Low testosterone has a wide range of causes, from normal aging to chronic diseases, medications, and genetic conditions. About 35% of men over 45 have clinically low levels, defined by the American Urological Association as a total testosterone below 300 ng/dL. Understanding what’s behind the drop matters because some causes are reversible and others require long-term management.
How Testosterone Production Works
Testosterone production depends on a chain of signals between your brain and your testes. A region of the brain called the hypothalamus releases a hormone that tells the pituitary gland to produce two signaling hormones. One of those signals then travels to the testes, telling specialized cells there to produce testosterone. A break anywhere in this chain leads to low levels, and where the break occurs determines the type of problem and how it’s treated.
When the testes themselves are damaged or dysfunctional, it’s called primary hypogonadism. When the brain’s signaling system is the issue, it’s called secondary hypogonadism. Some men have a combination of both.
Aging and Gradual Decline
Testosterone levels peak in early adulthood and then drop by roughly 1% per year after age 30. That may sound small, but by 55 or 60 the cumulative decline is significant. This gradual slide is normal biology, not a disease, though it can produce noticeable symptoms like reduced energy, lower sex drive, and loss of muscle mass. Whether age-related decline alone warrants treatment is a question you’d work through with a doctor based on symptoms and confirmed lab results.
Obesity and Type 2 Diabetes
Excess body fat is one of the most common and most reversible drivers of low testosterone. Studies from the U.S., U.K., and Australia consistently show that 30 to 50% of aging, obese men with type 2 diabetes have low total or free testosterone. Among men who are both over 65 and obese, that number climbs to roughly two-thirds.
The mechanism is straightforward. Fat tissue contains an enzyme that converts testosterone into estrogen. The more fat you carry, the more of that conversion happens. Rising estrogen levels then signal the brain to dial back its production commands, creating a feedback loop: more fat leads to less testosterone, and less testosterone makes it easier to accumulate fat. The good news is that meaningful weight loss can partially or fully reverse this effect. Even modest reductions in body fat have been shown to improve testosterone levels.
Medications That Suppress Testosterone
Several common drug classes lower testosterone by interfering with the brain-to-testes signaling chain. Opioids are the most well-documented culprit. A systematic review and meta-analysis found that men using opioids regularly had testosterone levels suppressed by nearly 50%, regardless of whether they were taking prescription painkillers or illicit opioids. This isn’t limited to one type of opioid; the suppression occurs across the entire drug class.
Corticosteroids (often prescribed for inflammation or autoimmune conditions) also dampen the brain’s hormone signals. Certain hormonal therapies, some antidepressants, and anabolic steroids can cause the same effect. With anabolic steroids, the irony is sharp: using synthetic testosterone or related compounds shuts down your body’s own production, and recovery after stopping can take months or longer.
Genetic and Structural Conditions
Some men are born with conditions that make low testosterone inevitable without treatment. The most common is Klinefelter syndrome, where a male has an extra X chromosome. Men with this condition typically have small testes that produce significantly less testosterone than normal. It affects roughly 1 in 600 males, though many go undiagnosed until they encounter fertility problems or unexplained fatigue in adulthood.
Iron overload disorders can also damage the testes or the pituitary gland over time, gradually impairing testosterone production. Undescended testes, physical injury to the testes, and mumps infection that spreads to the testes during or after puberty are other structural causes of primary hypogonadism.
On the brain side, pituitary tumors (usually benign) can compress the gland and disrupt its signaling. Kallmann syndrome, a rare condition present from birth, affects development of the hypothalamus and often comes alongside an impaired sense of smell or red-green color blindness.
Sleep Deprivation
Your body produces most of its testosterone during sleep, so severe sleep loss can drag levels down. A meta-analysis on this topic found that total sleep deprivation of 24 hours or more significantly reduces testosterone. Short-term partial sleep restriction, like getting five or six hours for a few nights, did not show a statistically significant effect in pooled data, though individual responses vary. The practical takeaway: occasional poor sleep probably won’t tank your levels, but chronically sleeping very little or pulling frequent all-nighters can.
Environmental Chemicals
A growing body of evidence links certain industrial chemicals to lower testosterone. Phthalates, found in plastics, food packaging, personal care products, and vinyl flooring, show some of the strongest negative associations. A study analyzing 17 endocrine-disrupting chemicals in U.S. males aged 12 to 19 found that phthalate exposure was inversely associated with total testosterone, free testosterone, and bioavailable testosterone. Combined exposure to multiple chemicals at once appeared to have a cumulative effect, reducing several measures of testosterone simultaneously.
These chemicals are difficult to avoid entirely because they’re embedded in everyday products. Reducing exposure through glass food storage, choosing fragrance-free personal care products, and minimizing plastic food contact during heating are practical steps, though the magnitude of benefit for any individual is hard to quantify.
Other Medical Causes
Chronic kidney disease, liver cirrhosis, and HIV/AIDS can all lower testosterone through various mechanisms, including direct damage to the testes and disruption of brain signaling. Significant physical stress, such as major surgery or critical illness, temporarily suppresses production as the body redirects resources. Testosterone levels typically recover once the acute illness resolves, which is why guidelines recommend against testing during hospitalization or acute sickness.
Excessive alcohol use damages both the testes and the liver’s ability to manage hormone metabolism. Chronic stress raises cortisol, which can suppress the brain signals that drive testosterone production, though the effect from everyday psychological stress is generally modest compared to disease or medication-related causes.
How Low Testosterone Is Confirmed
Because testosterone levels fluctuate throughout the day, accurate testing requires a morning blood draw, ideally before 10 a.m. or within three hours of waking. You should be fasting, sleeping on a normal schedule (not during jet lag or shift changes), and not acutely sick. Even under ideal conditions, two measurements taken on the same person can differ by up to 30% due to normal biological variation. That’s why guidelines require at least two separate low readings before making a diagnosis.
If your level comes back below 300 ng/dL on two properly timed morning tests and you have symptoms like persistent fatigue, reduced libido, erectile dysfunction, or unexplained loss of muscle and bone density, additional testing can help pinpoint whether the problem is in the testes, the brain’s signaling, or a reversible factor like obesity or medication.