Estrogen is the primary female sex hormone. It is responsible for the development and regulation of the female reproductive system and secondary sex characteristics. Estrogen may also refer to any substance, natural or synthetic, that mimics the effects of the natural hormone. The estrane steroid estradiol is the most potent and prevalent endogenous estrogen, although several metabolites of estradiol also have estrogenic hormonal activity.
Estrogens are used as medications as part of some oral contraceptives, in hormone replacement therapy for postmenopausal, hypogonadal, and transgender women, and in the treatment of certain hormone-sensitive cancers like prostate cancer and breast cancer. They are one of 3 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 estrogenic sex hormones have an ancient evolutionary history. The 3 major naturally occurring forms of estrogen in women are estrone (E1), estradiol (E2), and estriol (E3). Another type of estrogen called estetrol (E4) 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 (ERs) which in turn modulate the expression of many genes. Additionally, estrogens bind to and activate rapid-signaling membrane estrogen receptors (mERs), such as GPER (GPR30).