Low testosterone happens when the body can’t produce enough of the hormone, either because the testes themselves are failing or because the brain isn’t sending the right signals to trigger production. Most medical guidelines define low testosterone as a total level below 300 ng/dL. The causes range from natural aging to obesity, chronic disease, medications, sleep habits, and chemical exposures.
How Testosterone Production Works
Understanding the causes starts with a quick look at the system. Testosterone production depends on a chain of signals. The hypothalamus, a region deep in the brain, releases a hormone that tells the pituitary gland to produce two signaling hormones: LH and FSH. These travel through the bloodstream to the testes, where specialized cells called Leydig cells respond by making testosterone.
A break anywhere in this chain lowers testosterone. When the problem is in the testes themselves, it’s called primary hypogonadism. Testosterone drops, and the brain compensates by flooding the system with more LH and FSH, essentially shouting louder at testes that can’t respond. When the problem is in the brain’s signaling, it’s called secondary hypogonadism. LH and FSH are low or flat, so the testes never get the message to ramp up production. This distinction matters because the underlying cause, and the path to treatment, differs between the two.
Age-Related Decline
Testosterone levels peak in early adulthood and then gradually slide. In men between 40 and 70, total testosterone drops at roughly 0.4% per year. Free testosterone, the portion your body can actually use, declines faster at about 1.3% per year. This is slow enough that many men never notice it. But compounded over decades, the difference is real: a man in his late 60s may have meaningfully less circulating testosterone than he did at 30.
Age-related decline is considered a normal part of biology, not a disease. It becomes clinically relevant only when levels fall below the 300 ng/dL threshold and a man develops symptoms like persistent fatigue, low sex drive, loss of muscle mass, or mood changes. Aging alone rarely causes a dramatic drop; it usually works alongside other factors on this list.
Obesity and Metabolic Health
Excess body fat 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 fat a man carries, the more active this conversion becomes. The resulting rise in estrogen then signals the brain to dial back its production of LH and FSH, which in turn reduces testosterone output from the testes. It’s a feedback loop: more fat means less testosterone, and less testosterone makes it easier to accumulate more fat.
The numbers are striking. About 40% of obese men have below-normal free testosterone levels. Among men who are both obese and have type 2 diabetes, that figure climbs to 50%. Weight loss can partially or fully reverse this pattern by shrinking fat stores and reducing aromatase activity.
Type 2 Diabetes
Type 2 diabetes has a strong, well-documented link to low testosterone that goes beyond weight alone. Research consistently shows that 25% to 40% of men with type 2 diabetes have abnormally low free testosterone, with the vast majority fitting the secondary pattern where the brain’s signaling hormones are inappropriately low. Even after accounting for body weight, diabetes appears to independently suppress testosterone production. Among diabetic men, 44% had low free testosterone compared to 33% of age-matched men without diabetes. Insulin resistance and chronic inflammation likely play roles, though the exact mechanisms are still being studied.
Sleep Deprivation
The body produces most of its testosterone during sleep, particularly during deep sleep phases. Cutting sleep short has a fast and measurable effect. In a study of young, healthy men who slept only five hours per night for one week, daytime testosterone levels dropped by 10% to 15%. These weren’t older men with preexisting health problems. They were young and otherwise healthy, which makes the finding especially notable.
Chronic short sleep, anything consistently under six hours, can keep testosterone suppressed over time. Sleep apnea compounds the problem by repeatedly interrupting deep sleep cycles even when total time in bed seems adequate. Treating the sleep disorder often improves testosterone levels without any other intervention.
Medications That Suppress Testosterone
Several common medications can lower testosterone as a side effect. The most well-documented are opioid painkillers, which suppress the brain’s release of the master signaling hormone that kicks off the entire production chain. This means the testes receive weaker and weaker signals the longer a person takes opioids. The effect is dose-dependent: higher doses and longer use lead to greater suppression.
Corticosteroids (like prednisone), certain antidepressants, and some medications used to treat prostate conditions can also lower testosterone. If you’ve noticed symptoms that started after beginning a new medication, that connection is worth discussing with your prescriber. In many cases, adjusting the medication or switching to an alternative can help.
Alcohol and the Testes
Heavy alcohol use directly damages Leydig cells, the testosterone-producing cells in the testes. This is a primary form of hypogonadism: the testes themselves lose their ability to respond normally. Chronic heavy drinking also disrupts the brain’s hormonal signaling and can raise estrogen levels through effects on the liver. Moderate drinking doesn’t appear to cause the same degree of harm, but regular heavy consumption is a well-established cause of clinically low testosterone.
Nutritional Deficiencies
Two nutrients have the strongest evidence linking deficiency to lower testosterone: zinc and vitamin D. Zinc is essential for Leydig cell function, and even mild deficiency can reduce testosterone output. Vitamin D receptors and the enzymes that activate vitamin D are present throughout the male reproductive system, including the testes. Men with vitamin D levels below 25 nmol/L tend to have lower total and bioavailable testosterone than men with adequate levels, and genetic studies suggest this relationship is causal rather than coincidental.
Correcting a deficiency through diet or supplementation can improve testosterone, but megadosing beyond normal levels does not push testosterone higher. The benefit comes from moving out of deficiency, not from excess.
Endocrine-Disrupting Chemicals
Certain synthetic chemicals found in plastics, food packaging, and personal care products can interfere with testosterone production. The two most studied are bisphenol A (BPA) and phthalates. BPA blocks the androgen receptor, preventing testosterone from doing its job at the cellular level, and also interferes with the enzymatic steps that produce testosterone in the first place. Phthalates act through a similar combination of blocking androgen signaling and suppressing the enzymes responsible for steroid hormone synthesis.
Both compounds can also mimic estrogen, adding another layer of hormonal disruption. Lab studies show dose-dependent reductions in testosterone when testicular cells are exposed to these chemicals. The real-world impact of everyday exposure levels is harder to quantify, but reducing contact with plastic food containers (especially when heated), choosing fragrance-free products, and filtering drinking water are practical steps that lower exposure.
Stress and Cortisol
Chronic psychological stress raises cortisol, the body’s primary stress hormone. Cortisol and testosterone have an inverse relationship: when cortisol stays elevated for extended periods, the brain reduces its hormonal signals to the testes. This is another form of secondary hypogonadism. The body essentially deprioritizes reproduction during perceived emergencies. Short bursts of stress don’t cause lasting changes, but months or years of high stress from work, financial strain, or chronic illness can keep testosterone meaningfully suppressed.
Testicular Injury and Medical Treatments
Physical damage to the testes from injury, surgery, or infection (such as mumps affecting the testes after puberty) can permanently reduce testosterone production. Radiation therapy and chemotherapy, particularly for cancers in or near the pelvic region, can damage Leydig cells. This type of primary hypogonadism may be partial or complete depending on the extent of the damage. Men undergoing cancer treatment are sometimes offered fertility preservation beforehand because of this risk.
Conditions present from birth, such as Klinefelter syndrome (where a man carries an extra X chromosome), also cause primary hypogonadism. These are less common but account for a significant portion of cases diagnosed in younger men who have no obvious lifestyle or metabolic explanation for low levels.