Detailed definition
Estrogens are a class of steroid hormones derived from cholesterol and synthesized via the aromatization of androgens (testosterone and androstenedione) by the enzyme aromatase (CYP19A1). The three principal human estrogens are estradiol (E2), estrone (E1), and estriol (E3); a fourth, estetrol (E4), is produced only by the fetal liver. All four bind estrogen receptors α and β and the membrane G protein-coupled estrogen receptor (GPER) with differing affinities and tissue selectivity. Estrogens regulate the menstrual cycle, support pregnancy, build and maintain the female secondary sex characteristics, modulate bone remodeling, influence lipid profiles and vascular endothelial function, and contribute to neuroprotection and mood regulation. In men, smaller amounts of estrogen — produced largely by aromatization of testosterone — are essential for bone health and libido. Estrogen levels in women rise sharply at puberty, fluctuate cyclically through the reproductive years, and decline rapidly during the menopause transition.
Why it matters in menopause
The menopause transition is fundamentally an estrogen-deprivation state. Most of the symptoms women experience — hot flashes, night sweats, sleep disruption, mood lability, vaginal and bladder atrophy, accelerated bone loss, and shifts in lipid profile — trace back to losing this single hormone. Hormone replacement therapy works because it restores estrogen, particularly estradiol, to a physiologic range. Understanding that "estrogen" is a family of related molecules (not a single drug) helps explain why estradiol patches, vaginal estrogen, and conjugated equine estrogens can all be called "estrogen therapy" while behaving very differently in the body.