Origin of Estrogen and Regulation of Its Formation in Postmenopausal Women

Abstract

In premenopausal, ovulatory women, estrogen arises by two mechanisms. It is believed that the biologically more important of these is the synthesis of 17β-estradiol in the developing follicle. This obtains since 17β-estradiol is a more potent estrogen than that produced by the second mechanism, i.e., the extraglandular formation of estrone from circulating andro-stenedione. The rate of extraglandular estrone formation does not vary appreciably during the ovarian cycle (Fig. l-l).¹ On average, about 3 mg of androstenedione are produced each day in young women and 1.5% of plasma androstenedione is aromatized (the process of converting androgen to estrogen) to estrone in extraglandular tissues.^1–4 Thus, approximately 45 μg of estrone arise by this mechanism. At the extremes of the ovarian cycle, this amount of estrone may constitute 50% or more of the total estrogen produced in young women.¹ The rate of secretion of 17β-estradiol by the ovary, however, varies widely during the ovarian cycle (Fig. 1-1). It is estimated that the rate of secretion of 17β-estradiol varies from 20–40 μg/day early and late in the cycle to as much as 600–1000 μg/dayjust prior to the midcycle LH (luteinizing hormone) surge and ovulation.¹

Supported in part by USPHS Grant No. 2-P01-AG00306.