Measurement of sex hormone binding globulin in human amniotic fluid: its relationship to protein and testosterone concentrations, and fetal sex

Sex hormone binding globulin (SHBG) has been identified and quantified in human amniotic fluid. Identification was based on its electrophoretic mobility on polyacrylamide gels and its steroid binding characteristics, which were identical to those attributed to SHBG in pregnancy serum. Amniotic fluid SHBG binding capacity was measured by competitive saturation analysis using [3H]-5 alpha-dihydrotestosterone as the labelled ligand, after removal of endogenous steroids with dextran-coated charcoal. Similar amniotic fluid SHBG binding capacities were found in samples taken during early (13-20 weeks, 8.5 +/- 5.1 (SD) nmol/l, n = 10) and late (36-37 weeks, 8.7 +/- 3.0 nmol/l, n = 28) pregnancy. In comparison with pregnancy serum SHBG levels (390 +/- 140 nmol/l, n = 5), amniotic fluid SHBG was not enriched in relation to the relative concentrations of total proteins, albumin or transferrin. Amniotic fluid is therefore not a better source for the purification of SHBG than pregnancy serum. There were no differences in amniotic fluid SHBG levels with respect to fetal sex, but positive correlations were observed between SHBG binding capacities and testosterone concentrations in amniotic fluid from both male (r = 0.68, P less than 0.001) and female (r = 0.53, P less than 0.05) fetuses. It is suggested that SHBG may sequester free testosterone in amniotic fluid, and that measurements of SHBG in amniotic fluid may help to more accurately identify fetal sex in cases where borderline amniotic fluid testosterone concentrations are found.