Estrogen is a hormone that produces female physical traits and helps regulate a woman's menstrual cycle. In women, estrogen is produced in varying amounts throughout the menstrual cycle, mainly by the ovaries.

When a woman's estrogen levels reach a low enough point, she stops having monthly menstrual periods (menopause). This can happen either as part of natural aging or when a woman's ovaries have been removed or have been damaged by chemotherapy or radiation. After menopause, a woman's adrenal gland and the androgen from fat cells produce low levels of estrogen.

Men have low levels of estrogen in their bodies. Overweight and obese men and women have higher estrogen levels than those with lower body fat.