Low testosterone has many possible causes, ranging from normal aging to obesity, chronic disease, medications, and genetic conditions. The American Urological Association defines low testosterone as a total level below 300 ng/dL, and the causes fall into two broad categories: problems with the testicles themselves, and problems with the brain signals that tell the testicles to produce testosterone.
How Testosterone Production Works
Your brain and your testicles work as a connected system. The hypothalamus, a small region at the base of your brain, sends a chemical signal to your pituitary gland. The pituitary gland then releases hormones that travel through your bloodstream and tell your testicles to make testosterone. If something disrupts the testicles directly, that’s called primary hypogonadism. If something disrupts the brain signaling, it’s called secondary hypogonadism. Some conditions cause both at the same time.
Understanding which type you have matters because it points toward different causes and different treatment approaches.
Age-Related Decline
Testosterone peaks around age 17 and stays high for the next two to three decades. In most men, levels start dropping around age 40, declining just over 1% per year on average. This is gradual, nothing like the sharp hormonal drop women experience during menopause. Some men maintain high levels throughout life, while others experience a steeper decline that eventually produces symptoms like fatigue, reduced muscle mass, and lower sex drive.
Because the decline is slow, many men don’t notice it for years. A man in his 60s may have testosterone levels 20 to 30% lower than he did at 30, which can cross the threshold into clinical deficiency even without any other medical problem.
Obesity and Body Fat
Excess body fat is one of the most common and reversible causes of low testosterone. Fat tissue contains high levels of an enzyme that converts testosterone into estrogen. The more fat you carry, the more of this conversion happens. The resulting rise in estrogen then signals your brain to slow down production of the hormones that stimulate testosterone, creating a feedback loop: more fat leads to less testosterone, and less testosterone makes it easier to gain more fat.
Men with type 2 diabetes are particularly affected. In one study comparing diabetic men to healthy controls, roughly 71% of the men with type 2 diabetes had low testosterone, compared to about 25% of healthy men. Insulin resistance, which drives type 2 diabetes, appears to independently suppress testosterone production as well.
Sleep Deprivation
Your body produces most of its testosterone during sleep, which makes sleep loss a surprisingly powerful suppressor. A University of Chicago study put healthy young men (average age 24, lean, no medical problems) through a week of sleeping less than five hours per night. Their testosterone dropped 10 to 15% in just seven days. The researchers noted that this single week of poor sleep reduced testosterone by the same amount as aging 10 to 15 years. Levels were lowest in the afternoon and evening on sleep-restricted days.
Chronic short sleep, shift work, and untreated sleep apnea can all keep testosterone suppressed long term. For many men, improving sleep is the simplest intervention available.
Medications That Suppress Testosterone
Several widely prescribed drug classes lower testosterone, sometimes dramatically. Opioid painkillers are among the worst offenders. Opioids suppress the brain’s signaling to the testicles, and the effect begins within hours of taking them. Among men using daily opioid doses equivalent to 100 to 200 mg of oral morphine for more than a month, 50 to 100% develop some degree of testosterone suppression. Higher doses cause greater suppression. This condition, called opioid-induced androgen deficiency, is common but frequently goes undiagnosed.
Corticosteroids (often prescribed for inflammation and autoimmune conditions), certain antidepressants, and some medications used to treat prostate conditions also lower testosterone. If you’ve noticed symptoms after starting a new medication, that connection is worth exploring.
Genetic and Congenital Conditions
Some men are born with conditions that limit testosterone production. Klinefelter syndrome, where a man carries an extra X chromosome, is one of the most common genetic causes. The extra chromosome causes abnormal testicular development, which results in reduced testosterone output. Many men with Klinefelter syndrome aren’t diagnosed until they encounter fertility problems or symptoms of low testosterone in adulthood.
Undescended testicles, a condition present at birth, can also lead to long-term testosterone deficiency if not corrected in early childhood. Both testicles need to be in the scrotum to function properly, and prolonged time in the wrong position can permanently impair their ability to produce hormones.
Iron Overload and Chronic Disease
Hereditary hemochromatosis, a condition where the body absorbs too much iron from food, can damage the pituitary gland early in the course of the disease. Iron deposits accumulate in the specific pituitary cells responsible for sending testosterone-production signals to the testicles. This makes low testosterone one of the earlier symptoms of hemochromatosis, sometimes appearing before the liver damage and joint pain that are more commonly associated with the condition.
Other chronic illnesses that can lower testosterone include liver disease, kidney disease, and HIV/AIDS. Chronic illness creates stress on the body that can suppress the hormonal signaling chain at multiple points. Serious physical trauma or infections affecting the testicles (such as mumps orchitis) can directly damage the tissue that produces testosterone.
Nutritional Deficiencies
Two micronutrients play direct roles in testosterone production: zinc and vitamin D. Zinc is essential for the enzymatic processes that create testosterone, and a deficiency can measurably hinder production. Vitamin D helps regulate testosterone levels through its interaction with hormone receptors throughout the body.
The practical takeaway here is nuanced. If you’re already getting enough zinc and vitamin D, taking more won’t boost your testosterone above normal. But if you’re deficient, which is common with vitamin D especially in northern climates and in people who spend little time outdoors, correcting that deficiency can help restore testosterone to where it should be.
Environmental Chemical Exposure
Endocrine-disrupting chemicals interfere with hormone production and signaling. Phthalates, one of the most studied classes, are found in an enormous range of everyday products: food packaging, vinyl flooring, detergents, personal care products like shampoos and soaps, nail polish, and even medical tubing. Animal studies have shown that phthalate exposure during development lowers testosterone levels. These chemicals are so widespread in the environment that avoiding them entirely is nearly impossible, though reducing exposure by choosing fragrance-free products and minimizing plastic food contact can help.
Stress and the Cortisol Connection
Prolonged psychological or physical stress raises cortisol, your body’s primary stress hormone. Cortisol and testosterone have an inverse relationship: when cortisol stays elevated for extended periods, it suppresses the brain signals that drive testosterone production. This is the same secondary mechanism seen with opioids and obesity, where the brain dials down its hormonal output to the testicles.
Acute stress, like a tough workout or a bad day, doesn’t meaningfully affect testosterone. But chronic stress from ongoing financial pressure, relationship problems, overtraining, or untreated anxiety can keep cortisol elevated long enough to create a real suppressive effect. Combined with the poor sleep and weight gain that often accompany chronic stress, the impact on testosterone can compound quickly.