Men take testosterone primarily to treat a condition called hypogonadism, where the body doesn’t produce enough of the hormone on its own. The American Urological Association defines low testosterone as a total level below 300 ng/dL, and the FDA currently approves testosterone replacement only for men whose deficiency is tied to a known structural or genetic cause. That said, the reasons men seek treatment span a wide range of symptoms, from low sex drive and muscle loss to fatigue, mood changes, and metabolic problems that affect long-term health.
Low Testosterone as a Medical Diagnosis
Testosterone deficiency isn’t a single condition. It falls into two broad categories. Primary hypogonadism means the testes themselves are the problem, often due to genetic conditions like Klinefelter syndrome, injury, or infection. Secondary hypogonadism means the signal from the brain is faulty, usually because of pituitary or hypothalamic dysfunction, which can result from tumors, certain medications, or obesity.
Either way, the result is the same: the body can’t maintain adequate testosterone levels. Symptoms typically include persistent fatigue, reduced sex drive, difficulty with erections, loss of muscle and bone mass, increased body fat, depressed mood, and trouble concentrating. A diagnosis requires both a low blood level (drawn in the morning, when testosterone peaks) and the presence of symptoms. One low reading alone isn’t enough, and most guidelines require at least two separate morning blood draws to confirm.
Improving Sexual Function
Low sex drive is one of the most common reasons men seek treatment. A meta-analysis of 14 clinical trials involving nearly 2,300 men found that testosterone therapy significantly improved erectile function compared with placebo. It also improved libido, orgasm quality, satisfaction with intercourse, and overall sexual satisfaction. Men with more severe deficiency saw the biggest gains. Those with total testosterone below roughly 230 ng/dL experienced about twice the improvement in erectile function scores compared with men who had milder deficiency.
Testosterone isn’t a replacement for medications that treat erectile dysfunction directly. But for men whose sexual problems stem from genuinely low hormone levels, restoring testosterone to normal range often provides meaningful improvement across multiple dimensions of sexual health, not just erections.
Building and Maintaining Muscle
Testosterone plays a central role in building and preserving lean tissue. When levels drop, men lose muscle mass and strength, sometimes dramatically. In a clinical study of hypogonadal men treated for 10 weeks, fat-free mass increased by about 5 kg (11 pounds) on average. Body weight rose from roughly 175 to 185 pounds, and the gains were almost entirely lean tissue rather than fat.
This matters beyond appearance. Muscle mass supports metabolism, joint stability, and the ability to perform everyday activities. For older men with low testosterone, the progressive loss of muscle (sometimes called sarcopenia) accelerates frailty and raises the risk of falls. Restoring normal testosterone levels helps reverse that trajectory.
Bone Density and Fracture Risk
Testosterone is essential for bone health in men. During puberty, it drives the accumulation of peak bone mass. When testosterone drops later in life, bone breakdown speeds up while new bone formation slows down, a combination that leads to thinning bones and a higher chance of fractures. Men with untreated hypogonadism face a meaningfully elevated risk of osteoporosis, particularly as they age.
Multiple studies show that testosterone therapy improves bone mineral density. However, the relationship between treatment and actual fracture prevention is more complicated. A recent meta-analysis found that men on testosterone had a higher overall rate of clinical fractures compared to placebo, but those extra fractures were predominantly minor ones. The rates of major osteoporotic fractures (hip, spine, and wrist) showed no significant difference between treated and untreated men. The takeaway is that testosterone helps strengthen bone tissue, but it hasn’t been proven to reduce the fractures that matter most.
Blood Sugar and Metabolic Health
There’s a strong link between low testosterone and insulin resistance, which is why current endocrine guidelines recommend checking testosterone levels in all men with type 2 diabetes. Restoring normal levels appears to improve several metabolic markers simultaneously.
In a study of men with both low testosterone and type 2 diabetes, treatment produced a measurable drop in HbA1c, a key marker of long-term blood sugar control. Men with poorly controlled diabetes (HbA1c above 7.5%) saw the most benefit: a reduction of about 0.4% within six weeks, and nearly another half-percent over the following year. That magnitude of improvement is clinically meaningful and comparable to what some oral diabetes medications achieve. Alongside the blood sugar changes, studies have documented reductions in waist circumference, total cholesterol, LDL cholesterol, fasting glucose, and markers of inflammation.
How Testosterone Is Taken
Testosterone replacement comes in several forms, each with different schedules and trade-offs.
- Topical gels are applied daily to the shoulders, upper arms, or abdomen. They maintain steady hormone levels but carry a risk of transferring testosterone to women or children through skin contact. Covering the area with clothing and washing hands after application reduces that risk.
- Transdermal patches are applied each evening, typically to the back, abdomen, or upper arms. The application site needs to rotate daily, with at least seven days before reusing the same spot.
- Injections are the most common method. Short-acting versions are given every one to two weeks, while long-acting formulations are given roughly every 10 weeks after an initial loading period. Injections produce more fluctuation in hormone levels, with peaks shortly after the shot and troughs before the next one.
- Subcutaneous pellets are implanted under the skin every three to four months. They provide consistent levels without daily effort, but if side effects develop, the pellets must be physically removed to stop therapy.
Most men and their doctors choose based on lifestyle preference, cost, and how well each method maintains stable levels.
Cardiovascular Safety
For years, the biggest concern about testosterone therapy was heart risk. The TRAVERSE trial, the largest randomized study on this question, enrolled over 5,000 men who already had cardiovascular disease or were at high risk for it. After a mean follow-up of about three years, the rate of heart attack, stroke, and cardiovascular death was 7.0% in the testosterone group and 7.3% in the placebo group. That difference was not statistically significant, and the study met its goal of demonstrating that testosterone was not inferior to placebo for cardiovascular safety.
This doesn’t mean testosterone is risk-free for the heart. But it does mean the therapy, when used for legitimate deficiency, does not appear to increase the rate of major cardiac events in the population most vulnerable to them.
Side Effects and Monitoring
The most common side effect of testosterone therapy is an increase in red blood cell production. Your body responds to higher testosterone by making more red blood cells, which thickens the blood. Guidelines recommend regular blood draws to check hematocrit (the percentage of blood volume made up of red cells). If hematocrit reaches 54%, the standard recommendation is to lower the dose or pause therapy until levels come back down. Some clinicians use a more conservative cutoff of 52%.
Other potential side effects include acne, oily skin, fluid retention, breast tenderness, and sleep apnea. Prostate health also requires monitoring, though current evidence does not show that testosterone causes prostate cancer. Regular check-ins typically involve blood work every few months in the first year, then once or twice a year after that.
The Impact on Fertility
This is one of the most important and most overlooked consequences of testosterone therapy. Taking testosterone from an outside source signals the brain to shut down its own production, which simultaneously shuts down sperm production. The result is severely reduced sperm counts or, in many cases, a complete absence of sperm. For men who want to father children, starting testosterone therapy without understanding this risk can be a serious problem.
The good news is that the effect is usually reversible. A 2006 analysis of clinical trials found that after stopping testosterone, 67% of men recovered normal sperm counts within six months, 90% within a year, and 100% within two years. Recovery can be accelerated with medications that stimulate the body’s own hormone production, though even with treatment, it takes an average of four to five months to see improvement. Men who are considering having children should discuss alternative treatments with their doctor before starting testosterone.