Estrogen is made from cholesterol through a multi-step process that involves several organs, with the ovaries being the primary source during reproductive years. But estrogen production isn’t limited to one location. Fat tissue, the adrenal glands, the brain, bone, and even the testes in men all produce estrogen, each through slightly different mechanisms.
The Four Types of Estrogen
Your body makes four distinct forms of estrogen, each dominant at different life stages. Estradiol (E2) is the most potent and abundant form during the reproductive years. Estrone (E1) is weaker and becomes the primary estrogen after menopause. Estriol (E3) is produced in massive quantities during pregnancy, reaching levels up to 1,000 times higher than in non-pregnant women. Estetrol (E4) is made exclusively by the fetal liver during pregnancy.
From Cholesterol to Estrogen
All estrogen starts as cholesterol. The first step happens inside mitochondria, where an enzyme strips away part of the cholesterol molecule to create pregnenolone, a precursor shared by nearly every steroid hormone in the body. From there, pregnenolone moves through a series of enzymatic conversions, first into androgens like testosterone and androstenedione.
The final, defining step is aromatization. An enzyme called aromatase converts androgens into estrogens, specifically turning testosterone into estradiol and androstenedione into estrone. This conversion is what separates the estrogen pathway from testosterone production. Without aromatase, the process stops at androgens. This is why aromatase activity, and where it occurs in the body, determines how much estrogen you produce and where.
How the Ovaries Produce Estrogen
The ovaries are the dominant estrogen factory during reproductive years, and they rely on a teamwork system between two cell types. This is known as the “two-cell, two-gonadotropin” model, because it requires two kinds of cells and two pituitary hormones working together.
Here’s how it works: the brain’s hypothalamus releases a signaling hormone that tells the pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH acts on theca cells, the outer layer of the ovarian follicle, stimulating them to produce androgens like testosterone and androstenedione. These androgens then pass to the neighboring granulosa cells, where FSH activates aromatase to convert them into estradiol. Neither cell type can make estrogen alone.
Estradiol levels fluctuate dramatically across the menstrual cycle. During the early follicular phase, levels sit around 20 to 350 pg/mL. They climb sharply as ovulation approaches, peaking at 150 to 750 pg/mL during the midcycle surge. In the luteal phase after ovulation, levels range from about 30 to 450 pg/mL before dropping at the end of the cycle.
The Feedback Loop That Controls Production
Estrogen doesn’t just flow freely. The body uses a feedback system between the hypothalamus, pituitary gland, and ovaries to keep levels in check. For most of the menstrual cycle, rising estradiol suppresses the release of FSH and LH from the pituitary. This is negative feedback: more estrogen means less stimulation, which prevents overproduction.
But there’s a critical exception. When estradiol reaches a specific threshold near the end of the follicular phase, it flips from suppressing to stimulating the pituitary. Estradiol causes pituitary cells to build more receptors for the hypothalamic signal, making them increasingly sensitive. It may also prevent the breakdown of that signal within pituitary cells, lowering the threshold needed to trigger LH release. The result is a sudden, dramatic LH surge (levels spike roughly tenfold) that triggers ovulation. Once ovulation occurs and progesterone rises alongside estradiol in the luteal phase, the system reverts to negative feedback until both hormones drop at the end of the cycle, resetting the process.
Estrogen Production During Pregnancy
Pregnancy shifts estrogen production from the ovaries to the placenta, which becomes an estrogen powerhouse. By the end of pregnancy, estradiol and estrone levels are about 100 times higher than in non-pregnant women.
The placenta can’t do this alone, though. It lacks the enzymes to build androgens from scratch, so it relies on precursors supplied by both the mother’s and the fetus’s adrenal glands. The fetal adrenal cortex and fetal liver are particularly important: the fetus provides roughly 90% of the specific precursor needed to make estriol. The placenta then uses aromatase to convert these precursors into estrogens. This collaboration between fetal organs and the placenta is sometimes called the “fetal-placental unit.”
What Changes After Menopause
When the ovaries stop producing significant estrogen at menopause, the adrenal glands become the primary source of the raw materials for estrogen. The adrenals don’t make estrogen directly. Instead, they secrete androgens, particularly androstenedione and a precursor called DHEA, which are then converted into estrogens in peripheral tissues like fat, skin, and bone.
The dominant estrogen shifts from estradiol to estrone, a weaker form. Fat tissue is a major site of this conversion, using aromatase to turn adrenal androgens into estrone. This is one reason body composition influences estrogen levels after menopause: more fat tissue generally means more peripheral aromatization and higher circulating estrone. However, this relationship isn’t straightforwardly beneficial. Excess fat accumulation can impair how estrogen receptors function and alter the expression of the enzymes involved in synthesis.
Local Production in Other Tissues
Beyond the ovaries and fat tissue, several organs produce estrogen locally for their own use. Unlike ovarian estrogen, which enters the bloodstream and circulates throughout the body, estrogen made in these tissues mostly stays put. It acts right where it’s produced, functioning as a local chemical signal rather than a circulating hormone.
Bone cells produce estrogen locally to help regulate bone maintenance. Osteoblasts (the cells that build bone) show high levels of aromatase activity, synthesizing estrogen on-site. The brain is another notable producer. Neurons and a type of supporting brain cell called astrocytes can make estradiol from either circulating testosterone or cholesterol. Brain-produced estradiol is notably stable and isn’t converted to the weaker estrone form the way it is in other tissues. Skin and kidney tissue also contribute to local estrogen synthesis.
These local sources are the primary estrogen supply for people who don’t have active ovaries, including children before puberty and women after menopause.
Estrogen Production in Men
Men produce estrogen too, and it plays important roles in male biology. The testes produce sizable quantities of estradiol, primarily through Leydig cells in the testicular tissue. Aromatase in the testes converts testosterone into estradiol locally, where it helps regulate normal gonadal development and sperm maturation. The brain also produces estrogen in men through the same neuronal pathways as in women. Additional estrogen comes from peripheral aromatization of testosterone in fat and other tissues. The exact proportion contributed by each source remains unclear, but testicular and peripheral production both appear significant.