Testosterone is the primary sex hormone in males and plays a role in nearly every major system in the body, from building muscle and bone to regulating mood, metabolism, and red blood cell production. Women produce it too, at roughly one-twentieth the concentration, and it serves important functions in both sexes. Normal levels for adult men fall between 193 and 824 ng/dL, while women typically have less than 40 ng/dL.
How Testosterone Works Inside Cells
Testosterone is a signaling molecule. When it enters a target cell, it binds to a specialized receptor protein either directly or after being converted into a more potent form. That binding triggers a chain reaction: protective proteins fall away from the receptor, the receptor changes shape, and the whole complex moves into the cell’s nucleus. Once inside, it latches onto specific stretches of DNA, pairs up with a second receptor molecule, and recruits the machinery needed to read genes. The end result is that certain genes get switched on or off, leading to new proteins being built. Those proteins are what actually produce testosterone’s effects, whether that means thickening a muscle fiber, stimulating hair growth, or ramping up red blood cell production.
Puberty and Physical Development
The most visible effects of testosterone occur during male puberty, when levels surge dramatically. The hormone drives penile enlargement, deepening of the voice through growth of the larynx, and the development of body and facial hair. It also fuels the adolescent growth spurt by stimulating tissue growth at the ends of long bones, though it eventually signals those same growth plates to close, capping final adult height.
These changes are collectively called virilization, and they’re the reason testosterone is classified as an androgen. The timing and intensity of puberty depend largely on how much testosterone the testes produce and how sensitive a person’s tissues are to it.
Muscle Mass and Strength
Testosterone is one of the most powerful natural signals for building muscle. It stimulates protein synthesis directly, telling muscle cells to produce more of the structural proteins that make fibers larger and stronger. This is why men, on average, carry significantly more skeletal muscle than women, and why muscle mass tends to decline as testosterone drops with age.
The effect is dose-dependent. Higher testosterone levels within the normal range support greater capacity for muscle growth and repair, especially when combined with resistance exercise. When levels fall below normal, one of the earliest complaints is reduced physical performance, decreased endurance, and a noticeable loss of muscle mass.
Bone Density and Skeletal Health
Testosterone receptors are present on the cells responsible for building new bone tissue. When testosterone binds to these cells, it promotes their growth and differentiation, essentially telling them to lay down more bone. It also boosts the production of growth factors that further stimulate bone-building cells and suppress the natural cell death that would otherwise thin the skeleton over time.
This matters throughout life, not just during the growth years. Men with chronically low testosterone face a measurably higher risk of osteoporosis and fractures, particularly in the spine and hip. The hormone’s bone-protective effects work alongside estrogen (testosterone is actually converted into estrogen in bone tissue), which is one reason both hormones matter for skeletal health in men and women alike.
Reproductive Function and Sex Drive
Testosterone is essential for sperm production. Inside the testes, it supports the cells that nurture developing sperm through their maturation process. Without adequate testosterone, sperm counts drop or cease entirely, making it one of the most common hormonal causes of male infertility.
The hormone also drives libido. Reduced sexual desire and fewer spontaneous erections, particularly morning erections, are among the most reliable indicators of low testosterone. These effects are mediated by the brain, where testosterone influences the neural circuits that regulate arousal and sexual motivation.
Fat Distribution and Metabolic Health
Testosterone has a strong inverse relationship with body fat, particularly the visceral fat that accumulates around internal organs. Studies consistently show that men with higher testosterone carry less abdominal fat, while men with lower levels tend to accumulate more of it. This isn’t just cosmetic. Visceral fat is metabolically active and closely linked to insulin resistance and cardiovascular risk.
Restoring testosterone in middle-aged men with low levels has been associated with decreased visceral fat, lower blood glucose, and improved insulin sensitivity. The relationship runs both ways: excess body fat itself suppresses testosterone production, creating a cycle where weight gain and hormonal decline reinforce each other.
Red Blood Cell Production
Testosterone stimulates the kidneys to produce more erythropoietin, the hormone that tells bone marrow to make red blood cells. In clinical studies, testosterone administration raised erythropoietin levels by about 58% within the first month and kept them elevated for months afterward. The result is a higher red blood cell count and increased hemoglobin, which improves the blood’s capacity to carry oxygen.
This is why men naturally have higher hemoglobin levels than women, and it’s also why one of the most common side effects of testosterone therapy is an excessive rise in red blood cell count, a condition called polycythemia that can thicken the blood and increase clotting risk.
Mood, Energy, and Cognition
Low testosterone is closely associated with fatigue, depressed mood, irritability, and poor concentration. These aren’t vague complaints. They’re among the clinical criteria used to evaluate testosterone deficiency, and many men report significant improvement in energy and mental clarity when levels are restored.
The cognitive effects are subtler but real. Testosterone has been positively correlated with spatial reasoning ability in healthy men, and there’s evidence it helps regulate emotional processing by reducing the brain’s attentional bias toward negative stimuli. In practical terms, adequate testosterone levels appear to support a more even-keeled emotional baseline, while deficiency tilts the balance toward low motivation and heightened sensitivity to stress.
What Happens When Levels Drop
Testosterone naturally declines with age, typically falling about 1 to 2% per year after age 30. When levels drop below the normal range, the combination of symptoms is called hypogonadism. The most telling signs include reduced sex drive, fewer spontaneous and morning erections, unexplained fatigue, and shrinking testicular volume. Less specific but still common are increased body fat, loss of body hair, lower endurance, weight changes, and difficulty concentrating.
Some of these symptoms overlap with normal aging, depression, or thyroid disorders, which is why a blood test measuring total testosterone is the standard starting point for evaluation. Levels are typically drawn in the morning, when testosterone peaks in its daily cycle.
Testosterone in Women
Women produce testosterone in the ovaries and adrenal glands at much lower concentrations, but it still plays important roles. It contributes to bone density, muscle maintenance, and libido, and serves as the raw material for estrogen production. Many of the same symptoms that signal deficiency in men, including fatigue, low sex drive, and reduced bone strength, can also appear in women when testosterone drops too low, though this is less commonly tested for and less well studied.