A protein-binding radioassay for 6 beta-hydroxycortisol: detection in pregnancy urine and amniotic fluid

A major metabolic step in the excretion of cortisol during pregnancy is its hydroxylation to 6 beta-hydroxycortisol (6 beta-OHF), which is a prevalent unconjugated polar metabolite in maternal urine and amniotic fluid. The fetus may be a major contributor to the maternal pool of 6 beta-OHF. Objectives of this study were to establish levels of 6 beta-OHF before and during normal pregnancy. Until recently the lack of practical methods for routine measurement of 6 beta-OHF has limited the investigation of this polar metabolite as a potential diagnostic aid. A protein-binding radioassay for 6 beta-OHF has been developed which requires a preliminary extraction step and separation by column chromatography. Specificity, precision, and sensitivity of the method are reported. The mean normal 24-hour urine levels of 6 beta-OHF in male subjects (300 micrograms), female subjects not receiving estrogenic medication (313 micrograms), and female subjects using oral contraceptives (227 micrograms) did not differ significantly. However, during pregnancy, urinary levels increased to 430 micrograms during the first trimester and 863 micrograms during the third trimester. Amniotic fluid levels of 6 beta-OHF increased from a mean of 5 ng/ml during the first trimester to 19 ng/ml during the third trimester. The present assay offers a reliable clinical laboratory method for the specific measurement of 6 beta-OHF in high-risk pregnancies.