Low testosterone, clinically called hypogonadism, is when a man’s body produces less testosterone than it needs to function normally. The American Urological Association defines it as a total testosterone level below 300 ng/dL on a blood test. Testosterone plays a central role in muscle maintenance, bone strength, sex drive, and sperm production, so when levels drop too low, the effects can show up across your entire body.
What Testosterone Does in Your Body
Testosterone is best known as a sex hormone, but it works more like a master regulator that touches multiple systems at once. It drives the development of male physical traits during puberty, including facial hair, a deeper voice, and increased muscle mass. After puberty, it keeps those systems running. It promotes protein synthesis in muscle cells, which is why low levels often lead to noticeable loss of muscle size and strength. It directly influences the cells responsible for building and maintaining bone, helping bones reach their peak density during adolescence and preventing bone loss throughout adulthood.
Testosterone also regulates sex drive by acting on brain regions involved in sexual motivation and arousal. It’s essential for erectile function and for spermatogenesis, the process that produces sperm. Beyond the physical, testosterone influences mood, energy levels, and cognitive sharpness. When levels fall below what your body needs, you can feel the effects in ways that seem unrelated until you connect them.
How Low Testosterone Feels
Symptoms tend to develop gradually, which makes them easy to dismiss as stress or aging. The most common signs fall into three categories.
Sexual symptoms are often the first thing men notice: reduced sex drive, difficulty getting or maintaining erections, and lower semen volume. These tend to be what prompts a doctor visit.
Physical changes accumulate over months or years. You might notice increased body fat (especially around the midsection), loss of muscle mass even if your activity level hasn’t changed, decreased body hair, and fatigue that doesn’t improve with rest. Over time, reduced bone density raises the risk of fractures, particularly in the hips and spine.
Mental and emotional shifts include difficulty concentrating, depressed mood, irritability, and a general sense of reduced motivation. Some men describe it as feeling “flat,” lacking the drive or sharpness they’re used to.
No single symptom confirms low testosterone on its own. Many of these overlap with depression, thyroid problems, sleep disorders, and other conditions. A blood test is the only way to know for sure.
What Causes It
Low testosterone falls into two broad categories depending on where the problem originates. In primary hypogonadism, the testicles themselves aren’t producing enough testosterone. This can result from genetic conditions like Klinefelter syndrome, physical injury to the testicles, cancer treatment involving radiation or chemotherapy, or undescended testicles.
In secondary hypogonadism, the testicles are capable of producing testosterone, but the brain isn’t sending the right signals. The hypothalamus and pituitary gland normally release hormones that tell the testicles to ramp up production. When that signaling chain breaks down, testosterone output drops even though the testicles are physically fine. Causes include pituitary tumors, head injuries, certain medications (especially opioids), and significant weight gain.
The Obesity Connection
Obesity is one of the strongest modifiable risk factors for low testosterone. Fat tissue contains an enzyme called aromatase that converts testosterone into estrogen. The more body fat you carry, the more testosterone gets converted, and the elevated estrogen further suppresses the brain’s signals to produce more. This creates a feedback loop: low testosterone makes it easier to gain fat, and more fat drives testosterone even lower. Men with obesity also show reduced responsiveness in the cells that produce testosterone, even when the brain is sending appropriate signals. Insulin resistance, metabolic syndrome, and type 2 diabetes are all closely linked to lower testosterone levels.
How Testosterone Declines With Age
Testosterone levels begin a gradual decline starting around age 35. In men between 40 and 70, total testosterone drops at roughly 0.4% per year, while free testosterone (the portion your body can readily use) declines faster at about 1.3% per year. This means a man in his late 60s might have noticeably lower levels than he did at 30, but that alone doesn’t necessarily mean he has clinically low testosterone. Age-related decline is normal. Low testosterone as a diagnosis requires both a blood level below 300 ng/dL and symptoms that are affecting quality of life.
How It’s Diagnosed
Diagnosis requires a blood test, but the timing and repetition matter. Testosterone levels naturally fluctuate throughout the day, peaking in the early morning and dropping by afternoon. For men under 45, blood should be drawn in the early morning to capture the highest point. Men 45 and older have a flatter daily curve, so testing before 2 PM is acceptable.
A single low reading isn’t enough for a diagnosis. Up to 30% of men who test low on their first draw will have a normal result when tested again. Guidelines recommend confirming with a second morning blood test before moving forward with any treatment.
The standard test measures total testosterone, which includes both the testosterone bound to proteins in your blood and the small fraction circulating freely. Most testosterone is bound to a protein called SHBG, and bound testosterone isn’t as readily available for your body to use. If your total testosterone is borderline or your doctor suspects SHBG levels are unusually high or low (common with obesity, liver disease, or aging), a free testosterone test can give a clearer picture of what’s actually available to your tissues.
Treatment Options
Testosterone replacement therapy (TRT) is the primary medical treatment. It comes in several forms: injections given every one to two weeks, topical gels applied daily to the skin, adhesive patches, pellets implanted under the skin that release testosterone over two to three months, and oral capsules. Each method has tradeoffs in convenience, consistency of testosterone levels, and cost. Gels provide steady daily levels but require care to avoid transferring the hormone to others through skin contact. Injections are less frequent but can cause levels to spike and then drop between doses.
TRT typically improves energy, sex drive, erectile function, mood, and muscle mass over weeks to months. But it carries real risks that deserve serious consideration before starting.
TRT and Fertility
This is one of the most important and least understood consequences of testosterone therapy. Supplementing testosterone from outside the body shuts down the brain’s signals that drive sperm production. Within four to six months, 64% to 75% of men on TRT become azoospermic, meaning they produce zero sperm. One large study found suppression rates as high as 93% to 98%.
Stopping TRT does allow recovery in most cases. About 64% to 84% of men regain sperm production, with a median recovery time of roughly 110 days. However, some men take up to two years to recover, and older men or those who used TRT for extended periods are less likely to bounce back fully. In one study, 30% of men who had used testosterone could not achieve adequate sperm counts even after a year of recovery treatment.
If you’re considering fathering children now or in the future, this is a conversation to have before starting TRT, not after. Alternative treatments exist that can raise testosterone levels while preserving fertility.
Lifestyle Factors That Affect Levels
Not every case of low testosterone requires medication. Because obesity, poor sleep, and physical inactivity all suppress testosterone production, addressing these can meaningfully raise levels in some men. Losing excess body fat reduces the conversion of testosterone to estrogen in fat tissue and improves the brain’s hormonal signaling. Resistance training in particular stimulates testosterone production. Improving sleep quality matters too, since most daily testosterone release happens during deep sleep.
These changes won’t overcome a genetic condition or significant testicular damage, but for men whose low testosterone is driven by weight gain, sedentary habits, or metabolic dysfunction, they can be the difference between needing lifelong therapy and not.