Oestrogen is a steroid hormone that regulates reproduction, bone strength, heart health, and brain function in both women and men. While most people associate it with female biology, oestrogen influences nearly every organ system in the body. It exists in several forms, fluctuates dramatically across a person’s lifetime, and its decline is responsible for many of the health changes that accompany aging.
How Oestrogen Is Made
The ovaries are the primary source of oestrogen in premenopausal women, producing it in response to signals from the brain. But oestrogen isn’t created from scratch. An enzyme called aromatase converts testosterone and other androgens into oestrogen. This conversion happens in the ovaries, but also in fat tissue, the adrenal glands, bone, and even the brain. In men, aromatase activity in these tissues is the main source of oestrogen, since men have no ovaries but still need the hormone for normal body function.
This conversion process is why body fat influences oestrogen levels. More fat tissue means more aromatase activity, which means more oestrogen production. It also explains why certain medications work by blocking aromatase to reduce oestrogen in conditions like breast cancer.
The Four Types of Oestrogen
Your body produces four distinct forms of oestrogen, each dominant at different life stages.
- Estrone (E1) is the weakest form and becomes the primary oestrogen after menopause, produced mainly in fat tissue.
- Estradiol (E2) is the most potent and most abundant form during the reproductive years. It drives the menstrual cycle, maintains bone density, and protects cardiovascular health.
- Estriol (E3) is produced in large quantities by the placenta during pregnancy. Outside of pregnancy, levels are very low.
- Estetrol (E4) is made exclusively by the fetal liver during pregnancy. It has recently been developed into a contraceptive ingredient because of its weaker activity and potentially fewer side effects compared to synthetic oestrogens.
When doctors measure “oestrogen levels,” they typically measure estradiol, since it is the most biologically active form for most of adult life.
What Oestrogen Does in the Body
Oestrogen’s reach extends far beyond reproduction. It acts on receptors found in bone, blood vessels, the brain, skin, and the urinary tract, among other tissues.
In the reproductive system, oestrogen thickens the uterine lining each month in preparation for a potential pregnancy, regulates the menstrual cycle, and maintains vaginal lubrication and tissue health. During the menstrual cycle, estradiol levels climb steadily during the first half (the follicular phase), peaking above 200 picograms per milliliter just before ovulation. This surge triggers the release of an egg. Levels then fluctuate through the second half of the cycle before dropping if pregnancy doesn’t occur, prompting a period.
In bone, oestrogen slows the breakdown of old bone tissue, helping maintain density and strength. This is why bone loss accelerates sharply after menopause. In the cardiovascular system, oestrogen promotes blood vessel flexibility and helps regulate vascular tone, offering a degree of protection against heart disease during the reproductive years. In the brain, oestrogen supports learning and memory and appears to have neuroprotective effects, with research linking its decline to increased vulnerability to Alzheimer’s disease.
Oestrogen’s Role in Men
Men produce oestrogen too, just in much smaller amounts. Aromatase in fat, bone, and brain tissue converts a portion of testosterone into estradiol, and this small supply turns out to be essential. Oestrogen helps regulate male bone density, and low levels in men can lead to bone loss and osteoporosis, just as in women.
Oestrogen also plays a role in male fertility. Sperm cells carry oestrogen receptors, and locally produced oestrogen within sperm appears to protect sperm DNA from damage during transit. Research on men who genetically lack the ability to produce oestrogen shows that restoring normal estradiol levels, alongside testosterone, improves sexual desire and overall sexual function. Both hormones appear necessary for healthy male sexual behaviour, with estradiol specifically linked to sexual desire and activity.
Symptoms of Low Oestrogen
When oestrogen drops below normal levels, the effects are widespread. Common signs include hot flashes and night sweats, vaginal dryness, irregular or absent periods, dry skin, tender breasts, difficulty concentrating, moodiness, irritability, low sex drive, headaches around the time of a period, fatigue, insomnia, and weight gain concentrated around the belly.
These symptoms can appear at any age if oestrogen production is disrupted, whether from excessive exercise, very low body weight, certain medical treatments, or conditions affecting the ovaries. They also describe the experience of perimenopause and menopause for most women. In men, low oestrogen is associated with increased belly fat, reduced sex drive, and bone loss.
The Menopause Decline
Oestrogen levels don’t drop overnight at menopause. The decline begins years before periods stop, during perimenopause, when production becomes erratic. After menopause, circulating oestrogen gradually falls to negligible levels. The average age of natural menopause is around 51.
This loss of oestrogen has measurable consequences across multiple body systems. Cardiovascular risk increases, as the protective effects on blood vessels diminish. Bone density drops markedly, with an increase in bone turnover and reduced bone formation that raises fracture risk. Sleep disruption, temperature regulation problems, metabolic changes, and an elevated risk of neurological conditions all follow the decline. Bone density loss actually begins several years before menopause starts, accelerating afterward.
For women who experience premature ovarian insufficiency (menopause before age 40), the health risks of prolonged oestrogen deficiency are even greater, which is why hormone replacement is typically recommended until at least the average age of natural menopause.
Oestrogen Therapy
Hormone replacement therapy replenishes oestrogen to relieve menopausal symptoms and, in some cases, prevent long-term complications like osteoporosis. It is approved as a first-line treatment for moderate to severe hot flashes and night sweats and for preventing (though not treating) osteoporosis.
For vaginal dryness and urinary symptoms, local oestrogen applied directly to the affected area is generally preferred over whole-body treatment. Several formulations exist, including pills, patches, gels, and vaginal rings, each delivering oestrogen in slightly different ways depending on whether the goal is systemic relief or targeted symptom management.
Oestrogen also forms the backbone of most hormonal contraceptives. Older formulations use a potent synthetic oestrogen called ethinyl estradiol, which binds oestrogen receptors strongly and has pronounced effects on liver proteins and blood clotting factors. Newer options use natural estradiol or estetrol, which are less potent and may carry fewer side effects. Estetrol-based contraceptives, the most recent addition, require higher doses to suppress ovulation (15 mg compared to fractions of a milligram for the synthetic version) but show promise for better tolerability.