Studies on Phenolic Steroids in Human Subjects. VII. Metabolic Fate of Estriol and Its Glucuronide*

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Abstract
The metabolic fate of C14-estriol and C14-estriol-glucuronide has been studied following their intravenous injection into women, including subjects with biliary fistulas. The radioactivity following C14-estriol injection is excreted more rapidly in the urine than previously observed when C14-estrone or C14-estradiol was administered. The preponderant part of the radioactivity of the former steroid is excreted in the urine within 24 hours, whereas the peak excretion with the latter two steroids occurs 48 to 72 hours subsequent to injection. In all probability, the involvement of estrone and estradiol in an entero-hepatic circulation, owing to the fact that over half of their radioactivity is excreted in the bile, may be the major reason for the differences in the urinary excretory rates. Only 20% of the C14-estriol was excreted in the bile. C14-estriol was conjugated more rapidly than estrone or estradiol, and only a very small amount of the aglycone was metabolized. By reverse isotopic dilution, 16-ketoestradiol and 16-epiestriol could not be shown to be among the metabolites. C14-estriol-glucuronide is rapidly excreted in the urine, and no evidence was obtained that the conjugate had been split to any extent in the body. No biliary excretion of the estriol-glucuronide occurred.