estrogen
Estrogen is the family of steroid hormones that, more than any other, shapes the female body and reproductive system. Think of it as the lead architect of female development: it directs breast growth, the maturing of the uterus and vagina, the female pattern of fat distribution, and the readiness of the reproductive tract for each cycle. It is not a single molecule but a small group of closely related steroids.
The three main human estrogens are estradiol (the most potent, dominant during the reproductive years), estrone (weaker, the main estrogen after menopause), and estriol (a weak estrogen produced in large amounts during pregnancy). All are made from cholesterol through a chain of enzymatic steps, with the final and decisive step being the conversion of androgens into estrogens by the enzyme aromatase. In women this happens mainly in the ovarian follicle, but fat tissue, bone, brain, and the placenta also make estrogen.
Like other steroid hormones, estrogen is fat-soluble and slips through the cell membrane to bind intracellular estrogen receptors that act as gene switches, turning specific genes on or off. Beyond reproduction, estrogen protects bone density, influences blood lipids and vessel walls, and acts on the brain. A common misconception is that estrogen is exclusively female: men also make and need small amounts, largely from aromatization of testosterone.
In a teenage girl entering puberty, rising estrogen from awakening ovaries drives breast budding, widening of the hips, and the first menstrual periods, while also accelerating then closing the growth plates in the bones.
Estrogen drives female puberty and, paradoxically, ends the growth spurt by fusing the epiphyseal plates.
Estrogen exerts both negative and positive feedback on the hypothalamus and pituitary. For most of the cycle high estrogen suppresses LH and FSH, but a sustained high estrogen level just before ovulation flips to positive feedback and triggers the LH surge — one of the few clear examples of positive feedback in human physiology.