Low testosterone in men results from a wide range of causes, from normal aging to obesity, chronic disease, medications, and nutritional gaps. The American Urological Association defines low testosterone as a total level below 300 ng/dL, and reaching that threshold can involve one factor or several working together. Understanding what drives testosterone down is the first step toward figuring out what, if anything, can be done about it.
How Testosterone Production Works
Testosterone production depends on a chain of signals that starts in the brain. The hypothalamus sends a hormone signal to the pituitary gland, which then tells the testes to produce testosterone. A problem anywhere along this chain lowers output. When the testes themselves are damaged or dysfunctional, that’s called primary hypogonadism. When the brain’s signaling centers malfunction, it’s called secondary hypogonadism. The distinction matters because it changes what treatments are possible and what the underlying problem looks like.
Primary causes include genetic conditions like Klinefelter syndrome, physical injury to the testes, infections, radiation exposure, and certain autoimmune disorders. Secondary causes include tumors near the pituitary gland, head trauma, nutritional deficiencies, and medications that suppress brain signaling. Many men with low testosterone have a combination of factors rather than a single clear-cut cause.
Age-Related Decline
Testosterone levels drop by roughly 1% per year after age 30. That adds up. By 50, a man may have 20% less testosterone than he did at his peak. This gradual decline is a normal part of aging, not a disease, though it can produce symptoms like fatigue, reduced muscle mass, and lower sex drive that overlap with clinical testosterone deficiency. The challenge is distinguishing a natural, slow decline from something more significant that needs attention.
Obesity and Metabolic Health
Excess body fat is one of the strongest and most common drivers of low testosterone, and the mechanism is straightforward. Fat tissue contains an enzyme called aromatase that converts testosterone into estrogen. The more fat you carry, the more aromatase activity there is, and the more testosterone gets converted. Research published in the Journal of Clinical Endocrinology & Metabolism found that aromatase levels are significantly higher in the fat tissue of men with obesity compared to lean men, and that this elevated activity correlates with insulin resistance and higher blood sugar.
This creates a vicious cycle. Lower testosterone makes it easier to gain fat, and more fat drives testosterone down further. The relationship between obesity and testosterone is so strong that weight loss alone can meaningfully raise levels in many men without any other intervention.
Type 2 Diabetes and Insulin Resistance
Between 30% and 50% of men with type 2 diabetes have low total or free testosterone, even without any known problem with their testes or pituitary gland. That’s a striking number. Insulin resistance appears to be a key mechanism: when cells stop responding properly to insulin, it disrupts hormone signaling throughout the body, including the pathways that regulate testosterone. Men who are overweight but don’t have diabetes also show elevated rates of low testosterone (around 30% in one study), which reinforces that metabolic health broadly, not just a diabetes diagnosis, plays a role.
Medications That Lower Testosterone
Several common medication classes suppress testosterone production, sometimes dramatically. Opioid painkillers are among the worst offenders. Chronic opioid use suppresses the brain’s hormonal signaling to the testes, causing what researchers call opioid-induced androgen deficiency. All forms of opioids are implicated, though long-acting formulations appear to cause more harm than short-acting ones. This is clinically relevant for the millions of men on long-term pain management.
Glucocorticoids (steroids prescribed for inflammation, asthma, and autoimmune conditions) also suppress the same signaling pathway. And men who use anabolic steroids for muscle building face a well-documented rebound effect: when they stop, their natural testosterone production can remain suppressed for months or longer because the brain’s signaling system has been shut down by the external supply.
Sleep Deprivation
Most testosterone is produced during sleep, which makes sleep quality a surprisingly powerful lever. A study from the University of Chicago found that healthy young men who slept only five hours per night for one week saw their testosterone drop by 10% to 15%. The researchers noted this was equivalent to aging 10 to 15 years in terms of testosterone levels. That’s a massive effect from a single week of poor sleep, and it suggests that chronically short sleep could be a significant contributor for men who otherwise have no obvious medical cause.
Zinc and Nutritional Deficiencies
Zinc plays a direct role in testosterone production. The cells in the testes that manufacture testosterone need zinc to convert cholesterol-based precursors into active hormones. When zinc levels are low, these cells can’t complete the conversion process efficiently. A systematic review found a consistent positive relationship between serum zinc and testosterone levels in men, with zinc-deficient men showing lower testosterone and zinc supplementation improving levels in those who were deficient.
Zinc deficiency can also damage testicular tissue through oxidative stress, further impairing the organ’s ability to produce hormones. Men most at risk for zinc deficiency include those with poor diets, heavy alcohol use, or digestive conditions that impair nutrient absorption. Other nutritional gaps, particularly vitamin D and magnesium, have also been linked to lower testosterone, though the evidence for zinc is the most robust.
Environmental Chemicals
Certain industrial chemicals interfere with hormone production at the cellular level. Phthalates, found in plastics, personal care products, and food packaging, reduce a key protein that transports cholesterol into the part of the cell where testosterone is actually made. Without that transport step, production stalls. Phthalates also activate receptors in fat cells that increase aromatase expression, the same enzyme that converts testosterone to estrogen.
BPA, another chemical found in plastics and can linings, binds to androgen receptors and interferes with the signaling that triggers testosterone synthesis. It has a lower binding strength than natural hormones, but at the constant low-level exposures most people experience, it can still disrupt normal function. Even glyphosate, a widely used herbicide, has been shown to reduce testosterone by 35% at low, non-toxic concentrations in laboratory studies. The cumulative effect of daily exposure to multiple endocrine disruptors is difficult to measure, but the individual mechanisms are well established.
Other Medical Conditions
Liver and kidney disease both impair testosterone production, as the liver processes sex hormones and the kidneys influence the hormonal signaling chain. Hemochromatosis, a condition where the body absorbs too much iron, damages both the testes and the pituitary gland, making it a cause of both primary and secondary testosterone deficiency. It’s one of the more underdiagnosed causes because symptoms overlap with general fatigue and low energy.
Significant calorie restriction or rapid weight loss, including after bariatric surgery, can temporarily crash testosterone levels. The brain interprets severe energy deficit as a signal to shut down reproductive function. Eating disorders like anorexia produce the same effect. In most cases, testosterone recovers once nutrition stabilizes, but prolonged deficiency can cause lasting effects on bone density and muscle mass.
How Low Testosterone Is Diagnosed
Testosterone levels fluctuate throughout the day, peaking in the early morning and dropping as the day goes on. For this reason, blood tests are typically drawn before 10:00 AM to capture the highest point. A single low reading isn’t enough for diagnosis. Most guidelines require at least two separate morning blood draws showing levels below 300 ng/dL before confirming testosterone deficiency. Symptoms also matter: a man with a level of 280 ng/dL and no symptoms is in a very different situation than one at 280 ng/dL with fatigue, erectile dysfunction, and muscle loss.
Once low testosterone is confirmed, further testing can help pinpoint whether the problem originates in the testes or the brain’s signaling centers. Levels of luteinizing hormone, the pituitary signal that tells the testes to produce testosterone, help distinguish between the two. High levels suggest the brain is sending signals but the testes aren’t responding. Low levels suggest the brain isn’t sending adequate signals in the first place.