Clinical Significance:
Estradiol is a steroid hormone, which circulates predominantly protein-bound. In addition to estradiol, other natural steroidal estrogens include estrone, estriol and their conjugates. Natural estrogens are hormones secreted principally by the ovarian follicles and also by the adrenals, corpus luteum, and placenta and, in males, by the testes. Exogenous estrogens (natural or synthetic) elicit, to varying degrees, all the pharmacologic responses usually produced by endogenous estrogens.
Estradiol is metabolized to estrone and estriol by various target tissues. Estrogenic hormones are secreted at varying rates during the menstrual cycle throughout the period of ovarian activity. The gonadotropins of the anterior pituitary regulate secretion of the ovarian hormones, estradiol and progesterone. Hypothalamic control of pituitary gonadotropin production is in turn regulated by plasma concentrations of the estrogens and progesterone. During pregnancy, the placenta becomes the main source of estrogens. At the menopause, ovarian secretion of estrogens declines at varying rates.
