Estrogens (U.S., otherwise oestrogens or œstrogens) are a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone.

Estrogens are used as part of some oral contraceptives, in estrogen replacement therapy of postmenopausal women, and in hormone replacement therapy for transwomen.

Like all steroid hormones, estrogens readily diffuse across the cell membrane; inside the cell, they interact with estrogen receptors. Additionally, estrogens have been shown to activate a G protein-coupled receptor, GPR30.

Types

The three major naturally occurring estrogens in women are estradiol, estriol, and estrone. In the body these are all produced from androgens through actions of enzymes.

• From menarche to menopause the primary estrogen is 17β-estradiol. In postmenopausal women more estrone is present than estradiol.
• Estradiol is produced from testosterone and estrone from androstenedione.
• Estrone is weaker than estradiol.

Premarin, a commonly prescribed estrogenic drug, contains the steroidal estrogens equilin and equilenin, in addition to estrone sulfate. A range of synthetic and natural substances have been identified that also possess estrogenic activity. Synthetic substances of this kind are known as xenoestrogens, plant products with estrogenic activity are called phytoestrogens, and those produced by fungi are known as mycoestrogens. Unlike estrogens produced by mammals, these substances are not necessarily steroids.

Estrogen production

Testosterone is synthesized during steroidogenesis, with cholesterol as the starting molecule.

Estrogen is produced primarily by developing follicles in the ovaries, the corpus luteum, and the placenta. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the production of estrogen in the ovaries. Some estrogens are also produced in smaller amounts by other tissues such as the liver, adrenal glands, and the breasts. These secondary sources of estrogen are especially important in postmenopausal women.

Synthesis of estrogens starts in theca interna cells in the ovary, by the synthesis of androstenedione from cholesterol. Androstenedione is a substance of moderate androgenic activity. This compound crosses the basal membrane into the surrounding granulosa cells, where it is converted to estrone or estradiol, either immediately or through testosterone. The conversion of testosterone to estradiol, and of androstenedione to estrone, is catalyzed by the enzyme aromatase.