An estrogen is any of a group of female sex hormones, produced principally by the ovaries but also by the adrenal glands and (in men) the testes. While estrogens (also spelled oestrogens) are present in both men and women, they are usually present at significantly higher levels in women of reproductive age. They promote the development of female secondary sex characteristics, such as breast enlargement and development, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle (see menstruation). Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) regulate the production of estrogen in ovulating women.

 

Among the principal estrogens are estrone (shown in the diagram), estriol (with hydroxyl groups at C-16 and C-17), and 17(β)-estradiol (unsubstituted at C-16).