Low endogenous estrogen levels--analytical problems and tissue sensitivity.

Measurement of low peripheral concentrations of estradiol-17 beta with radioimmunoassay techniques has frequently produced extremely variable results, due to methodological problems. One such problem is variable non-specifically bound radioactivity (NSB) in methods using precipitation for separation of free and bound radioactivity. This problem can be overcome either by substituting the precipitation technique with dextran coated charcoal separation which drastically reduces NSB or by correcting for individual NSB values in samples and calibrators. These simple modifications of an existing commercial kit make convenient determination of estradiol-17 beta at low concentrations in postmenopausal women possible. Estrone sulfate is the quantitatively most important circulating estrogen in non-pregnant individuals. Studies of the metabolism of tritiated estrone sulfate in vitro evidently show that estrone sulfate is converted into estradiol-17 beta in several estrogen target tissues, including the vaginal mucosa. This conversion is probably important for the estradiol-17 beta supply to target organs in individuals with low circulating levels of estradiol-17 beta such as postmenopausal women.