Low testosterone develops when something disrupts the body’s ability to produce the hormone, whether that’s aging, excess body fat, a medical condition, or certain medications. The American Urological Association defines low testosterone as a total level consistently below 300 ng/dL, confirmed on at least two blood draws taken in the early morning. Understanding the different causes helps explain why this condition affects such a wide range of men, from otherwise healthy 35-year-olds to older adults managing chronic illness.
The Natural Decline With Age
Testosterone levels in most men begin dropping around age 40, declining at an average rate of just over 1% per year. Unlike menopause in women, this isn’t a sudden shift. It’s a slow, steady slide that some men barely notice while others feel significantly. Over two decades, that gradual loss can add up to a 20% or greater reduction from peak levels. Some men maintain high testosterone well into old age, so aging alone doesn’t guarantee you’ll develop symptoms. But it does make every other risk factor on this list hit harder.
When the Testicles Are the Problem
The most direct cause of low testosterone is damage to or dysfunction of the testicles themselves. This is sometimes called primary hypogonadism, and it means the testicles can’t produce adequate testosterone even when they’re receiving the right signals from the brain.
Several conditions fall into this category. Klinefelter syndrome, a genetic condition where a man is born with an extra X chromosome, is one of the more common congenital causes. Physical injuries to the testicles, undescended testicles that weren’t corrected in childhood, and mumps infection that spreads to the testicles (mumps orchitis) can all cause permanent damage. Cancer treatments, particularly chemotherapy and radiation directed at or near the groin, can impair testosterone production temporarily or permanently. Hemochromatosis, a condition that causes iron to build up in the body, can also cause the testicles to fail.
When the Brain’s Signaling Breaks Down
Your brain controls testosterone production through a chain of command. The hypothalamus signals the pituitary gland, which releases hormones that tell the testicles to make testosterone. If anything disrupts this signaling chain, the testicles may be perfectly healthy but still underperform because they never get the message to ramp up production.
Pituitary tumors are one of the more common culprits. Even benign growths can suppress the hormones that drive testosterone production, and treating those tumors with surgery or radiation can cause further damage to the gland. Kallmann syndrome, a rare condition present from birth, causes abnormal development of the hypothalamus and often comes with an impaired sense of smell. Inflammatory conditions like sarcoidosis and tuberculosis can also interfere with this brain-to-testicle communication. HIV/AIDS can affect the system at multiple levels, impairing the hypothalamus, pituitary, and testicles simultaneously.
Obesity and the Estrogen Conversion Problem
Carrying excess body fat is one of the most common and most reversible causes of low testosterone. Fat tissue contains an enzyme that converts testosterone into estrogen. The more body fat you have, the more of this conversion takes place. The result is a double hit: your circulating testosterone drops while your estrogen levels rise. That elevated estrogen then signals the brain to slow down testosterone production even further, creating a cycle that feeds on itself.
This is why weight loss often produces measurable improvements in testosterone levels without any other intervention. The effect is dose-dependent: the more excess fat you’re carrying, the greater the suppression, and the more dramatic the improvement when you lose it.
Chronic Illness
Type 2 diabetes and chronic kidney disease both independently lower testosterone. Chronic kidney disease disrupts levels of luteinizing hormone, the pituitary signal that tells the testicles to produce testosterone. When that hormone drops, testosterone follows. Diabetes compounds this problem by damaging blood vessels and nerves throughout the body. Liver cirrhosis, obstructive sleep apnea, and chronic inflammatory conditions can all suppress testosterone through overlapping mechanisms. Men managing any of these conditions often have multiple factors driving their levels down at once.
Medications That Suppress Testosterone
Opioid painkillers are among the most potent suppressors of testosterone in everyday medicine. They inhibit the brain’s signaling hormones while also directly reducing testosterone synthesis in the testicles. In one study of men on sustained-release opioid formulations, 74% had abnormally low testosterone levels. Among those taking higher doses (equivalent to 100 mg of methadone daily), every single participant had subnormal sex hormone levels. The effect is dose-dependent: higher opioid intake means greater suppression.
Corticosteroids used for inflammation, some antifungal medications, and certain drugs used to treat prostate conditions can also lower testosterone. If you’ve noticed symptoms that started around the time you began a new medication, that connection is worth exploring with your prescriber.
Anabolic Steroids and Testosterone Supplements
This is one of the most counterintuitive causes. Using synthetic testosterone or anabolic steroids floods the body with androgens, which tells the brain there’s already more than enough. The hypothalamus responds by shutting down its signaling to the pituitary, which in turn stops telling the testicles to produce testosterone. Over time, the testicles shrink and their internal testosterone concentration, normally about 100 times higher than what’s in the bloodstream, plummets.
When someone stops using these substances, the brain’s signaling system doesn’t always bounce back quickly. Some men experience prolonged suppression that takes months or even years to resolve. The longer and heavier the use, the greater the risk that natural production won’t fully recover.
Sleep Deprivation
Testosterone production follows a daily rhythm, with most of it happening during sleep. A meta-analysis looking at the relationship between sleep loss and testosterone found that total sleep deprivation (staying awake for 24 hours or more) significantly reduces testosterone levels. Shorter bouts of poor sleep, like getting five hours a night for several days, showed a less clear-cut effect in pooled research, though individual studies have found reductions. The practical takeaway: consistently poor sleep probably chips away at your levels over time, even if a single bad night doesn’t cause a dramatic drop.
Environmental Chemical Exposure
Certain industrial chemicals that act as hormone disruptors can measurably lower testosterone. Phthalates, found in plastics, food packaging, and personal care products, interfere with the enzymes that produce testosterone. Large population studies show that men with the highest phthalate exposure have roughly 12 to 15% lower testosterone levels than men with the lowest exposure. BPA, another chemical common in plastics and can linings, has been linked to a 10 to 15% decrease in testosterone in cross-sectional studies. Animal research suggests even larger effects, with some experiments showing up to 40% reductions in testosterone from phthalate exposure. While individual chemical exposures may seem modest, the cumulative effect of daily contact with multiple hormone-disrupting compounds is harder to quantify.
How Low Testosterone Is Confirmed
A single blood test isn’t enough for a diagnosis. Guidelines call for at least two separate blood draws, both taken in the early morning when testosterone peaks, both showing levels below 300 ng/dL. Using the same lab and testing method for both draws reduces the chance of misleading results. If levels come back low, the next step is usually checking the brain’s signaling hormones (LH and FSH) to figure out whether the problem originates in the testicles or in the brain’s command center. That distinction shapes the entire treatment approach.