESIGN: Retrospective. RESULTS: As expected, during hMG stimulation there was a significant increase in total and free E 2 (28 to 1,986 pg/mL, P<.001; and 0.3 to 20.8 pg/mL, P<.001, respectively) and total T levels (40.3 vs. 78.3 ng/dL, P<.001) from basal to late stimulation. Free T levels increased, but the difference did not reach significance. The binding capacity of SHBG for both E 2 and T increased in a proportional manner (980±340 vs. 1,434±449 nmol/L, P<.009; and 352±190 vs. 512±128 nmol/L, P<.02; respectively) since the ratio of SHBG binding to E 2 and T was unchanged. Although the SHBG molar concentration appeared increased, the difference did not reach significance (821±542 to 1,099+254 nmol/L). CONCLUSION: A short-term, although profound, increase in circulating E 2 does not seem to be associated with an increase in the relative binding capacity of the carrier protein for either E 2 or T, although an overall increase in binding for both steroids was observed. It is possible that longer periods of exposure to E 2 may be necessary to demonstrate a change in the differential binding of this carrier protein with an alteration in the hormonal milieu.