estrogen estrogen estrogen or oestrogen (see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. there are three major endogenous estrogens that have estrogenic hormonal activity: estrone (e1), estradiol (e2), and estriol (e3). estradiol, an estrane, is the most potent and prevalent. another estrogen called estetrol (e4) is produced only during pregnancy. this article is about estrogens as hormones. for their use as medications, see estrogen (medication). estrogendrug classestradiol, the major estrogen sex hormone in humans and a widely used medication.class identifiersusecontraception, menopause, hypogonadism, transgender women, prostate cancer, breast cancer, othersatc codeg03cbiological targetestrogen receptors (erα, erβ, mers (e.g., gper, others))