The single most common cause of low testosterone in men is aging, with levels dropping roughly 1% per year after age 40. But aging alone rarely tells the whole story. In most cases, low testosterone results from a combination of age-related decline and one or more contributing factors, with obesity being the most significant accelerator. A total testosterone level below 300 ng/dL is the widely accepted threshold for diagnosis, and an estimated 2% to 30% of men between 40 and 79 are affected, depending on how strictly it’s measured.
How Testosterone Production Breaks Down
Your body produces testosterone through a chain of signals. The brain’s hypothalamus sends a hormonal signal to the pituitary gland, which then tells the testes to produce testosterone. A problem anywhere in this chain can lower your levels, and where the breakdown happens determines what type of low testosterone you have.
When the testes themselves are damaged or dysfunctional, it’s called primary hypogonadism. The brain is sending the right signals, but the testes can’t respond. This happens with physical injury, infection, certain cancer treatments, or genetic conditions. When the problem starts in the brain, either at the hypothalamus or pituitary gland, it’s called secondary hypogonadism. The testes are capable of making testosterone, but they’re not receiving adequate instructions to do so. Secondary hypogonadism is far more common in the general population and is the type most often driven by obesity, metabolic problems, and lifestyle factors.
Why Obesity Is the Biggest Modifiable Cause
Excess body fat, particularly around the midsection, actively lowers testosterone through a self-reinforcing cycle. Fat tissue contains high levels of an enzyme called aromatase, which converts testosterone into estrogen. The more fat you carry, the more testosterone gets converted. That rising estrogen then signals the brain to reduce its hormonal instructions to the testes, which means even less testosterone gets produced. Less testosterone, in turn, makes it easier to gain more fat, because testosterone normally helps block fat cells from absorbing and storing fatty acids. This creates a downward spiral: more fat leads to less testosterone, which leads to more fat.
The damage goes deeper than hormonal conversion. Low testosterone impairs the ability of muscle tissue to process energy efficiently. Since muscle accounts for up to 70% of the body’s insulin sensitivity, this contributes to insulin resistance, a hallmark of metabolic syndrome and type 2 diabetes. The connection between diabetes and low testosterone is striking: roughly 37% of men with type 2 diabetes have testosterone levels below the diagnostic threshold.
The Role of Aging
Age-related testosterone decline is gradual and inevitable. After 40, most men lose about 1% of their total testosterone per year. By 60 or 70, that cumulative drop can be significant, but the rate is slow enough that many men never develop symptoms. Aging lowers testosterone partly because the testes become less responsive over time and partly because the brain’s signaling becomes less precise. Men who stay lean, active, and healthy tend to experience a milder decline than those who don’t, which is why researchers view aging as a baseline factor that other causes build on top of.
Sleep, Stress, and Daily Habits
Sleep is one of the most underappreciated factors in testosterone production. A study published in JAMA found that healthy young men who slept only five hours per night for one week saw their daytime testosterone levels drop by 10% to 15% compared to when they slept eight hours or more. The effect was most pronounced in the afternoon and evening. At least 15% of the U.S. working population routinely gets this little sleep, which means millions of men may be suppressing their testosterone through sleep habits alone.
Chronic stress raises cortisol, which competes with testosterone production at multiple levels. Heavy alcohol use and sedentary behavior also contribute. None of these factors typically cause severe deficiency on their own, but stacked together, or combined with aging and weight gain, they can push levels below the threshold where symptoms appear.
Genetic Conditions
Klinefelter syndrome is the most common sex-chromosome disorder in men and a well-known genetic cause of low testosterone. Men with this condition carry an extra X chromosome (47,XXY instead of the typical 46,XY). Starting in fetal development, the testes gradually lose function, and by adulthood, most men with Klinefelter syndrome have testosterone in the low-normal or below-normal range. The condition affects roughly 1 in 600 men, though many go undiagnosed because symptoms can be subtle. Characteristic signs include tall stature, reduced body hair, and small testes, but the presentation varies widely.
Medications That Lower Testosterone
Several commonly prescribed medications can suppress testosterone production. Opioid painkillers are among the most significant offenders, as they directly reduce the brain’s hormonal signaling to the testes. Long-term corticosteroids, sometimes prescribed for autoimmune conditions or chronic inflammation, have a similar effect. Ironically, anabolic steroids used for muscle building also cause low testosterone: when synthetic testosterone floods the body, the brain shuts down its own production signals. Once the steroids are stopped, natural production can take months or even years to recover, and in some cases it never fully returns.
Environmental Chemicals
A growing body of evidence links endocrine-disrupting chemicals to declining testosterone levels in men. These chemicals are found in plastics (BPA, phthalates), pesticides, flame retardants, and everyday products like soaps and toothpastes containing triclosan. They interfere with testosterone production through several mechanisms, including disrupting the enzymes responsible for building steroid hormones and mimicking estrogen in the body. Population-level studies have found correlations between higher exposure to these chemicals and lower sperm counts and testosterone levels, with men in heavily agricultural or industrial areas showing more pronounced effects. While individual exposure on any given day is small, the concern centers on decades of cumulative, low-level contact with multiple chemicals simultaneously.
How Low Testosterone Is Identified
The American Urological Association defines low testosterone as a total level below 300 ng/dL, a threshold shared by most major medical societies worldwide (though some use cutoffs ranging from 230 to 350 ng/dL). Because testosterone fluctuates throughout the day, peaking in the morning, testing is typically done with an early morning blood draw. Most guidelines require at least two separate low readings before confirming a diagnosis, since a single result can be thrown off by a bad night’s sleep, recent illness, or normal daily variation.
Symptoms that commonly prompt testing include persistent fatigue, reduced sex drive, difficulty maintaining erections, loss of muscle mass, increased body fat, and mood changes like irritability or low motivation. None of these symptoms is unique to low testosterone, which is why the blood test matters. Some men have levels below 300 ng/dL with minimal symptoms, while others feel significant effects at levels closer to the borderline.