Estrogen and testosterone are sex hormones that regulate far more than reproduction. Both hormones exist in every human body regardless of sex, influencing bone strength, muscle mass, fat distribution, mood, and heart health. The difference between males and females isn’t which hormones they have, but how much of each they produce.
Where These Hormones Come From
Testosterone is produced primarily in the testes in males and in the ovaries and adrenal glands in females. Women make roughly 5 to 10% of the testosterone men do, but it still plays a meaningful role in their health. Estrogen follows a similar pattern in reverse: the ovaries are the main source in premenopausal women, while men produce smaller amounts in the testes and adrenal glands.
There’s also a surprising link between the two hormones. An enzyme called aromatase, found in fat tissue, blood vessels, and other organs, converts testosterone directly into estrogen. This conversion happens locally in tissues throughout the body, which means your cells can fine-tune their own estrogen supply from circulating testosterone. In men, this process is one of the primary ways estrogen is produced at all.
Estrogen Is Actually Three Hormones
Estrogen isn’t a single molecule. It’s a group of three related hormones, each dominant at a different stage of life. Estradiol is the most potent form and the primary estrogen during the reproductive years. Estriol rises sharply during pregnancy. After menopause, the body shifts to producing mostly estrone, a weaker form. This transition from estradiol to estrone is one reason menopause brings such noticeable physical changes.
Beyond reproduction, estrogen helps maintain bone density, supports blood vessel flexibility, and plays a role in brain function. It also directs where the body stores fat, favoring the hips and thighs over the abdomen. Research on transgender women receiving estrogen therapy illustrates this clearly: after 12 months, subcutaneous fat at the hip and thigh increased by about 84% from baseline, with smaller gains in abdominal fat. The hormone essentially reshapes fat distribution toward a pattern associated with lower cardiovascular risk.
What Testosterone Does Beyond Reproduction
Testosterone is best known for building muscle and deepening the voice during puberty, but its roles extend well beyond that. It drives muscle protein synthesis, strengthens bones, supports red blood cell production, and helps maintain mood stability. Men with low testosterone often experience irritability, poor concentration, and depression. Women with low levels report similar symptoms, along with fatigue, reduced muscle tone, thinning hair, and low sex drive.
Testosterone also has a strong influence on body composition. Studies using MRI scans on individuals receiving testosterone therapy show marked increases in thigh muscle area and reductions in subcutaneous fat across the body. Interestingly, in men, low testosterone is associated with abdominal obesity, and supplementing it tends to reduce belly fat. In women, however, higher-than-typical testosterone levels are linked to a more abdominal, “male-pattern” fat distribution.
One of testosterone’s lesser-known benefits involves the cardiovascular system. Research published in the Proceedings of the National Academy of Sciences found that testosterone protects against early plaque buildup in arteries, but not through its own direct action. The protection comes from its conversion to estrogen by aromatase in blood vessel walls. When researchers blocked that conversion with an aromatase inhibitor, the protective effect disappeared. This means some of testosterone’s heart benefits are really estrogen benefits in disguise.
Free vs. Bound Testosterone
Not all the testosterone circulating in your blood is available for your body to use. Most of it travels bound to proteins, particularly one called sex hormone binding globulin (SHBG), which is made in the liver. While bound to SHBG, testosterone is essentially inactive. Only the small “free” fraction, plus a portion loosely bound to another protein called albumin, can enter cells and do its work.
This is why a standard blood test measuring total testosterone doesn’t always tell the full story. Two people with identical total testosterone levels can feel very different if one has much higher SHBG, leaving less hormone in its active, free form. SHBG binds estrogen as well, so the same principle applies to both hormones.
What Happens When Levels Drop
Estrogen and testosterone both decline with age, but the timelines differ. Women experience a relatively abrupt drop in estrogen during menopause, typically in their late 40s or early 50s. Estradiol levels, which can range widely during reproductive years (roughly 20 to over 100 pg/mL depending on the point in the menstrual cycle), fall to around 15 to 20 pg/mL after menopause. This sharp decline drives hot flashes, bone loss, vaginal dryness, and sleep disruption.
Testosterone in men declines more gradually, roughly 1 to 2% per year starting around age 30. There’s no single “male menopause” event, but the cumulative loss over decades can produce low energy, reduced muscle mass, increased body fat, depressed mood, and lower sex drive. In women, testosterone also decreases with age, and the effects can include fatigue, loss of muscle tone, brittle skin, difficulty sleeping, and trouble with concentration or mood.
How the Two Hormones Work Together
Estrogen and testosterone aren’t opposites. They’re collaborators. Both contribute to bone strength: testosterone stimulates bone growth directly, while estrogen slows bone breakdown. Losing either one accelerates osteoporosis. Both influence mood and cognitive function, and both play roles in maintaining sex drive in all genders.
The aromatase conversion pathway means the two hormones are biochemically linked. Your body uses testosterone as raw material to produce estrogen locally in tissues that need it, from brain cells to blood vessel walls. This is why conditions that dramatically lower testosterone in men can also produce symptoms of estrogen deficiency, and why blocking aromatase (as some bodybuilders do) can lead to joint pain, mood changes, and bone thinning despite high testosterone levels.
The balance between the two matters as much as the absolute level of either one. Too much estrogen relative to testosterone in men can cause breast tissue growth and weight gain. Too much testosterone relative to estrogen in women is associated with acne, excess body hair, and irregular periods. Hormonal health isn’t about maximizing one or minimizing the other. It’s about maintaining the ratio your body needs.