Low testosterone has many causes, ranging from normal aging to chronic disease, excess body fat, medications, and sleep habits. A total testosterone level below 300 ng/dL, confirmed on at least two early-morning blood draws, is the clinical threshold used to diagnose the condition. Understanding what drives testosterone down helps clarify whether the cause is reversible or something that needs ongoing management.
Normal Age-Related Decline
Testosterone levels peak in early adulthood and then drop by about 1% per year after age 30. That slow decline is a normal part of aging, not a disease. Over decades, though, the cumulative loss adds up. A man in his 60s or 70s may have significantly less testosterone than he did at 25, even without any underlying condition. The key distinction is whether symptoms accompany the decline. Many men with modestly lower levels feel fine and don’t need treatment.
Problems With the Testes
The most direct cause of low testosterone is damage to or dysfunction of the testes themselves, since that’s where testosterone is produced. Several conditions can impair the testes:
- Klinefelter syndrome: A genetic condition where a male is born with an extra X chromosome. It’s one of the most common genetic causes of low testosterone and often isn’t diagnosed until puberty or adulthood.
- Undescended testes: When one or both testes don’t descend into the scrotum before birth, long-term testosterone production can be affected even after surgical correction.
- Mumps orchitis: A mumps infection that spreads to the testes can permanently damage the tissue responsible for making testosterone.
- Testicular torsion or injury: Physical damage from twisting, trauma, or surgery can reduce the testes’ ability to produce hormones.
- Testicular cancer and its treatment: Both the cancer itself and treatments like surgery or radiation can lower testosterone output.
- Varicocele: Enlarged veins in the scrotum raise local temperature and can impair testosterone production over time.
When the testes are the root problem, the brain’s hormone signals are usually working normally. It’s the factory that’s broken, not the control center.
Problems With the Brain’s Hormone Signals
Testosterone production depends on a chain of signals that starts in the brain. The hypothalamus tells the pituitary gland to release hormones that then instruct the testes to make testosterone. If anything disrupts that signaling chain, testosterone drops even though the testes themselves are healthy.
Pituitary tumors are a common culprit. Even benign growths can crowd out the cells responsible for sending those signals. Treatment for brain tumors, including surgery and radiation near the pituitary, can cause lasting damage to this system. Kallmann syndrome, a rare condition present from birth, involves abnormal development of the hypothalamus. People with Kallmann syndrome often also have an impaired sense of smell or difficulty distinguishing red from green.
Inflammatory diseases like sarcoidosis and tuberculosis can also interfere with hormone signaling. HIV/AIDS affects testosterone through multiple pathways, disrupting the hypothalamus, the pituitary, and the testes simultaneously.
How Excess Body Fat Lowers Testosterone
Obesity is one of the most common and most reversible causes of low testosterone. Fat tissue contains an enzyme called aromatase that converts testosterone into estrogen. The more body fat you carry, the more aromatase activity there is, and the more testosterone gets converted. The resulting rise in estrogen then signals the brain to slow down its hormone output, creating a feedback loop: more fat leads to less testosterone, and less testosterone makes it easier to gain more fat.
This pattern shows up clearly in men with type 2 diabetes and metabolic syndrome. Between 25% and 40% of men with type 2 diabetes have clinically low testosterone levels. The relationship runs both directions. Low testosterone promotes insulin resistance and belly fat, while those metabolic problems further suppress testosterone. Losing weight can partially or fully reverse this type of hormone decline, since reducing fat tissue lowers aromatase activity and allows the brain’s signaling system to recover.
Chronic Inflammation
When the body is in a state of ongoing inflammation, whether from autoimmune disease, chronic infection, or metabolic stress, the immune system’s signaling molecules can directly suppress testosterone production. This happens through two routes: inflammation damages the reproductive tissue itself, and it disrupts the brain’s hormonal control system. Inflammatory conditions also tend to increase oxidative stress, which further impairs the cells in the testes that manufacture testosterone.
This helps explain why men with chronic illnesses often have low testosterone even when there’s no obvious problem with their testes or pituitary gland. The inflammation acts as an invisible drag on the entire hormone system.
Medications That Suppress Testosterone
Several widely used medications can significantly lower testosterone levels, and the effect is often underrecognized.
Opioid painkillers are the biggest offender. They suppress the brain’s hormone signaling within hours of use, and the suppression deepens with higher doses over longer periods. Among patients taking the equivalent of 100 to 200 mg of oral morphine daily for more than a month, anywhere from 50% to 100% develop some degree of testosterone deficiency. This happens with all forms of opioids, including patches and sustained-release formulations. Buprenorphine, used in addiction treatment, appears to have a milder effect on testosterone than full-strength opioids, though long-term data is limited.
Corticosteroids (anti-inflammatory steroids prescribed for conditions like asthma and arthritis) and anabolic steroids used for bodybuilding both suppress the brain’s signal to the testes. With anabolic steroids, the external testosterone tells the brain to shut down its own production. When someone stops using them, it can take months for natural production to restart, and in some cases it never fully recovers.
Certain hormonal treatments, including those used for prostate cancer, are designed to suppress testosterone deliberately.
Sleep Deprivation
Sleep has a surprisingly powerful effect on testosterone. Most daily testosterone release happens during sleep, so cutting sleep short directly reduces production. A study at the University of Chicago found that healthy young men who slept only five hours per night for one week saw their testosterone levels drop by 10% to 15%. That’s a meaningful decline from just modest sleep loss, and it happened in men with no underlying health problems.
Chronic short sleep, shift work, and untreated sleep apnea can all keep testosterone persistently low. Sleep apnea is especially relevant because it fragments sleep throughout the night, preventing the deep sleep phases when testosterone is released. Treating sleep apnea often improves testosterone levels without any other intervention.
Environmental Chemical Exposure
A growing body of evidence links certain industrial chemicals to lower testosterone. Phthalates, found in plastics, food packaging, and personal care products, are among the most studied. Research on males aged 12 to 19 in the United States found that several phthalate byproducts in urine were inversely associated with testosterone levels, meaning higher chemical exposure correlated with lower hormones. When researchers modeled the combined effect of 17 different endocrine-disrupting chemicals together, the collective exposure was associated with reduced total testosterone, free testosterone, and bioavailable testosterone. Animal studies have shown that exposure to multiple chemicals at once, such as phthalates and BPA combined, suppresses testosterone more than either chemical alone.
These exposures are difficult to eliminate entirely since the chemicals are widespread in everyday products. Reducing use of plastic food containers, choosing fragrance-free products, and avoiding heating food in plastic can lower your exposure, though the individual impact on testosterone levels is hard to quantify.
When Multiple Causes Overlap
In practice, low testosterone rarely comes from a single cause. A man in his 50s who carries extra weight, takes an opioid for back pain, sleeps poorly, and has type 2 diabetes has four separate forces pushing his testosterone down at once. Each one may be modest on its own, but together they can produce a level well below 300 ng/dL. This is also why treatment often involves addressing the reversible factors first. Losing weight, improving sleep, and reviewing medications with a doctor can raise testosterone substantially before hormone replacement is ever considered.