Estrogens are a group of female hormones essential for the reproductive process and for the development of the uterus, breasts, and other physical changes associated with puberty. Estrogens have an effect on about 300 different tissues throughout a woman's body -- not only those involved in the reproductive process, such as the uterus, breasts, and external genitalia -- but also tissues in the central nervous system (including the brain), the bones, the liver, and the urinary tract. Estrogens determine the characteristic female distribution of body fat on the hips and thighs, which develops during adolescence.

As the number of follicles (small sacs inside the ovary that contain a developing egg) decline in the ovary, hormone production decreases and eventually stops. The symptoms and medical consequences of menopause are related to the reduction in circulating estrogen levels throughout the body. In addition to estrogen, the ovaries also produce other hormones including progesterone and androgens. The production of progesterone is related to ovulation and can fluctuate during the perimenopausal years, accounting for some of the bleeding changes that occur at this time. Androgens, typically thought of as male hormones (because they are produced by the testes and circulate in higher levels in the male), are produced by the ovary and the adrenal gland (gland on top of the kidneys that produces hormones) in the female. Androgen production, which includes testosterone, declines in women as a consequence of aging and menopause. The effects of decreased androgens in women have not been defined well and may contribute to menopausal symptoms like fatigue, loss of sexual desire, decrease in muscle mass and bone loss. Circulating levels of androgens do not decrease as much as estrogen and can convert to estrogen in different tissues of the body, particularly fat. Circulating levels of estrogen can, therefore, vary with body fat composition even though ovarian production of estrogen stops with menopause. This partially explains the variability of estrogen-mediated symptoms in women and the association of estrogen-dependent tumors with obesity.