Origin of serum estradiol in postmenopausal women.

In postmenopausal women, the circulating levels of estrone (E1) and estradiol (E2) may be of clinical importance. The origin of E1, but not of E2, has been defined. To examine the source of the latter, the serum concentrations, metabolic clearance rates, conversion ratios, and production rates of testosterone (T), androstenedione (A), E2, and E1 were measured in 20 postmenopausal subjects. For E2, the mean +/- SE CRTE2 was 0.0014 +/- 0.0005; thus, the contribution of circulating T to the circulating E2 pool was minimal (2.5%). The contribution of circulating A to E2 was also insignificant, whereas the CRE1E2 was appreciable (0.065 +/- 0.011), accounting for 21.5% of the E2 pool. For E1, the major contribution was the peripheral conversion of A, accounting for 24.6% of circulating E1. The contribution of peripheral conversion of T (unmeasurable) and E2 (2.9%) to the E1 pool were minimal. These data are consistent with the concept that in postmenopausal women the major contribution of peripheral conversion to the circulating E2 pool is from E1, which in turn is the product of peripheral aromatization of circulating A.