Es. Knochenhauer et al., DIFFERENTIAL BINDING OF ESTRADIOL AND TESTOSTERONE TO SHBG - RELATIONTO CIRCULATING ESTRADIOL LEVELS, Journal of reproductive medicine, 43(8), 1998, pp. 665-670
OBJECTIVE: Sex hormone-binding globulin (SHBG) binds testosterone (T)
to a greater degree than it does estradiol (E-2), acting as an amplifi
er of E-2 action. However; it is not known whether the relative capaci
ty of SHBG for E-2 vs. T is altered by the hormonal milieu. We hypothe
sized that an increase in circulating E-2 levels results in a compensa
tory increase in the relative binding capacity of SHBG for these hormo
nes, dampening the E-2 amplification effect in hyperestrogenic conditi
ons. STUDY DESIGN: Retrospective. RESULTS: As expected, during hMG sti
mulation there was a significant increase in fetal 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 reac
h significance. The binding capacity of SHBG for both E-2 and T increa
sed in a proportional manner (980 +/- 340 vs. 1,434 +/- 449 nmol/L, P
< .009; and 352 +/- 290 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 no
t reach significance (821 +/- 542 to 1,099 +/- 254 nmol/L). CONCLUSION
: A short-term, although profound, increase in circulating E-2 does no
t seem to be associated with an increase in the relative binding capac
ity of the carrier protein for either E-2 or T, although an overall in
crease in binding for both steroids ions observed. It is possible that
longer periods of exposure to E-2 may be necessary to demonstrate a c
hange in the differential binding of this carrier protein with an alte
ration in the hormonal milieu.