Estrogen

Estrogen is a group of steroid hormones that regulate the development and function of the female reproductive system and influence secondary sexual characteristics. In humans, the principal estrogens are estradiol (E2), estrone (E1), and estriol (E3). Estradiol is the most potent and prevalent during reproductive years; estrone becomes more prominent after menopause; estriol is abundant during pregnancy. Estrogens are mainly produced by the ovaries, with contributions from the placenta during pregnancy, adipose tissue, and to a lesser extent the adrenal glands and testes. The enzyme aromatase converts androgens into estrogens in various tissues.

Estrogen actions are mediated by estrogen receptors, primarily ERα and ERβ, which function as transcription factors and also initiate rapid non-genomic signaling. Estrogen is synthesized under the control of the hypothalamic-pituitary-gonadal axis: follicle-stimulating hormone stimulates ovarian estrogen production, while luteinizing hormone supports ovulation and corpus luteum activity. Estrogen affects a wide range of tissues, including the reproductive tract, breast tissue, bone, liver, cardiovascular system, and brain, contributing to regulation of the menstrual cycle, fertility, bone density, lipid metabolism, and aspects of mood and cognition.

Physiologically, estrogens drive puberty, regulate the menstrual cycle, support pregnancy, and help maintain bone density. They influence cardiovascular risk factors and metabolic processes. Clinically, estrogen levels decline at menopause, leading to vasomotor symptoms and reduced bone density. Estrogen therapy is used for some menopausal symptoms and preventive indications but carries risks that vary with age, duration, and use pattern. Abnormally high or low estrogen levels can reflect endocrine disorders or estrogen-secreting tumors and may alter cancer risk depending on tissue context.