Testosterone is the primary male sex hormone, and it influences nearly every system in a man’s body. It drives muscle growth, bone strength, fat distribution, red blood cell production, sexual function, and mood. Normal levels for adult men fall in a range around 450 to 600 ng/dL, with anything below 300 ng/dL generally considered low. After age 30, levels drop about 1% per year on average, which means the effects of testosterone become increasingly noticeable as men age.
Muscle Mass and Bone Strength
Testosterone is one of the main signals that tells your body to build and maintain muscle. It increases muscle mass and strength by promoting protein activity within muscle fibers. This is why men with higher testosterone levels generally carry more lean mass, and why men with low testosterone often notice their muscles getting smaller or weaker even without changes in activity.
The effects on bone are more complex. Testosterone acts directly on bone-building cells called osteoblasts, enhancing the production and mineralization of new bone. It also regulates the cells responsible for breaking down old bone, keeping the balance between bone creation and bone removal in check. Some of testosterone’s bone benefits actually come from its conversion to estrogen inside the body, which also plays a protective role. The result is thicker, denser bones. When testosterone drops significantly, men face a real increase in fracture risk, particularly in the spine and hips.
Fat Distribution and Metabolism
Testosterone has a powerful effect on where your body stores fat. Research published in the Journal of Clinical Endocrinology and Metabolism found that when testosterone levels drop below normal, men gain fat in both the midsection and the limbs. Specifically, men with suppressed testosterone saw a 32% increase in deep fat stored between the muscles of the thigh. Conversely, higher testosterone levels caused significant reductions in these deeper fat stores, with losses of 14% to 28% depending on the dose studied.
This matters because deep abdominal and intermuscular fat is closely linked to insulin resistance, a precursor to type 2 diabetes. Fat stored between muscles has been connected to reduced ability to process blood sugar and decreased activity of enzymes involved in energy storage. In theory, testosterone’s ability to reduce these fat deposits should improve insulin sensitivity, though clinical measurements suggest the relationship isn’t that straightforward. Still, the shift in body composition from fat toward lean tissue is one of the most consistent effects of healthy testosterone levels.
Sexual Function and Libido
Testosterone is the primary driver of sex drive in men. It directly regulates libido, and when levels are adequate, sexual desire stays consistent. When they fall, interest in sex often drops noticeably, sometimes before any other symptoms appear.
Beyond desire, testosterone plays a mechanical role in erections. It helps regulate the production of nitric oxide, a molecule that relaxes blood vessel walls inside the penis. During arousal, nitric oxide signals these vessels to widen, allowing blood to flow in and produce an erection. When testosterone is too low, nitric oxide production drops, blood flow decreases, and achieving or maintaining an erection becomes harder. This is one reason erectile difficulties in men over 40 often coincide with declining hormone levels, though blood vessel health and other factors also play a role.
Red Blood Cell Production
Testosterone stimulates the production of red blood cells, the cells responsible for carrying oxygen throughout your body. A study in the American Journal of Physiology found that over 12 months, testosterone treatment increased red blood cell count by 9%, hemoglobin by 8%, and hematocrit (the percentage of blood volume made up of red cells) by 4%. It also suppressed hepcidin, a hormone that limits iron absorption, by 57%, essentially freeing up more iron for new red blood cell production.
This is why men naturally have higher red blood cell counts than women. It’s also why one of the first things doctors monitor during testosterone therapy is hematocrit. Too many red blood cells thicken the blood, which can increase the risk of clotting. For men with normal testosterone, this system works in balance. For men on supplemental testosterone, it requires monitoring.
Mood and Mental Function
Testosterone has a direct effect on brain chemistry. It crosses into the brain and binds to receptors in areas that regulate emotion, particularly the amygdala and the limbic system. It also interacts with serotonin and dopamine pathways, two neurotransmitter systems central to mood, motivation, and reward.
Men with clinically low testosterone frequently develop depressive symptoms, including low energy, irritability, poor concentration, and reduced motivation. These symptoms often track closely with hormone levels. Research in neuropsychopharmacology has shown that testosterone has an antidepressant-like effect, largely through its influence on the emotional processing centers of the brain. On the other end of the spectrum, extremely high testosterone, such as from anabolic steroid use, can trigger mood swings, aggression, and in some cases psychosis. The relationship between testosterone and mood is dose-dependent: too little creates problems, too much creates different ones.
Hair Growth and Hair Loss
Testosterone itself doesn’t cause the hair changes men experience. That job belongs to DHT (dihydrotestosterone), a more potent hormone that your body creates from testosterone using a specific enzyme. DHT is responsible for facial hair growth during and after puberty, including beard development.
Paradoxically, the same hormone that thickens your beard can thin the hair on your scalp. Elevated DHT activity at scalp hair follicles causes those follicles to shrink and shortens the hair growth cycle, eventually producing finer, shorter hairs until the follicle stops producing visible hair altogether. Genetics determine how sensitive your follicles are to DHT, which is why male pattern baldness runs in families. Two men with identical testosterone levels can have completely different experiences with hair loss based on their genetic sensitivity to this conversion.
What Low Testosterone Feels Like
Because testosterone touches so many systems, low levels rarely produce just one symptom. Men typically notice a cluster: reduced sex drive, fatigue that doesn’t improve with sleep, difficulty building or keeping muscle, increased belly fat, low mood or irritability, and sometimes trouble concentrating. These symptoms overlap with many other conditions, including depression, sleep apnea, and thyroid problems, which is why a blood test is the only way to confirm low testosterone.
The American Urological Association defines low testosterone as a total level below 300 ng/dL, measured in the morning when levels are highest. A single low reading isn’t enough for a diagnosis. Levels fluctuate day to day, so repeated testing is standard. Age-related decline is normal and doesn’t automatically require treatment. The question is whether the decline is causing symptoms that affect quality of life, and whether the numbers support what a man is experiencing.