Testosterone is a steroid hormone that drives the development of male sex organs and secondary sexual characteristics like muscle mass, body hair, and a deeper voice. While it’s often called the “male sex hormone,” women produce it too, and it plays important roles in both sexes. In men, healthy levels typically fall between 350 and 575 ng/dL depending on age, while women have much smaller amounts circulating in their blood.
What Testosterone Does in the Body
Testosterone belongs to a class of hormones called androgens. It’s classified as an anabolic steroid, meaning it promotes tissue building, particularly muscle and bone. The hormone works by entering cells and switching on specific genes that control growth, protein production, and sexual development.
Once testosterone reaches a cell, it can be converted into a more potent form called DHT, which has stronger effects on tissues like the prostate and hair follicles. It can also be converted into estradiol, a form of estrogen. This means testosterone serves as a raw material for multiple hormones, not just one.
Testosterone’s Role in Men
In men, testosterone is responsible for a long list of functions that go well beyond reproduction. It stimulates sperm production, fuels sex drive, and maintains erectile function. It also drives protein synthesis in skeletal muscle, which is why men naturally carry more muscle mass than women. Bone density depends on it too. When testosterone drops, bones weaken, and the risk of osteoporosis climbs.
The hormone’s influence starts before birth. In the womb, testosterone directs the formation of male reproductive organs. During puberty, it triggers the voice to deepen, facial and body hair to grow, and the penis and testicles to develop. In adulthood, it maintains energy levels, mood stability, and body composition. Men with very low testosterone often report fatigue and depression before they notice any physical changes.
Testosterone’s Role in Women
Women produce testosterone in smaller quantities, primarily in the ovaries and adrenal glands. It’s an essential hormone for women, not a male “extra.” Testosterone supports bone strength, muscle maintenance, and cognitive function in women. Its most well-studied role in female health is sexual desire. Loss of libido is the primary reason testosterone is sometimes prescribed to women, particularly after menopause. Clinical trials have also found that it can improve musculoskeletal health and cognitive performance in postmenopausal women.
Normal Testosterone Levels by Age
Testosterone levels aren’t static. They peak in early adulthood and gradually decline with age. A large study published in The Journal of Urology mapped out what normal looks like for men at different ages:
- Ages 20 to 24: 409 to 558 ng/dL
- Ages 25 to 29: 413 to 575 ng/dL
- Ages 30 to 34: 359 to 498 ng/dL
- Ages 35 to 39: 352 to 478 ng/dL
- Ages 40 to 44: 350 to 473 ng/dL
The American Urological Association uses 300 ng/dL as a general cutoff for diagnosing low testosterone in men. But as the data above shows, what counts as “low” for a 22-year-old (below 409 ng/dL) is quite different from what’s low for a 42-year-old (below 350 ng/dL).
How Testosterone Is Measured
If you get a blood test, you’ll typically see one of three measurements. A total testosterone test captures all the hormone in your blood, both the portion bound to proteins and the portion floating freely. A free testosterone test measures only the unbound form, which is more readily available for your body to use in building muscle and bone. A bioavailable testosterone test captures free testosterone plus the portion loosely attached to a protein called albumin, which your body can still access relatively easily.
Most of your testosterone is tightly bound to a protein called sex-hormone-binding globulin (SHBG). When it’s locked to SHBG, your body can’t use it as readily. This is why two people with the same total testosterone can feel very different. One may have plenty of free testosterone available, while the other’s supply is mostly bound up.
Signs of Low Testosterone
Low testosterone, called hypogonadism, shows up differently depending on when it occurs. In adult men, the earliest signs are often reduced sex drive, lower energy, and depression. These can be easy to dismiss or attribute to stress. Over time, more noticeable physical changes develop: difficulty getting or maintaining erections, loss of muscle mass, less facial and body hair, and fertility problems.
During puberty, low testosterone can delay or stall development. Boys may not see normal growth of the penis and testicles, their voice may not deepen, and they may develop breast tissue (a condition called gynecomastia). Their arms and legs may grow disproportionately long relative to their torso.
Signs of Too Much Testosterone
Excess testosterone is a more common clinical concern in women than in men. The most frequent symptom is hirsutism, which means hair growing in a typically male pattern on the face, chest, or back. Other signs include acne (testosterone increases oil production in the skin), thinning hair on the scalp in a male pattern, and irregular or absent periods.
The most common cause of excess testosterone in women is polycystic ovary syndrome (PCOS), which affects roughly 10% of women of reproductive age. PCOS develops gradually, typically over a year or more. When symptoms appear suddenly, over just a few months, and include voice deepening or significant muscle gain, that pattern is more concerning for an androgen-producing tumor in the ovaries or adrenal glands. These tumors are more common in postmenopausal women.
Lifestyle Factors That Affect Levels
Your daily habits have a surprisingly large influence on how much testosterone your body produces. Body weight is one of the biggest levers. Excess body fat, especially around the abdomen, suppresses testosterone production. Losing weight through diet and exercise can boost levels by up to 30%, according to Harvard Health Publishing.
Sleep is another critical factor. Most testosterone release happens during sleep, particularly during the REM stage. Chronic sleep deprivation directly lowers levels. Sleep apnea, a condition where breathing repeatedly stops during sleep, also impairs hormone production. Aiming for seven to nine hours of quality sleep per night supports healthy testosterone output.
Regular exercise, particularly resistance training, is one of the most effective natural ways to maintain testosterone. Chronic stress works in the opposite direction, as the stress hormone cortisol tends to suppress testosterone when it stays elevated for long periods.