Testosterone is used medically to treat low testosterone levels (hypogonadism) in men, as part of gender-affirming hormone therapy for transgender men, and occasionally to address bone loss in postmenopausal women. Beyond prescribed uses, testosterone plays a central role in the body’s everyday operations: building muscle, maintaining bone strength, producing red blood cells, and regulating sex drive, mood, and energy.
What Testosterone Does in the Body
Testosterone is the primary sex hormone in men, though women produce smaller amounts too. Its most visible role begins during puberty, when it drives the adolescent growth spurt by working alongside growth hormone to lengthen bones and vertebrae. It deepens the voice, triggers facial and body hair growth, and promotes development of the reproductive organs.
In adults, testosterone keeps several systems running. It stimulates muscle protein synthesis, which is why men carry roughly 50% more muscle mass than women, with the effect concentrated in the upper body. It maintains bone mineral density by supporting the cells that build new bone tissue. And it directly stimulates red blood cell production in two ways: acting on bone marrow and triggering the kidneys to release a hormone that further boosts red cell output. This is why men naturally have higher red blood cell counts than women.
Testosterone also influences mood, motivation, and cognitive sharpness. When levels drop, men commonly report fatigue, irritability, difficulty concentrating, and reduced interest in sex, sometimes well before any physical changes become apparent.
Medical Conditions Treated With Testosterone
The FDA has approved testosterone replacement therapy only for men with hypogonadism tied to a known structural or genetic cause. That includes conditions like Klinefelter syndrome, pituitary tumors, or damage to the testes from injury, infection, or chemotherapy. In these cases, the body either can’t produce enough testosterone on its own or the signaling pathway from the brain to the testes is broken.
The American Urological Association defines low testosterone as a total level below 300 ng/dL, measured on at least two separate mornings (testosterone peaks in the early hours and drops throughout the day). Blood levels alone aren’t enough for a diagnosis. Symptoms like low libido, erectile difficulties, fatigue, depressed mood, or loss of muscle mass need to be present alongside the lab result.
The FDA has also signaled interest in expanding the indication to cover men with low libido caused by idiopathic hypogonadism, meaning low testosterone without a clear structural cause. This broader group makes up a large share of the men currently prescribed testosterone off-label.
Gender-Affirming Hormone Therapy
Testosterone is a cornerstone of masculinizing hormone therapy for transgender men. The goal is to raise testosterone into the typical male range of 300 to 1,000 ng/dL, producing changes like voice deepening, facial hair growth, fat redistribution, increased muscle mass, and cessation of menstrual cycles. Delivery options include weekly injections, daily topical gels, or skin patches, with the specific formulation tailored to patient preference and response.
Bone Loss in Women
Though less common, testosterone pellet therapy has been used in postmenopausal women to combat bone loss. In one study, all patients saw either improved bone mineral density or a halt in further decline, with average gains of 6.2% at the spine and 1.6% at the hip.
How Testosterone Therapy Is Delivered
Testosterone comes in several forms, and the choice depends on convenience, insurance coverage, and how steady you want your hormone levels to be. The most common options are:
- Injections: Testosterone cypionate or enanthate, typically given weekly or every two weeks. A common starting point is 75 mg subcutaneously once a week, adjusted based on blood work.
- Topical gels: Applied daily to the shoulders or upper arms. The 1% gel comes in doses ranging from 2.5 to 10 grams per day.
- Skin patches: Worn daily, delivering 2.5 to 7.5 mg over 24 hours.
- Implantable pellets: Small pellets placed under the skin every few months, providing a slow, steady release.
Injections tend to produce more noticeable peaks and valleys in how you feel throughout the week, while gels and patches keep levels more stable. Gels carry the risk of transferring testosterone to others through skin contact, so you need to cover the application site or wash your hands thoroughly.
How Testosterone Builds Muscle
Testosterone’s muscle-building effect goes beyond simply “increasing strength.” At the cellular level, it ramps up the machinery that cells use to assemble new proteins. Research published in The Journal of Clinical Endocrinology & Metabolism found that testosterone supplementation increases the ribosomal capacity inside muscle cells, essentially giving them more assembly lines to produce protein. It also improves synthetic efficiency by redirecting amino acids already inside the cell toward building new tissue rather than being broken down for energy.
This effect is especially pronounced during calorie deficits. The same research showed that testosterone simultaneously slows protein breakdown while boosting synthesis, which explains why men on testosterone therapy can maintain or even gain lean mass while losing fat. For someone not on therapy, a calorie deficit would normally cause muscle loss alongside fat loss.
Timeline for Noticing Changes
If you’re starting testosterone therapy, improvements don’t arrive all at once. Here’s a realistic week-by-week picture of what most men experience:
Mood changes come first. Within the first two weeks, many men notice reduced fatigue and slightly better focus. By weeks three and four, stress feels more manageable and irritability fades. Mental steadiness continues improving through week eight before settling into a stable baseline around weeks nine through twelve.
Libido follows a similar but slightly slower curve. Morning erections and sexual interest pick up around weeks three to four. Erectile function continues improving through weeks seven and eight, then levels off into a more predictable pattern by week twelve.
Body composition is the slowest to change. You’re unlikely to see anything in the mirror during the first month, though gym performance may start improving around weeks three to four. By weeks seven and eight, clothes may fit differently, especially around the waist and chest. Measurable changes in fat mass and lean muscle typically become clear around week twelve and continue stabilizing over the following six to twelve months. Consistent strength training and adequate protein intake dramatically influence how much recomposition you actually see.
Risks and Side Effects
Testosterone therapy carries real trade-offs. The most common side effects include acne and oily skin, which are the same androgen-driven skin changes that affect teenagers during puberty. Sleep apnea can worsen on therapy, so if you already snore heavily or have been diagnosed with sleep apnea, your doctor will monitor this closely.
The most frequently flagged lab concern is polycythemia, an excessive increase in red blood cells. Because testosterone directly stimulates red cell production, levels can climb high enough to thicken the blood and raise the risk of clotting. Regular blood draws to check your red blood cell count are a standard part of monitoring.
Testosterone can also stimulate growth of prostate tissue. It doesn’t appear to cause prostate cancer, but it can accelerate the growth of existing prostate cancer, which is why a screening is typically done before starting therapy.
Cardiovascular Safety
For years, the biggest open question was whether testosterone therapy increased the risk of heart attacks and strokes. The TRAVERSE trial, the largest randomized study to address this, followed over 5,000 men for an average of about 22 months. Its primary finding: testosterone replacement was noninferior to placebo for major adverse cardiac events, meaning it did not increase heart attack or stroke rates compared to men receiving a dummy gel.
That said, the trial had notable limitations. More than 60% of participants in both groups stopped using their assigned treatment before the study ended, and about 18% were lost to follow-up entirely. The average treatment duration was under two years, leaving long-term cardiovascular effects beyond that window still uncertain. The results are reassuring for the medium term but not a blanket guarantee of safety over decades of use.