Estradiol is the most potent natural estrogen in humans. It regulates reproductive function in females, and, with progesterone, maintains pregnancy. Most estradiol is secreted by the ovaries (non-pregnant women), although the testes (in men) and adrenal cortex (in men and women) secrete small amounts. During pregnancy, the placenta produces most of the circulating estradiol Estradiol and estrone interconvert in vivo. In normal non-pregnant women, estradiol synthesized by the ovary is the predominant source of both estrone and estriol.
Virtually all circulating estradiol is protein-bound. Reported association constants for estradiol with sex hormone binding globulin and serum albumin are, respectively, 6.8 x 108 and 6 x 104. One consequence of this binding is that the conditions of any assay for serum estradiol must release this steroid quantitatively from its binding partners. The amount and proportion of protein-bound and free estradiol vary by gender, and with pregnancy and menstrual phase in women.
Normal estradiol levels are lowest at menses and into the early follicular phase (25-75 pg/mL) and then rise in the late follicular phase to a peak of 200-600 pg/mL just before the LH surge, which is normally followed
immediately by ovulation. As LH peaks, estradiol begins to decrease before rising again during the luteal phase (100-300 pg/mL). If conception does not take place, estradiol falls further to its lowest levels, and menses
begins shortly thereafter. If conception occurs, estradiol levels continue to rise, reaching levels of 1,000-5,000 pg/mL during the first trimester, 5,000-15,000 pg/mL during second trimester, and 10,000-40,000 pg/mL during third trimester.At menopause, estradiol levels remain low.
Because the ovaries produce most estradiol in normal women, estimation of this hormone is sometimes a gauge of ovarian function.In addition, monitoring estradiol levels is important in evaluating amenorrhea, precocious puberty, the onset of menopause, and infertility in men and women. Monitoring estradiol levels is essential during in vitro fertilization, because the timing of recovery of oocytes depends on follicular development, which in turn depends on the estradiol level.
Manufacturer’s Reagent package insert Architect Estradiol, May 2019, Abbott Ireland Diagnostic Division, Lisnamuck, Longford Co.,Longford,Ireland