estrogen, any one of a group of hormones synthesized by the reproductive organs and adrenal glands in females and, in lesser quantities, in males. The estrogens cause the thickening of the lining of the uterus and vagina in the early phase of the ovulatory, or menstrual, cycle (see menstruation); in lower animals cyclical estrogen secretion also induces estrus, or "heat." The estrogens are also responsible for female secondary sex characteristics such as, in humans, pubic hair and breasts, and they affect other tissues including the genital organs, skin, hair, blood vessels, bone, and pelvic muscles.

The major estrogen secreted by the ovary is 17β-estradiol; this is converted to estrone in the blood. Estriol is the principal estrogen formed by the placenta during pregnancy. These three compounds, 17β-estradiol, estrone, and estriol, account for most of the estrogenic activity in humans.

The ability of estrogens to suppress secretion of follicle-stimulating hormone (FSH) by the pituitary gland and thereby inhibit ovulation makes estrogen and estrogenlike compounds major components in oral contraceptives. Estrogen replacement therapy (ERT) uses synthetic estrogen (e.g., Premarin), typically given with progestins (e.g., Provera) to treat the physical changes of menopause, including hot flashes and vaginal dryness. ERT also retards the development of osteoporosis in postmenopausal women but increases the risk of breast cancer, heart attack (see infarction), stroke, and, when not given with progestins, uterine cancer. Estrogens are also used to treat prostate cancer.

Any of a class of hormones that primarily influence the female reproductive system's development, maturation, and function. The three major estrogens—estradiol, estrone, and estriol—are produced mainly by the ovaries and placenta; the adrenal glands and the testes secrete smaller amounts. Estrogens affect the ovaries, vagina, fallopian tubes, uterus, and mammary glands and play crucial roles in puberty, menstruation, pregnancy, and parturition (labour). They also influence the structural differences between female and male bodies. In experimental animals, loss of estrogens diminishes mating desires and other behavioral patterns.

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