The Renal Clearance of Endogenous Estrogens in Late Pregnancy*

Although several studies have been published describing the renal excretion of certain steroid hormones (2, 3), or groups of hormones and hormone metabolites (4-8), an analysis of the re- nal clearance of endogenous estrogens has not so far been described. This has been due principally to the lack of adequate chemical methods for the determination of estrogens in plasma. Methods for the determination of plasma estrogens in late pregnancy, however, have recently become avail- able (9, 10), and it therefore appeared feasible to investigate the renal excretion of endogenous estrogens in this condition. An inspection of the general level of estrogen concentrations in ran- dom samples obtained from subjects in late preg- nancy had suggested that the concentration ratio of the three estrogens estrone (1,3,5-estratrien-3ol-17-one), estradiol-17,8 (1,3,5-estratriene-3, 17,3diol), and estriol (1,3,5-estratriene-3,f, 16a, 17,8triol) in plasma might differ from their concen- tration ratio in urine (11).