People take testosterone for a range of reasons, but most fall into two broad categories: replacing testosterone the body no longer makes enough of, or aligning the body’s physical characteristics with a person’s gender identity. A smaller number use it to counteract muscle loss during severe illness or weight loss conditions like HIV. Whatever the reason, testosterone therapy changes the body in measurable ways, and understanding those changes helps explain why millions of people use it.
Low Testosterone Is the Most Common Reason
The most frequent medical reason for testosterone therapy is a condition called hypogonadism, where the body produces too little testosterone on its own. This can originate in the testes (primary hypogonadism) or in the brain’s signaling system that tells the testes to produce testosterone (secondary hypogonadism). Either way, the result is the same: testosterone levels drop below the threshold where the body functions normally.
The American Urological Association defines low testosterone as a total level below 300 ng/dL, measured through a morning fasting blood draw on at least two separate occasions. The treatment goal is typically to bring levels into the 450 to 600 ng/dL range, which sits in the middle of what most labs consider normal. Diagnosis requires both consistently low blood levels and symptoms. A number alone isn’t enough.
Hypogonadism can develop at any age. In younger men, it’s often caused by genetic conditions, injuries to the testes, or pituitary problems. In older men, testosterone naturally declines with age, roughly 1 to 2 percent per year after 30, and some eventually cross into the range where symptoms appear. Conditions like obesity, type 2 diabetes, and chronic opioid use can also suppress production.
What Low Testosterone Actually Feels Like
The symptoms of low testosterone overlap heavily with depression, which is one reason the condition often goes unrecognized. Persistent fatigue, low motivation, irritability, mood swings, difficulty focusing, disrupted sleep, social withdrawal, and a drop in sex drive are common to both. Some people are misdiagnosed with clinical depression when the underlying issue is actually low testosterone.
Beyond mood, low testosterone affects the body in ways that become noticeable over months and years. Muscle mass decreases, body fat increases (particularly around the midsection), bone density drops, and erections become less reliable. Some men notice thinning body hair or breast tissue development. These changes tend to be gradual, which makes them easy to dismiss as normal aging.
What Testosterone Therapy Does for the Body
Once prescribed, testosterone therapy reverses many of these changes by restoring hormone levels to the normal range. The effects span several systems in the body.
Muscle and Fat
Testosterone has a direct effect on lean body mass. In a controlled trial where men underwent severe calorie restriction and intense exercise, those receiving testosterone gained an average of 2.5 kg more lean mass than those on placebo. Fat loss, interestingly, was similar between groups, suggesting testosterone’s primary body composition benefit is protecting and building muscle rather than burning fat directly.
Bone Density
Testosterone plays a key role in maintaining bone strength. Clinical trials have found that therapy increases bone mineral density at the spine and other sites. In the T-Trial Bone Study, men receiving testosterone saw a 7.5% increase in spinal bone density compared to just 0.8% in the placebo group. Whether this translates to fewer fractures is still unclear, though. The largest trial to examine fracture rates, the TRAVERSE study, actually found a slightly higher fracture rate in the testosterone group (3.5% vs. 2.46%), and other studies have produced mixed results.
Sexual Function
Improved libido is one of the earliest and most consistent effects of testosterone therapy. Guidelines from the American College of Physicians note that it can improve sexual function in some men, particularly desire and arousal. Erectile function may improve as well, though men with erectile dysfunction caused by blood vessel problems or nerve damage may still need additional treatment.
One important trade-off: testosterone therapy suppresses sperm production and can shrink the testes. Men who want to have children in the near future are typically advised against starting treatment, or they use alternative approaches that stimulate the body’s own testosterone production while preserving fertility.
Gender-Affirming Hormone Therapy
Testosterone is also prescribed as part of masculinizing hormone therapy for transgender men and some nonbinary individuals. The goal is to produce the physical changes typically driven by male hormones during puberty: a deeper voice, facial and body hair growth, redistribution of body fat, increased muscle mass, and cessation of menstrual periods.
These physical changes serve a psychological purpose. Masculinizing hormone therapy has been shown to improve psychological and social well-being, ease emotional distress related to gender, improve satisfaction with sex, and raise overall quality of life. For many people, the physical alignment between their body and their gender identity is the primary benefit.
How Testosterone Is Taken
Testosterone therapy comes in several forms, and the choice depends on convenience, cost, and how steady the hormone levels need to be. The main options are topical gels, injectable solutions, transdermal patches, nasal gels, and implantable pellets.
Topical gels are applied daily to the shoulders, upper arms, or stomach. They require leaving the gel on the skin for at least five hours before showering or swimming, and there’s a risk of transferring the hormone to others through skin contact. Injections are given every one to two weeks (or longer with slow-release formulations) and produce more pronounced peaks and valleys in hormone levels. Pellets are implanted under the skin every few months and provide the steadiest release, but they require a minor in-office procedure.
Risks and Side Effects
Testosterone therapy is not without downsides, and monitoring is a standard part of treatment.
The most common lab abnormality is a rise in red blood cell concentration, called erythrocytosis. In one study, 57% of patients on testosterone saw their hematocrit (the percentage of blood made up of red cells) climb above 0.46, and 23% exceeded 0.50. Higher body mass index and higher starting hematocrit both predicted greater increases. Thicker blood raises the risk of blood clots, which is why regular blood tests are part of ongoing care.
The TRAVERSE trial, the largest cardiovascular safety study of testosterone therapy, enrolled over 5,200 men with either existing heart disease or high cardiovascular risk. The primary finding was reassuring: rates of heart attack, stroke, and cardiovascular death were nearly identical between testosterone and placebo groups (7.0% vs. 7.3%). However, three specific problems occurred more often with testosterone: atrial fibrillation (3.5% vs. 2.4%), acute kidney injury (2.3% vs. 1.5%), and pulmonary embolism (0.9% vs. 0.5%). Men with a history of blood clots are advised to use testosterone with caution.
Prostate health is another consideration. Testosterone doesn’t appear to cause prostate cancer, but men over 55 are typically screened before starting therapy and monitored during treatment. The Endocrine Society recommends against testosterone in men with active breast or prostate cancer, uncontrolled heart failure, severe sleep apnea, or recent heart attack or stroke within the past six months.
Who Should Not Take Testosterone
Beyond the medical contraindications, testosterone therapy is specifically not recommended as a treatment for type 2 diabetes. While men with diabetes often have lower testosterone levels, clinical guidelines recommend against using testosterone to improve blood sugar control, as the evidence doesn’t support that use. It’s also not appropriate for men trying to conceive, since it suppresses the hormonal signals needed for sperm production. This effect is reversible after stopping therapy, but recovery can take months.
People sometimes seek testosterone for general anti-aging purposes or athletic performance, but these uses fall outside medical guidelines. Testosterone prescribed for someone with normal levels carries the same side effects without the benefit of correcting a deficiency, and supraphysiologic doses used for performance enhancement carry substantially greater risks for blood clots, liver damage, and hormonal disruption.