Testosterone and estrogen are the two primary sex hormones in the human body. Both are present in everyone, regardless of sex, and they do far more than drive reproduction. They shape your bones, muscles, brain chemistry, metabolism, and mood throughout your entire life.
What Testosterone Does
Testosterone is a steroid hormone produced mainly in the testes in men and in smaller amounts in the ovaries and adrenal glands in women. It’s often called the “male hormone,” but that label undersells its importance for everyone. In both sexes, testosterone regulates sex drive by acting on brain regions involved in sexual motivation and arousal. It promotes protein synthesis in muscle cells, driving muscle growth and repair. And it directly stimulates the bone-building cells in your skeleton, helping bones reach their peak mass during puberty and maintaining density throughout adulthood.
Beyond those headline roles, testosterone influences energy levels, fat distribution, red blood cell production, and even mental focus. Men with clinically low testosterone often notice reduced energy, depressed mood, and difficulty concentrating before they notice any physical changes. Over time, low levels can lead to loss of muscle mass, increased body fat, weaker bones, reduced body hair, and erectile dysfunction.
What Estrogen Does
Estrogen is actually a group of three related hormones. Estradiol is the most potent form and dominates during the reproductive years. Estrone takes over after menopause as the body’s primary estrogen. Estriol rises mainly during pregnancy. When people refer to “estrogen” in everyday conversation, they usually mean estradiol.
Estrogen’s most visible job is regulating the menstrual cycle and supporting pregnancy, but it also protects bones, influences cholesterol levels, and keeps vaginal and bladder tissues healthy. In fact, estrogen is more powerful than testosterone at slowing bone breakdown in both men and women. Men who lack the enzyme that converts testosterone into estradiol show dramatically low bone density, which tells us that even in the male body, estrogen is essential for skeletal health.
How the Body Makes Both Hormones
Testosterone and estrogen aren’t produced in isolation. They share a biochemical pathway, and your body actually builds estrogen from testosterone. An enzyme called aromatase strips and reshapes part of the testosterone molecule, converting it into estradiol. This happens in the ovaries, testes, fat tissue, brain, and other organs. In men, both the cells that produce testosterone in the testes and the developing sperm cells themselves contain aromatase and actively manufacture estradiol. So men are always producing estrogen, and women are always producing testosterone.
This conversion process is one reason body fat matters for hormone balance. Fat tissue is rich in aromatase, so carrying more body fat tends to shift the ratio toward higher estrogen and lower testosterone, especially in men.
Normal Levels and How They Shift
Testosterone levels in men are roughly 10 to 20 times higher than in women. In women, estradiol levels fluctuate dramatically across the menstrual cycle: they can range from around 20 pg/mL in the early follicular phase to 150 to 750 pg/mL at mid-cycle ovulation, then drop back down. After menopause, estradiol typically falls to 20 pg/mL or lower, and estrone becomes the dominant form.
For both hormones, levels decline gradually with age. Testosterone in men drops slowly starting around age 30. Estrogen in women declines more abruptly during the transition into menopause, usually between the mid-40s and early 50s. These shifts drive many of the symptoms people associate with aging.
How They Affect Your Brain and Mood
Both hormones interact with the brain’s chemical messaging systems in ways that directly shape mood, motivation, and cognition. Estrogen has a well-documented facilitating effect on dopamine, the neurotransmitter involved in reward, motivation, and working memory. It also boosts serotonin, the neurotransmitter most associated with stable mood, by increasing its production and slowing its removal from the gaps between nerve cells. This means serotonin stays active longer when estrogen levels are healthy.
These interactions help explain why hormonal transition periods carry emotional weight. During perimenopause, when estrogen levels swing unpredictably, many women experience mood swings, irritability, and an increased risk of depression. Similarly, men with low testosterone commonly report low mood, reduced motivation, and mental fog as early symptoms, sometimes before any physical changes become obvious.
What Happens When Levels Are Too Low
Low testosterone in men produces a recognizable pattern. Early signs tend to be reduced sex drive, lower energy, and depressed mood. Over time, more visible changes appear: loss of muscle mass, increased belly fat, thinning body and facial hair, breast tissue growth, and weakening bones. Men with low testosterone face roughly triple the rate of hip bone loss compared to men with normal levels (22.5% versus 8.6% in one large study of older men).
Low estrogen in women, most commonly during perimenopause and menopause, brings its own set of changes. Hot flashes and trouble sleeping are the most recognizable. Vaginal dryness and discomfort during sex result from thinning tissues. Bone loss accelerates, and cholesterol levels can shift in unfavorable directions. These aren’t subtle. About 75% of women going through menopause experience hot flashes, and bone density loss can begin years before the final menstrual period.
What Happens When Levels Are Too High
Excess estrogen in men is closely linked with obesity. Carrying more fat tissue increases aromatase activity, which converts more testosterone into estradiol. This creates a feedback loop: higher estrogen suppresses the brain signals that tell the testes to produce testosterone, leading to even lower testosterone levels, which promotes further fat gain. The result is a self-reinforcing cycle of rising estrogen, falling testosterone, and increasing body fat.
In women, excess testosterone can cause acne, abnormal hair growth on the face and body, thinning hair on the scalp, and irregular periods. Polycystic ovary syndrome (PCOS) is the most common condition associated with elevated testosterone in women and affects an estimated 6 to 12% of women of reproductive age.
How Bone Health Depends on Both
Bone health is one area where the partnership between testosterone and estrogen is especially clear. Testosterone directly stimulates the cells that build new bone. When testosterone drops, it triggers increased activity from the cells that break bone down, accelerating bone loss. But estrogen converted from testosterone plays an equally critical role. In men, estrogen is actually more effective than testosterone at slowing bone breakdown. Men who are unable to convert testosterone to estrogen, due to a genetic lack of aromatase, develop severely weakened bones despite having normal testosterone levels.
Testosterone replacement therapy in men with clinically low levels has been shown to improve bone density at the lumbar spine by about 3.7% compared to placebo, based on a meta-analysis of 29 randomized controlled trials involving over 1,000 participants. That improvement comes from both the direct effects of testosterone and the estrogen produced from it through aromatase conversion.