Testosterone is the primary male sex hormone, and it influences nearly every system in the body. It drives the physical changes of puberty, maintains muscle and bone strength throughout life, regulates sex drive and sperm production, stimulates red blood cell production, and plays a role in mood and energy levels. For adult men aged 19 and older, normal total testosterone falls between 240 and 950 ng/dL, though levels gradually decline with age.
How Testosterone Works Inside Your Cells
Testosterone travels through the bloodstream and enters target cells, where it binds to androgen receptors. Once bound, the receptor changes shape and moves into the cell’s nucleus, where it attaches to specific stretches of DNA and switches genes on or off. This is how testosterone produces its effects: not by doing something directly to a muscle fiber or bone cell, but by reprogramming what that cell builds and how it behaves. In some tissues, testosterone is first converted into a more potent form called DHT, which binds to the same receptor with greater strength and drives effects like body hair growth and prostate development.
Puberty and Physical Development
Testosterone is the engine behind male puberty. Rising levels trigger a cascade of changes that typically unfold over several years, beginning as early as age 9 and continuing into the mid-to-late teens. The sequence follows a roughly predictable pattern.
In the earliest stage (around ages 9 to 14), sparse hair begins growing around the penis and under the arms, and the testes start to enlarge. Between ages 10 and 16, pubic hair darkens and coarsens, the voice begins cracking as the larynx grows, and height increases accelerate. By ages 11 to 16, body hair reaches near-adult levels, the voice deepens further, and the penis and prostate gland continue to grow. In the final stage, pubic hair may extend to the thighs and up toward the belly button, and growth in height tapers off.
These changes are permanent. Once testosterone has masculinized the skeleton, vocal cords, and hair follicles during puberty, those traits persist even if testosterone levels later decline.
Muscle Mass and Bone Strength
Testosterone is a powerful driver of muscle growth. At puberty, boys gain roughly 35% more muscle mass than girls, a gap that persists into adulthood. The hormone works by stimulating the precursor cells that build muscle fibers and increasing the number of satellite cells (the repair cells that help muscles recover and grow after use). This promotes protein synthesis, which is why muscle mass and strength are closely tied to testosterone levels.
Bone responds to testosterone as well. The hormone supports the cells responsible for laying down new bone tissue, helping maintain bone mineral density throughout life. When testosterone drops significantly, as it often does in men over 70, both muscle and bone tend to weaken. Men with total testosterone below 320 ng/dL are considered hypogonadal, and by the eighth decade of life, many men fall into this range.
Sex Drive, Erections, and Fertility
Testosterone is essential for sexual function on multiple levels. It is a primary driver of libido, and when levels fall, reduced sex drive is often one of the first noticeable symptoms. The hormone also supports the physiological processes behind erections, so low testosterone can contribute to erectile difficulties.
For fertility, testosterone plays a critical but sometimes misunderstood role. Sperm production requires testosterone, but the levels needed inside the testes are many times higher than what circulates in the bloodstream. This is why taking supplemental testosterone from outside the body can actually harm fertility: it signals the brain to stop stimulating the testes, which causes local testosterone production to plummet and sperm counts to drop. Men trying to conceive should be aware that testosterone therapy and sperm production often work against each other.
Red Blood Cell Production
Testosterone stimulates the production of red blood cells through a chain of signals. It triggers an initial rise in erythropoietin (EPO), the hormone that tells bone marrow to make more red blood cells. It also suppresses hepcidin, a protein that regulates iron availability, freeing up more iron for red blood cell assembly. This is why men generally have higher red blood cell counts than women, and it is also why testosterone therapy can sometimes push red blood cell levels too high. On average, testosterone treatment raises hematocrit (the percentage of blood volume occupied by red blood cells) by up to about 4 percentage points, which in some men is enough to increase the risk of blood clots.
Body Fat and Metabolism
Testosterone influences where and how much fat the body stores. Men with healthy testosterone levels tend to carry less visceral fat (the deep abdominal fat that surrounds organs and raises cardiovascular risk). When testosterone declines, visceral fat tends to increase, which can create a feedback loop: fat tissue converts testosterone into estrogen, further lowering testosterone and encouraging more fat storage.
The relationship between testosterone and insulin sensitivity is complex and differs between the sexes. In men, low testosterone is associated with insulin resistance and a higher risk of type 2 diabetes. Restoring testosterone to normal ranges generally improves how the body handles blood sugar. The metabolic benefits of testosterone in men appear closely tied to its effects on body composition, since less visceral fat typically means better insulin function.
Mood, Energy, and Cognition
Low testosterone is linked to fatigue, irritability, and depressive symptoms. Many men with hypogonadism report a general loss of motivation and well-being that improves when levels are restored. This connection is strong enough that testosterone levels are sometimes checked during evaluations for depression in men, particularly those over 50.
The effects on cognition are less clear. A major trial involving 493 men aged 65 and older with age-related memory impairment found that one year of testosterone treatment produced no improvement in verbal memory, visual memory, executive function, or spatial ability compared to placebo. So while testosterone clearly affects mood and energy, it does not appear to sharpen thinking or protect against cognitive decline in older men.
Normal Levels and How They Change With Age
For men 19 and older, total testosterone between 240 and 950 ng/dL is considered the normal range. The American Urological Association uses a threshold of 300 ng/dL as the cut-off for diagnosing low testosterone, with the important caveat that the diagnosis requires two separate blood draws, both taken in the early morning when levels are naturally highest.
Free testosterone, the small fraction not bound to proteins in the blood, declines more predictably with age. A man in his early 20s may have free testosterone up to about 1.08 ng/dL, while a man in his 70s tops out around 0.82 ng/dL. By the 80s and 90s, the upper limit drops below 0.76 ng/dL. This gradual decline is normal and happens in virtually all men, though the rate varies. Significant drops, especially below 300 ng/dL total, are more likely to produce symptoms like reduced muscle mass, increased body fat, low libido, fatigue, and mood changes.