Men take testosterone when their bodies stop making enough of it on their own, a condition called hypogonadism or testosterone deficiency. The most common threshold doctors use to diagnose low testosterone is a blood level below 300 ng/dL, though some medical societies set the cutoff anywhere from 230 to 350 ng/dL. When levels fall below that range and symptoms are present, testosterone therapy can restore sex drive, energy, mood, bone strength, and muscle mass.
What Causes Low Testosterone
The FDA approves testosterone products only for men who have low levels tied to a specific medical condition. Those conditions fall into two categories: problems with the testicles themselves, or problems with the brain structures that tell the testicles to produce testosterone.
Direct testicular causes include genetic conditions like Klinefelter syndrome, physical injury, and damage from chemotherapy or radiation. These are grouped under “primary hypogonadism,” meaning the testicles simply can’t produce enough hormone even when they receive the right signals.
The second category involves the hypothalamus and pituitary gland, two brain structures that regulate testosterone production. When these are disrupted, the testicles never get the signal to ramp up. Causes include pituitary tumors, head injuries, surgery near the brain, long-term opioid use, high-dose steroid medications, severe nutritional changes (rapid weight gain or loss), chronic infections, iron overload disorders, and even prolonged severe stress. Long-term opioid use is an increasingly recognized cause that many men don’t connect to their symptoms.
How Common It Is by Age
Testosterone naturally declines with age, but not every man with lower levels qualifies as deficient. The Baltimore Aging Longitudinal Study found that roughly 10% of men in their 50s had low testosterone, rising to 20% by their 60s and 70% by their 70s. Similar patterns appeared in large American, European, and Asian studies.
However, low testosterone on a blood test isn’t the same as clinical hypogonadism. When researchers in the European Male Ageing Study required both low blood levels and sexual symptoms to make the diagnosis, the numbers were much smaller: 0.1% of men aged 40 to 49, 0.6% in their 50s, 3.2% in their 60s, and 5.1% in their 70s. A large Chinese study of over 6,000 men using the same criteria found about 8% prevalence overall. The takeaway is that age-related decline is common, but the kind of deficiency that warrants treatment is far less so.
Symptoms That Lead Men to Get Tested
The symptoms that typically bring men to a doctor overlap with many other conditions, which is part of why low testosterone often goes unrecognized. The most consistent red flags are sexual: reduced desire, fewer spontaneous erections, and difficulty with erectile function. These sexual symptoms are actually what most diagnostic guidelines rely on to distinguish true hypogonadism from a simple dip in blood levels.
Beyond sexual health, men with testosterone deficiency commonly report persistent fatigue, low motivation, depressed mood, difficulty concentrating, and a general loss of vitality. Physical changes can include increased body fat (especially around the midsection), loss of muscle mass and strength, and reduced body hair. Over time, low testosterone also weakens bones significantly, which is one of the less visible but more dangerous consequences.
What Testosterone Therapy Improves
Treatment doesn’t produce instant results. Different symptoms respond on different timelines, and knowing what to expect helps set realistic goals.
Sexual interest is one of the first things to improve, typically within three weeks, plateauing around six weeks. Erections and ejaculation may take up to six months to fully respond. Mood improvements, including reduced depressive symptoms, become noticeable after three to six weeks but continue building for up to 30 weeks. General quality of life tends to shift within three to four weeks.
Body composition changes are slower. Fat loss and muscle gain begin around 12 to 16 weeks, stabilize between 6 and 12 months, and can continue marginally for years. Bone density follows an even longer arc. A study following 72 men with hypogonadism for up to 16 years found that the biggest jump in spinal bone density happened in the first year of treatment, with previously untreated men seeing their bone density rise from about 95 to 120 mg/cm³. Long-term treatment maintained bone density within the normal age-related range for all 72 men, regardless of whether their deficiency originated in the testicles or the brain. For men at risk of osteoporosis, this is one of the most important benefits of treatment.
An eight-month study of men with testosterone deficiency found that treatment significantly improved erectile function scores and lowered both depression and aging symptom scores compared to an untreated control group. Interestingly, cognitive function scores didn’t improve across the board, but men who started with measurable cognitive impairment did see significant gains.
How It’s Taken
Testosterone comes in several forms, and each has a different routine and hormone profile.
- Topical gels are applied daily to the skin, usually the shoulders or upper arms. They maintain relatively steady hormone levels but require daily attention and care to avoid transferring the gel to others through skin contact.
- Injections are typically given weekly, using 100 to 200 mg of testosterone cypionate or enanthate. Injections produce the highest overall testosterone levels of the three main options, though levels fluctuate more between doses.
- Implantable pellets are placed under the skin every three to six months (usually 10 to 14 small pellets per session), targeting a peak level of 500 to 800 ng/dL. They offer the longest gap between treatments but require a minor in-office procedure each time.
All three methods are effective at raising testosterone. The practical differences come down to convenience, cost, how steady you want your levels to be, and your tolerance for needles or daily routines.
Risks and Who Shouldn’t Take It
Testosterone therapy is not appropriate for everyone with a low number on a blood test. It’s contraindicated in men with untreated prostate cancer or breast cancer. Men considered high-risk for prostate cancer, including those with a first-degree relative who had it or African-American men with an elevated PSA level above 3 ng/mL, are also generally excluded.
One of the most common side effects is an increase in red blood cell production. This is a normal biological response to testosterone, but if the hematocrit (the proportion of red blood cells in your blood) rises above 54%, treatment is paused until levels normalize. Unchecked, thickened blood raises the risk of clotting. This is why regular blood monitoring is a standard part of treatment, typically every few months in the first year.
Testosterone also increases estrogen levels in all delivery methods, since the body naturally converts some testosterone into estrogen. Injectable forms tend to produce the most sustained estrogen elevation. For most men this doesn’t cause noticeable problems, but it’s one of the values doctors track over time.
Why Some Men Take It Without a Diagnosis
Not every man taking testosterone has a clinical diagnosis. Some pursue it for anti-aging purposes, athletic performance, or general vitality, often through clinics that operate outside traditional medical guidelines. The FDA has specifically noted that testosterone products are approved only for men with documented low levels tied to a medical condition, not for age-related decline alone. The distinction matters because the risk-benefit calculation changes substantially when you’re adding testosterone to a body that’s already producing adequate amounts. Higher-than-normal levels amplify side effects like red blood cell overproduction and hormonal imbalance without the clear symptom relief that deficient men experience.