Testosterone is a hormone that affects nearly every system in your body, from how much muscle you build to how you feel on a given day. While it’s often thought of as a “male hormone,” testosterone plays essential roles in both men and women, influencing everything from bone strength and fat storage to sex drive, red blood cell production, and mood. Here’s what it actually does and why the levels matter.
How It Builds and Maintains Muscle
Testosterone is one of the strongest natural signals your body has for building muscle tissue. It works by ramping up the rate at which your muscles create new protein, the basic process behind muscle growth and repair. In a study published in the American Journal of Physiology, healthy men who received a single testosterone injection saw their muscle protein synthesis rate double within five days, with no corresponding increase in protein breakdown. That means testosterone doesn’t just help you build muscle faster; it tips the balance toward net growth.
This is why people with higher testosterone levels generally carry more lean mass, and why a significant drop in testosterone often leads to noticeable muscle loss over time. It also explains why resistance training, which temporarily raises testosterone, is so effective at maintaining strength as you age.
Fat Distribution and Metabolism
Testosterone influences where your body stores fat and how efficiently you process blood sugar. In men, adequate testosterone levels help keep visceral fat (the deep abdominal fat surrounding organs) in check. When testosterone drops, visceral fat tends to increase, which in turn raises the risk of metabolic problems like type 2 diabetes and cardiovascular disease.
The relationship between testosterone and metabolism is sex-specific, though. In women, excess testosterone can actually impair how fat cells respond to insulin. Research in the Journal of Endocrinology found that when fat cells from women were exposed to testosterone, their ability to absorb glucose in response to insulin was significantly reduced. This selective insulin resistance was driven by the androgen receptor and may help explain the metabolic complications seen in conditions like polycystic ovary syndrome (PCOS), where testosterone levels are elevated.
Sex Drive and Erectile Function
Testosterone is the primary hormonal driver of libido in both men and women. It doesn’t work like an on/off switch, but when levels fall below a certain threshold, sexual desire typically drops noticeably. This is one of the most common symptoms people report when testosterone is low.
In men, testosterone also plays a direct role in the mechanics of erection. It helps maintain the activity of an enzyme that produces nitric oxide in penile tissue. Nitric oxide relaxes blood vessels and allows blood flow into the penis during arousal. Animal studies have shown that removing testosterone reduces this enzyme’s activity by about 45%, and restoring testosterone prevents that loss. So while erections depend on blood flow and nerve signals, testosterone is a key upstream factor keeping that system functioning.
Red Blood Cell Production
Your body uses testosterone as a signal to produce red blood cells, the cells that carry oxygen through your bloodstream. Testosterone stimulates the kidneys to release erythropoietin (EPO), the same hormone that triggers the bone marrow to make more red blood cells. It also appears to act directly on blood-forming stem cells and increases iron availability by adjusting the hormones that regulate iron absorption.
This is one reason men typically have higher red blood cell counts than women. It’s also why testosterone therapy can sometimes push red blood cell levels too high, a condition called erythrocytosis. A meta-analysis of clinical trials found that testosterone treatment increased the risk of erythrocytosis more than eightfold compared to placebo. When the proportion of red blood cells in the blood (hematocrit) exceeds about 54%, it raises the risk of blood clots, which is why doctors monitor blood counts during testosterone therapy.
Bone Density
Testosterone helps maintain bone strength through two pathways. It acts directly on bone cells, and it also converts into estradiol, a form of estrogen that is critical for bone mineral density in both sexes. As testosterone declines with age, bone thinning accelerates.
This effect is well documented in women too. Research published in The Lancet Diabetes & Endocrinology found that women in their late reproductive years with lower free testosterone levels experienced a statistically significant decline in bone mineral density of more than 1% per year. Clinical trials have also shown that testosterone supplementation improves musculoskeletal health in postmenopausal women, though it’s not a standard treatment for osteoporosis.
Mood, Energy, and Cognition
Testosterone interacts with the brain chemicals that regulate mood, including serotonin, dopamine, and norepinephrine. By modulating the activity of these neurotransmitters, testosterone helps stabilize mood and can reduce symptoms of depression. This is why low testosterone is frequently associated with irritability, fatigue, low motivation, and a general sense of emotional flatness that people sometimes describe as “just not feeling like myself.”
The cognitive effects are subtler but real. Testosterone appears to support spatial reasoning and memory, particularly in older adults. Some men with low testosterone report difficulty concentrating or a sense of mental fog that improves with treatment, though the research on cognition is less definitive than the evidence for mood effects.
Skin, Hair, and Appearance
Testosterone controls several visible traits. It increases sebum production in the skin’s oil glands, which is why men tend to have oilier skin and larger pores than women. When excess oil gets trapped in hair follicles, it forms comedones, the primary lesions of acne. This connection between testosterone and sebum explains why acne often flares during puberty, when testosterone levels surge, and why hormonal acne in women tends to correlate with androgen levels.
Hair growth is more complicated. Testosterone’s more potent metabolite, DHT, is responsible for growing thicker terminal hair on the face, chest, and other androgen-sensitive areas. But in the scalp, DHT does the opposite: it causes hair follicles to miniaturize over time, producing progressively thinner and shorter hairs until the follicle stops producing visible hair altogether. This is the mechanism behind male-pattern baldness, and it can happen even when testosterone levels are completely normal. The difference comes down to the individual genetic sensitivity of your hair follicles to DHT, which is why some men with high testosterone keep a full head of hair while others begin losing theirs in their twenties.
Testosterone in Women
Women produce testosterone in smaller quantities, primarily in the ovaries and adrenal glands, but it’s no less important for their physiology. Testosterone serves as an obligatory precursor for estradiol, meaning the body can’t make its primary form of estrogen without it. Beyond that conversion role, testosterone directly supports bone density, lean muscle mass, and sexual desire in women.
Both excess and deficiency cause problems. Too much testosterone, as seen in PCOS, can lead to acne, excess body hair, irregular periods, and metabolic disruption. Too little, which commonly occurs after menopause or surgical removal of the ovaries, can cause persistent low libido, fatigue, and loss of muscle and bone strength.
Normal Levels and Age-Related Decline
For adult men, the American Urological Association defines low testosterone as a total level below 300 ng/dL, measured on two separate mornings (testosterone peaks in the early morning and drops through the day). The therapeutic target for men receiving treatment is typically 450 to 600 ng/dL, the middle of the normal range.
Testosterone begins declining gradually after about age 30, averaging just over 1% per year. That might sound small, but it compounds. By age 50 or 60, many men have lost 20 to 30% of their peak levels. Not all of them will have symptoms, because the threshold for problems varies from person to person. According to the Endocrine Society, men over 50 with total testosterone below 200 ng/dL are considered clearly deficient and should have their pituitary function evaluated. Those between 200 and 400 ng/dL are in a gray zone that warrants additional testing before considering treatment. Men above 400 ng/dL generally don’t need further workup.
For women, there is no universally agreed-upon clinical reference range, partly because female testosterone levels are much lower and partly because the symptoms of deficiency overlap with other hormonal changes during menopause. Diagnosis in women tends to rely more on symptoms than on specific lab numbers.