Estrogen

Estrogen, or oestrogen, is the primary female sex hormone. It is responsible for the development and regulation of the female reproductive system and secondary sex characteristics.

Estrogens are synthesized in all humans, regardless of sex. The three major naturally occurring forms of estrogen in AFAB individuals are estrone (E1), estradiol (E2), and estriol (E3). The estrane steroid estradiol is the most potent and prevalent of these. Another type of estrogen called estetrol (E4) is produced only during pregnancy. Quantitatively, estrogens circulate at lower levels than androgens in all human sexual variations. While naturally occuring estrogen levels are significantly lower in AMAB individuals compared to AFAB ones, estrogens nevertheless also have important male physiological roles.

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).

In addition to their role as naturally produced hormones, estrogens are used as medications regarding sexual health for Male-to-Female Hormone Replacement Therapy, but also in menopausal hormone therapy and as hormonal birth control.