The ovaries are the primary source of estrogen in females before menopause. They produce the most potent form, estradiol, through a coordinated process involving two distinct cell types and signals from the brain. But the ovaries aren’t the only source. Fat tissue, the adrenal glands, and during pregnancy, the placenta all contribute to estrogen levels at different stages of life.
How the Ovaries Make Estrogen
Estrogen production inside the ovary depends on a teamwork system between two types of cells in the follicle, the small fluid-filled sac that holds a developing egg. This process is sometimes called the “two-cell, two-gonadotropin system” because it requires both cell types and two hormones from the pituitary gland to work.
Here’s how it works. The outer layer of the follicle contains theca cells, which respond to luteinizing hormone (LH) by producing androgens, including testosterone and androstenedione. These androgens then pass to the inner layer of granulosa cells. The granulosa cells, stimulated by follicle-stimulating hormone (FSH), contain an enzyme called aromatase that converts those androgens into estradiol. Neither cell type can make estrogen on its own. The theca cells make the raw material, and the granulosa cells do the final conversion.
This is why estrogen levels rise and fall so dramatically across the menstrual cycle. During the follicular phase (the first half of the cycle), estradiol typically ranges from 20 to 350 pg/mL. As a dominant follicle matures and grows, estradiol climbs to a midcycle peak of 150 to 750 pg/mL, which triggers ovulation. After ovulation, during the luteal phase, levels settle to roughly 30 to 450 pg/mL before dropping if pregnancy doesn’t occur.
The Brain Controls the Whole Process
The ovaries don’t decide on their own when to ramp up estrogen. That signal starts in the hypothalamus, a small region at the base of the brain that releases gonadotropin-releasing hormone (GnRH) in pulses. These pulses travel to the pituitary gland, which responds by secreting FSH and LH into the bloodstream. The speed of those pulses matters: slower pulses favor FSH production, while faster pulses favor LH.
This system runs on a feedback loop. As the dominant follicle produces more estradiol, that rising estrogen actually suppresses FSH release, preventing other follicles from maturing. But when estradiol sustains a level of roughly 200 to 300 pg/mL for about 48 hours, the feedback flips. Instead of suppressing the brain’s signals, it triggers a surge of GnRH, which causes the LH spike responsible for ovulation. After ovulation, FSH stays low through the rest of the cycle, keeping new follicles from developing until the next cycle begins.
Three Types of Estrogen, Three Different Roles
The body produces three main forms of estrogen, each dominant at a different life stage:
- Estradiol (E2) is the strongest and most abundant estrogen during the reproductive years. The ovaries are its main source, though small amounts are also made in other tissues by converting testosterone.
- Estrone (E1) is a weaker estrogen. The ovaries produce it during reproductive years, but it becomes the dominant form after menopause, when fat tissue takes over production by converting adrenal hormones.
- Estriol (E3) is the weakest of the three and is mainly produced by the placenta during pregnancy. It’s barely detectable outside of pregnancy.
Fat Tissue as an Estrogen Factory
Fat cells contain aromatase, the same enzyme found in the ovary’s granulosa cells. This means adipose tissue can convert circulating androgens into estrogen independently of the ovaries. Before menopause, this contribution is relatively minor compared to the ovaries. After menopause, it becomes the body’s primary estrogen source.
The amount of estrogen fat tissue produces scales with body fat. More fat tissue means more aromatase activity and higher estrogen conversion. Estrogen concentrations inside fat tissue can be more than double the level found in the bloodstream, which means local effects within the tissue itself can be significant even when blood levels appear modest. This local production plays a role in metabolism, influencing how the body handles fats and blood sugar.
The Adrenal Glands Provide Raw Materials
The adrenal glands, which sit on top of each kidney, don’t produce much estrogen directly. What they do produce are androgen precursors, particularly DHEA and its sulfated form DHEAS, in a region called the zona reticularis. These precursors circulate through the bloodstream and get converted into estrogen by aromatase in fat tissue, skin, and other organs. Think of the adrenals as suppliers of building blocks rather than finished product. This supply chain becomes especially important after menopause and during pregnancy.
The Placenta During Pregnancy
Pregnancy creates an entirely new estrogen source. After the first trimester, the placenta becomes the major producer of circulating estradiol, estrone, and estriol. But the placenta has an interesting limitation: it lacks the enzymes needed to build estrogen from scratch. It can’t convert progesterone-related compounds into androgens on its own.
Instead, the placenta relies on androgen precursors supplied by both the mother’s adrenal glands and the fetal adrenal glands. By the end of pregnancy, roughly equal amounts of estrogen come from maternal and fetal DHEAS. The fetal liver adds another step, providing a chemical modification that the placenta needs to produce estriol specifically. This is why estriol is sometimes used as a marker of fetal health: it first appears in maternal blood around 9 weeks of gestation, closely tracking the development of the fetal adrenal glands, and its continued production depends on a living fetus.
What Changes After Menopause
When the ovaries stop releasing eggs and their follicles are depleted, the two-cell production system shuts down. Estradiol levels drop to 20 pg/mL or less. The body doesn’t stop making estrogen entirely, but it shifts to a much weaker form. Estrone, synthesized in fat tissue from adrenal DHEA, becomes the dominant estrogen. This is a fraction of what the ovaries previously produced, and it’s a less potent hormone.
This shift explains many of the changes associated with menopause. The drop in estradiol affects bone density, cardiovascular health, and metabolism. In the liver, where estrogen helps regulate over 1,000 genes related to fat processing, reduced estrogen signaling promotes fat accumulation and can contribute to insulin resistance. The liver doesn’t produce estrogen, but it is one of the most responsive organs to estrogen’s effects, and the loss of that signaling after menopause has wide-reaching metabolic consequences.
The Full Picture
Estrogen production in females is not a single-organ operation. Before menopause, the ovaries dominate, producing estradiol through a tightly regulated cycle controlled by the brain. Fat tissue and the adrenal glands play supporting roles. During pregnancy, the placenta takes over as the primary source, working in partnership with both the mother and the fetus. After menopause, fat tissue becomes the last reliable producer, converting adrenal androgens into the weaker estrone. Each transition reflects how the body adapts its estrogen supply to match reproductive demands at different life stages.