Serum unconjugated estriol in the menstrual cycle and early pregnancy

A radioimmunoassay for serum unconjugated estriol in the menstrual cycle with a sensitivity of about 5 pg/ml is described. 8 cycles were studied. In 2 cycles, single spikes of 22 and 30 pg/ml were obtained. In 3 cycles, concentrations of 4-5 pg/ml were found whereas in the other 3 studies, no estriol was detected. In general, peaks of estriol corresponded to peaks in estradiol plus estrone. Patients in 4-12 weeks of gestation were also studied. Concentrations as high as 262 pg/ml were found but in isolated instances, no estriol was detected. The results support the view that in contrast to the pregnant state, in the normal menstrual cycle, the bulk of the estriol produced is conjugated before release into the blood.