Estrogen or oestrogen is the primary female sex hormone as well as a medication. It is responsible for the development and regulation of the reproductive system. Estrogen may also refer to any substance, natural or synthetic, the estrane steroid estradiol is the most potent and prevalent endogenous estrogen, although several metabolites of estradiol also have estrogenic hormonal activity. They are one of three types of sex hormones, the others being androgens/anabolic steroids like testosterone and progestogens like progesterone, estrogens are synthesized in all vertebrates as well as some insects. Their presence in both vertebrates and insects suggests that sex hormones have an ancient evolutionary history. The three major naturally occurring forms of estrogen in women are estrone, estradiol, and estriol, another type of estrogen called estetrol is produced only during pregnancy. Quantitatively, estrogens circulate at lower levels than androgens in both men and women, while estrogen levels are significantly lower in males compared to females, estrogens nevertheless also have important physiological roles in males. Like all steroid hormones, estrogens readily diffuse across the cell membrane, once inside the cell, they bind to and activate estrogen receptors which in turn modulate the expression of many genes. Additionally, estrogens bind to and activate rapid-signaling membrane estrogen receptors, the three major naturally occurring estrogens in women are estrone, estradiol, and estriol. Estradiol is the predominant estrogen during reproductive years both in terms of serum levels as well as in terms of estrogenic activity. During menopause, estrone is the predominant circulating estrogen and during pregnancy estriol is the predominant circulating estrogen in terms of serum levels. Though estriol is the most plentiful of the three estrogens it is also the weakest, whereas estradiol is the strongest with a potency of approximately 80 times that of estriol. Thus, estradiol is the most important estrogen in non-pregnant females who are between the menarche and menopause stages of life, however, during pregnancy this role shifts to estriol, and in postmenopausal women estrone becomes the primary form of estrogen in the body. Another type of estrogen called estetrol is produced only during pregnancy, all of the different forms of estrogen are synthesized from androgens, specifically testosterone and androstenedione, by the enzyme aromatase. Estradiol, estrone, and estriol have all been approved as drugs and are used medically. Estetrol is currently under development for medical indications, but has not yet approved in any country. A variety of synthetic estrogen esters, such as estradiol valerate, estradiol cypionate, estradiol acetate, estradiol undecylate, polyestradiol phosphate, the aforementioned compounds behave as prodrugs to estradiol, and are longer-lasting in comparison. Esters of estrone and estriol also exist and are employed in clinical medicine, ethinylestradiol is a more potent synthetic analogue of estradiol that is used widely in hormonal contraceptives
Image: Estradiol during menstrual cycle
Estradiol, the major estrogen sex hormone in humans and a widely used medication.