4-OH-E2, or 4-hydroxyestradiol, is a naturally occurring metabolite of estradiol, which is one of the primary and most potent forms of estrogen produced in the ovaries in females and in smaller amounts in the testes in males.

Estrogens, including 4-OH-E2, have essential roles in various physiological processes, such as the development of secondary sexual characteristics, regulation of the menstrual cycle, and maintenance of bone density. They also influence the growth and function of reproductive tissues.

The "4-OH" in 4-OH-E2 indicates the presence of a hydroxyl group (OH) on the fourth carbon of the estrogen molecule. Different forms of estrogen metabolites, like 4-OH-E2, can have diverse effects on the body and may be associated with different health outcomes. These metabolites can be used as markers in medical research and clinical practice to study estrogen metabolism and its potential implications for health.