4-OH-E1, or 4-hydroxyestrone, is another natural metabolite of estrogen in the human body. It is produced through the metabolism of estrone, one of the primary estrogen hormones in both males and females.

Estrogens, including 4-OH-E1, are vital for various physiological functions, including the development of secondary sexual characteristics, regulation of the menstrual cycle, and maintenance of bone density. These hormones also have an impact on the growth and function of reproductive tissues.

The "4-OH" in 4-OH-E1 indicates the presence of a hydroxyl group (OH) on the fourth carbon of the estrogen molecule. Different forms of estrogen metabolites, such as 4-OH-E1, can have distinct effects on the body and may be associated with different health consequences. In medical research and clinical practice, these metabolites are used as markers to study estrogen metabolism and its potential implications for health.

It's worth noting that the balance and levels of various estrogen metabolites can be important in understanding and managing conditions related to estrogen, such as breast cancer and other hormone-related disorders