Low testosterone is generally defined as a total testosterone level below 300 ng/dL, the threshold most commonly used in clinical practice. The Endocrine Society’s harmonized reference range for healthy, nonobese men aged 19 to 39 places the normal range between 264 and 916 ng/dL, with 300 ng/dL often serving as the practical cutoff. But a number alone isn’t a diagnosis. Low testosterone, formally called male hypogonadism, requires both low lab results and symptoms.
The Numbers That Define Low Testosterone
Most doctors use 300 ng/dL as the working threshold for low total testosterone. This comes from large-scale reference data: in healthy, nonobese young men aged 19 to 39, the 2.5th percentile (the bottom of the range) is 264 ng/dL, and the 5th percentile is 303 ng/dL. In practice, a result under 300 is treated as the starting point for concern, not an automatic diagnosis.
When levels drop below 150 ng/dL, that’s considered severe. At that point, doctors typically investigate whether the problem originates in the brain’s signaling system rather than the testicles themselves, and imaging of the pituitary gland may be recommended to rule out tumors or other structural issues.
Total testosterone is the standard first test, but it doesn’t tell the whole story. Only about 2 to 3 percent of your testosterone circulates freely in the blood, unattached to proteins. The rest is bound to two proteins: one that grips it tightly and essentially makes it inactive, and another (albumin) that holds it loosely enough that it can still act on tissues. Free testosterone and bioavailable testosterone (the free portion plus the loosely bound portion) can help clarify the picture when total testosterone is borderline or when conditions that affect binding protein levels are present.
Why Total Testosterone Can Be Misleading
A protein called SHBG (sex hormone binding globulin) has an outsized influence on your total testosterone reading. SHBG binds testosterone tightly and carries it through the bloodstream, and total testosterone levels are strongly correlated with SHBG levels. That means if your SHBG is unusually high, your total testosterone can look normal even though very little of it is available to your body. If SHBG is low, your total testosterone may read low even though your free testosterone is adequate.
SHBG tends to be lower in men who are overweight. In one study, 13 out of 14 untreated men with testosterone below 300 ng/dL also had low SHBG. Higher body weight, insulin resistance, and metabolic conditions all push SHBG down. This is one reason doctors sometimes order free or bioavailable testosterone when the total number doesn’t match the symptoms.
Symptoms That Matter for Diagnosis
A low number on a lab report is not enough by itself. Diagnosis requires symptoms alongside low levels. The most specific symptoms are sexual: reduced sex drive, loss of morning or spontaneous erections, and difficulty getting or maintaining an erection. These are the symptoms most tightly linked to testosterone deficiency rather than other conditions.
Other symptoms that strongly suggest hypogonadism include loss of body hair (especially in the armpits and pubic area), shrinking testicles, hot flashes, and a very low or absent sperm count. Less specific but common complaints include fatigue, depressed mood, difficulty concentrating, and loss of muscle mass. These overlap with many other conditions, which is why the sexual symptoms carry more diagnostic weight.
How Testosterone Changes With Age
Testosterone levels naturally decline as men get older, but the drop is more gradual than many people assume. In large studies of nonobese men, the median total testosterone by age group looks like this:
- Ages 19 to 39: 531 ng/dL (median)
- Ages 40 to 49: 481 ng/dL
- Ages 50 to 59: 477 ng/dL
- Ages 60 to 69: 477 ng/dL
The upper end of the range stays remarkably stable across decades, hovering around 840 to 850 ng/dL at the 95th percentile regardless of age. The lower end shifts more noticeably: the 5th percentile drops from 304 ng/dL in men under 40 to 252 in men in their 70s and 218 in men over 80. So while average levels dip modestly after 40, the floor keeps dropping with age, meaning more older men fall into the low range.
How Body Weight Affects Your Levels
Obesity is one of the strongest modifiable drivers of low testosterone. Roughly 20 to 50 percent of obese men meet the criteria for testosterone deficiency, depending on how severe the obesity is. Fat tissue actively converts testosterone into estrogen, which reduces both free and total testosterone levels. Abdominal fat is particularly impactful: studies analyzing nearly 6,000 men found that higher body fat percentage, especially around the midsection, correlates with lower testosterone.
The relationship runs in both directions. Low testosterone promotes fat accumulation, and more fat further suppresses testosterone, creating a cycle that can be difficult to break without deliberate intervention. Weight loss, on the other hand, can meaningfully raise testosterone levels, sometimes enough to bring a borderline-low result back into the normal range without any other treatment.
Getting Tested the Right Way
Testosterone levels fluctuate throughout the day, peaking in the early morning and dropping later. For men under 45, blood should be drawn between 7 and 9 AM to capture the highest point of the cycle. Men 45 and older show much less daily variation, so testing anytime before 2 PM is considered reliable.
One low result doesn’t confirm a diagnosis. Up to 30 percent of men who test low the first time will have a normal result on a repeat test. For this reason, a second early-morning blood draw is standard before any diagnosis is made. Illness, poor sleep, stress, and certain medications can temporarily suppress testosterone, so retesting helps filter out these transient dips.
Primary vs. Secondary Low Testosterone
Once low testosterone is confirmed, doctors determine whether the problem starts in the testicles or in the brain. The distinction matters because the causes and treatment options differ.
Primary hypogonadism means the testicles themselves aren’t producing enough testosterone. The brain responds by ramping up its signaling hormones (LH and FSH) to try to stimulate more production, so blood tests show low testosterone with high LH and FSH. Common causes include genetic conditions, injury to the testicles, or damage from chemotherapy or radiation.
Secondary hypogonadism means the brain isn’t sending adequate signals to the testicles. Testosterone is low, and LH and FSH are also low or inappropriately normal. This form is more common and is frequently linked to obesity, opioid use, pituitary disorders, or chronic illness. These hormones are best measured in the early morning, between 8 and 10 AM, for the most accurate reading.
Long-Term Health Risks of Untreated Low Testosterone
Low testosterone isn’t just about sexual function or energy. Men with untreated hypogonadism face a higher risk of weakened bones, with low bone mineral density and osteoporosis being well-documented consequences. Loss of bone and muscle mass together, sometimes called osteosarcopenia, increases fracture risk and reduces physical capacity over time.
Cardiovascular risk is also elevated. Low testosterone levels are associated with major adverse cardiac events and higher mortality from cardiovascular disease. The hormone plays a role in blood vessel function and metabolic health, and prolonged deficiency contributes to unfavorable changes in body composition, insulin sensitivity, and cholesterol profiles. Comorbidities like obesity and diabetes, which are already more common in men with low testosterone, compound these risks further.