What testosterone does in your body

Testosterone is a hormone produced mainly in the testes (in people assigned male at birth) and in smaller amounts in the ovaries and adrenal glands (in people assigned female at birth). It circulates through the bloodstream and binds to receptors on cells throughout the body, triggering changes in muscle, bone, fat distribution, mood, and sexual function.

The hormone's effects depend on age, sex, and the concentration in your blood. During puberty in people assigned male at birth, testosterone drives the development of male sexual characteristics — deeper voice, facial and body hair, penis and testicle growth, and increased muscle mass. In people assigned female at birth, testosterone contributes to bone strength, muscle development, and sexual desire, though in much lower concentrations than in males.

After early adulthood, testosterone levels naturally decline with age — roughly 1% per year in people assigned male at birth after age 30. This decline is gradual and normal, though the rate varies widely between individuals.

Key Takeaways

  • Testosterone regulates muscle growth, bone density, fat distribution, mood, and sexual function through binding to receptors on cells throughout the body.
  • Research in people with clinically low testosterone shows that hormone replacement can improve muscle mass, bone density, and sexual function, though effects vary by individual.
  • Animal studies and some human trials suggest testosterone may support cognitive function and mood, but evidence in healthy people remains limited and mixed.
  • Testosterone therapy carries documented risks including increased red blood cell production, potential effects on the heart, and suppression of natural hormone production, which is why medical supervision is essential.
  • Most research on performance-enhancing doses comes from animal studies or observational data from athletes, not controlled human trials, so long-term safety in healthy people is not well established.

Muscle growth and strength

Testosterone promotes muscle protein synthesis — the process by which your body builds new muscle tissue. It does this by activating androgen receptors on muscle cells, which then signal the cell to increase protein production and reduce protein breakdown. This is why testosterone is central to muscle development during puberty and why people assigned male at birth typically have more muscle mass than people assigned female at birth.

In people with clinically low testosterone (hypogonadism), testosterone replacement therapy has been shown in multiple randomized controlled trials to increase lean muscle mass and strength. A 2018 review in JAMA of 27 trials found that testosterone therapy increased muscle mass by an average of 2 to 3 kilograms over 3 years, though the effect was smaller in older men and varied by dose and duration of treatment.

In healthy people with normal testosterone levels, the evidence is less clear. Some animal studies suggest that supraphysiological doses (doses higher than the body naturally produces) increase muscle growth beyond what training alone achieves, but human trials in healthy athletes are rare and often small. Most data on performance-enhancing doses comes from observational studies of athletes who use testosterone illegally, which cannot control for diet, training intensity, or other variables.

Bone density and fracture risk

Testosterone supports bone density by stimulating osteoblasts — the cells that build bone tissue — and by slowing bone loss. People assigned male at birth typically have denser bones than people assigned female at birth, partly because of higher testosterone levels. As testosterone declines with age, bone density decreases, which increases fracture risk.

In people with low testosterone, hormone replacement therapy increases bone mineral density, particularly in the spine and hip. A 2020 meta-analysis in The Lancet of 37 studies found that testosterone therapy increased bone density by 1 to 3% over 1 to 3 years. Whether this translates to fewer fractures in older men remains uncertain, because long-term fracture data are limited.

In people with normal testosterone levels, the relationship between testosterone and bone density is less direct. Bone health depends on multiple factors — weight-bearing exercise, calcium and vitamin D intake, and overall metabolic health — and testosterone is one piece of a larger picture.

Sexual function and desire

Testosterone is essential for sexual desire and erectile function in people of all sexes. It acts on the brain to increase sexual motivation and on blood vessels and nerves in the penis to support erections. In people with low testosterone, sexual desire and erectile function often decline.

Testosterone replacement therapy in people with clinically low testosterone improves sexual desire and erectile function in most cases, according to multiple randomized trials. The effect usually appears within weeks to months of starting treatment. However, erectile dysfunction has many causes — cardiovascular disease, diabetes, depression, relationship stress, and other medications — and testosterone alone does not address all of them.

In people with normal testosterone levels, further increases do not necessarily improve sexual function. Some observational data suggest that very high testosterone levels may actually impair sexual function, though this is not well studied in humans.

Mood, cognition, and mental health

Testosterone receptors are present throughout the brain, including in regions involved in mood, memory, and motivation. Animal studies show that testosterone influences behavior, aggression, and social dominance. Some human observational data suggest that people with low testosterone report higher rates of depression and lower mood.

In people with clinically low testosterone, some studies show that hormone replacement improves mood and reduces depressive symptoms, though the effect is modest and not universal. A 2016 review in Psychoneuroendocrinology found mixed results: some trials showed improvement in depression and anxiety, while others found no significant effect.

Whether testosterone improves mood or cognition in people with normal levels is not well established. A few small studies suggest potential benefits for memory and processing speed, but these are preliminary and have not been replicated in large, rigorous trials. The idea that testosterone universally improves mood or mental clarity in healthy people is not supported by strong evidence.

Cardiovascular effects and safety concerns

Testosterone therapy carries documented risks that require medical supervision. The most consistent finding is an increase in red blood cell production (polycythemia), which thickens the blood and can increase the risk of blood clots and stroke. This is why people on testosterone therapy need regular blood tests to monitor red blood cell count.

Testosterone may also affect the heart directly. Some observational studies in older men suggest a link between testosterone therapy and increased heart attack risk, though other studies have not found this association. The evidence is mixed and contested, which is why major medical organizations recommend careful cardiovascular screening before starting testosterone therapy and ongoing monitoring during treatment.

Testosterone therapy also suppresses the body's natural production of testosterone and sperm. In people assigned male at birth, this can lead to testicular shrinkage and infertility, which may or may not reverse after stopping treatment. Acne, male-pattern baldness (in genetically predisposed individuals), and liver stress are also documented side effects.

Performance-enhancing doses and long-term safety

Athletes and bodybuilders often use testosterone doses 10 to 100 times higher than therapeutic doses. Most evidence on these supraphysiological doses comes from animal studies, case reports, and observational data from athletes — not controlled human trials. This means the long-term safety of high-dose testosterone in healthy people is not well established.

Animal studies show that very high testosterone doses can damage the heart, liver, and kidneys, and can increase aggression and impulsive behavior. In humans, case reports and observational data link high-dose testosterone use to heart attacks, strokes, liver tumors, and psychiatric symptoms, but these studies cannot prove causation or quantify risk in the general population.

The lack of rigorous human data on long-term safety is itself important information: if you are considering testosterone therapy for any reason, the decision should be made with a doctor who can assess your individual risk factors and monitor you over time.

Frequently Asked Questions

Does testosterone increase aggression?

Animal studies show that very high testosterone doses increase aggressive behavior, but human evidence is mixed. Some observational data suggest a link between high testosterone and aggression, while other studies find no clear relationship. Aggression is influenced by many factors — genetics, upbringing, social context, and mental health — and testosterone is only one piece of a complex picture.

Can women benefit from testosterone therapy?

Some women with low testosterone report improved sexual desire and energy after hormone replacement, though research in women is much more limited than in men. Testosterone therapy in women carries risks including virilization (development of male characteristics) and effects on the heart and liver. Any consideration of testosterone therapy in women should involve careful discussion with a doctor about risks and benefits.

Does natural testosterone decline with age require treatment?

Natural testosterone decline is a normal part of aging and does not automatically require treatment. Treatment is typically considered only when testosterone levels are clinically low and symptoms are present — such as low sexual desire, fatigue, or mood changes — and when other causes have been ruled out. Many older men with lower testosterone levels function well without therapy.

What is the difference between testosterone therapy and testosterone supplements?

Testosterone therapy (injections, gels, patches) delivers pharmaceutical-grade hormone under medical supervision, with regular blood testing to monitor levels and side effects. Testosterone supplements sold over the counter typically contain plant extracts or precursors that claim to boost natural testosterone production, but most have not been rigorously tested in humans and their effects are usually small or unproven.

How long does it take to see effects from testosterone therapy?

Sexual function and mood may improve within weeks. Muscle and strength gains typically appear over 3 to 6 months. Bone density changes take 1 to 3 years to measure. The timeline varies by individual, dose, and which effects you are tracking.