Testosterone is a hormone that drives muscle growth, bone strength, sex drive, red blood cell production, and mood regulation in both men and women. While it’s often thought of as a “male hormone,” women produce it too, and it plays essential roles across the body regardless of sex.
How Testosterone Works in Your Body
Testosterone belongs to a class of hormones called androgens. When it enters a cell, it binds to a receptor that carries it into the cell’s nucleus, where it switches specific genes on or off. This triggers the cell to produce new proteins, which is why testosterone has such a powerful effect on muscle tissue, bone, and organ development. In some tissues, the body converts testosterone into a more potent form before it acts, and in others it gets converted into estrogen, which is partly how it supports bone health in both sexes.
Muscle, Bone, and Body Composition
Testosterone directly stimulates protein synthesis in skeletal muscle, which is why it’s so closely linked to strength and muscle mass. People with higher testosterone levels tend to carry more lean tissue and less body fat. When levels drop, the balance shifts: muscle shrinks, strength declines, and fat accumulates, particularly around the midsection.
Bone density depends heavily on testosterone as well. The hormone helps maintain the mineral content that keeps bones hard and resistant to fractures. In men with low testosterone, bone loss is a recognized complication. Clinical trials in postmenopausal women have also shown that testosterone supplementation improves musculoskeletal health, confirming it isn’t just a male concern.
Sexual and Reproductive Function
In men, testosterone is the primary driver of sexual development from before birth through adulthood. During fetal development, it triggers the formation of male reproductive organs. At puberty, rising testosterone levels cause the testes and penis to enlarge, sperm production to begin, and libido to increase. Voice deepening, facial and body hair growth, and the adolescent growth spurt are all testosterone-dependent changes. The hormone also closes the growth plates in bones later in puberty, which is why boys stop growing taller.
In adult men, testosterone maintains sperm production and sex drive on an ongoing basis. A significant drop in the hormone commonly leads to reduced libido and erectile difficulties.
In women, testosterone is produced in smaller amounts by the ovaries and adrenal glands. It contributes to libido, arousal, and overall sexual satisfaction. The hormone also acts as a building block for estrogen production throughout the body, making it a key part of the female hormonal system even though levels are far lower than in men.
Red Blood Cell Production
Testosterone plays a surprisingly important role in blood health. It stimulates the kidneys to produce more erythropoietin (EPO), the signal that tells bone marrow to make red blood cells. In clinical studies, testosterone administration increased hemoglobin by about 7% and boosted the red blood cell volume by roughly 10%. It also suppressed hepcidin, a hormone that locks up iron stores, by nearly half, freeing iron for new red blood cell production.
This is why men naturally have higher red blood cell counts than women, and why anemia can be a symptom of very low testosterone. It’s also why doctors monitor blood thickness in anyone receiving testosterone therapy, since too many red blood cells can raise cardiovascular risks.
Mood, Memory, and Mental Health
Testosterone’s effects reach well beyond the physical. Research consistently shows that the hormone reduces anxiety in both men and women, with the effect appearing to be dose-dependent. Low testosterone in older men is associated with increased depressive symptoms, and the same pattern holds in women: those with depression tend to have lower testosterone than healthy controls. Supplementation has improved depressive symptoms in both sexes, particularly in men with clinically low levels.
Memory and cognitive function also benefit. Even a short six-week course of testosterone treatment improved spatial and verbal memory in older men in one study. In women, a single dose was enough to enhance spatial memory performance. Postmenopausal women and men going through age-related hormonal decline both show measurable cognitive improvements with testosterone supplementation. The relationship between testosterone and spatial reasoning is complex, though. In women, higher levels correlate with better performance, while in men, the association is actually curvilinear, meaning moderately lower levels sometimes correspond with stronger spatial skills.
Normal Testosterone Levels by Age
Testosterone is measured through a blood test, with results reported in nanograms per deciliter (ng/dL). For adult men (18 and older), the normal range is roughly 193 to 824 ng/dL. During the peak of male puberty (ages 11 to 15), levels can climb as high as 830 ng/dL as the body drives rapid development.
For adult women, normal levels sit below 40 ng/dL. During female adolescence (ages 11 to 17), levels can reach up to about 79 ng/dL. These ranges vary somewhat between laboratories and testing methods, so any individual result needs to be interpreted against the specific lab’s reference range.
What Low Testosterone Feels Like
Some people with low testosterone have no noticeable symptoms at all. Others experience a combination of physical and psychological changes:
- Low sex drive and difficulty with erections in men
- Loss of muscle size and strength, often noticeable in the arms and legs
- Increased body fat, especially around the belly
- Bone loss, raising fracture risk over time
- Fatigue and sleep problems, including insomnia
- Depression, trouble concentrating, and a general feeling of mental fog
- Low sperm count, which can affect fertility
These symptoms overlap with many other conditions, which is why a blood test is necessary to confirm whether testosterone is actually the issue. Levels naturally decline with age in men, dropping gradually after age 30, so some degree of change is expected.
When Testosterone Is Used as Treatment
Testosterone replacement is FDA-approved for men who have both documented low blood levels (confirmed on at least two separate tests) and symptoms of deficiency. The most common underlying cause is hypogonadism, a condition where the testes don’t produce enough of the hormone. This can result from problems with the testes themselves or from damage to the pituitary gland or hypothalamus caused by tumors, trauma, or radiation.
For women, testosterone therapy is not FDA-approved for treating low libido, though some clinicians prescribe it off-label. The evidence for its benefits in women is growing, particularly around cognitive function and musculoskeletal health after menopause, but standardized guidelines for female treatment remain limited.