Most doctors use 300 ng/dL as the cutoff for diagnosing low testosterone and considering testosterone replacement therapy (TRT). But a number alone won’t get you a prescription. You also need documented symptoms, and your blood has to be drawn under specific conditions, typically twice, before a diagnosis is confirmed.
The reality is more nuanced than a single number. Different medical organizations, insurance plans, and clinics use slightly different thresholds, and borderline cases often come down to additional lab work and how significantly symptoms affect your daily life.
The 300 ng/dL Threshold
The American Urological Association sets the standard most U.S. doctors follow: a total testosterone level below 300 ng/dL supports a diagnosis of low testosterone. This cutoff was chosen to balance the potential benefits of treatment against the risks. It’s not arbitrary. It’s based on population data showing where symptoms tend to cluster and where treatment produces meaningful improvement.
The Endocrine Society uses a slightly different reference point. Their harmonized normal range for healthy, non-obese young men (ages 19 to 39) starts at 264 ng/dL at the 2.5th percentile and 303 ng/dL at the 5th percentile. In practice, this means the two major guideline bodies land in roughly the same zone: somewhere between 264 and 300 ng/dL marks the floor of normal.
Medicare and some private insurers set their own bar. Medicare’s coverage criteria reference 280 ng/dL as the level below which further testing is warranted, and some plans won’t consider coverage until levels fall below 200 ng/dL for older men. If you’re pursuing TRT through insurance rather than a cash-pay clinic, your insurer’s specific threshold matters as much as the clinical guidelines.
A Number Alone Isn’t Enough
Every major guideline requires symptoms alongside low lab values. You can’t walk into a clinic with a testosterone level of 250 ng/dL, feel perfectly fine, and get a prescription. The diagnosis of hypogonadism requires both consistently low testosterone and clinical signs that something is wrong.
The symptoms doctors look for are specific. Sexual symptoms carry the most diagnostic weight: reduced sex drive, weaker erections, and fewer morning erections. A large European study of men aged 40 to 79 found that these three sexual symptoms were the strongest predictors of true hypogonadism when paired with low testosterone. Other recognized symptoms include persistent fatigue, loss of muscle strength or endurance, depressed mood, reduced enjoyment of life, and declining work or athletic performance. Screening questionnaires used in primary care typically flag a problem if you report three or more of these symptoms, or if you specifically report decreased sex drive or weaker erections.
How the Blood Test Has to Be Done
Testosterone levels fluctuate throughout the day, peaking in the early morning and dropping by as much as 30% later in the afternoon. To get an accurate reading, blood must be drawn before 10:00 AM, or within three hours of waking up, ideally in a fasting state.
One low reading isn’t enough. Both the AUA and Medicare require at least two separate blood draws on two different days, from the same lab using the same testing method. Day-to-day biological variation alone can cause readings to differ by up to 30%, so a single result below 300 ng/dL could simply reflect a bad night’s sleep, recent illness, or normal fluctuation. Two consistent results confirm the pattern.
When Free Testosterone Changes the Picture
Total testosterone is the standard first test, but it doesn’t always tell the full story. Much of the testosterone in your blood is bound to a protein called SHBG, which makes it unavailable to your tissues. Only the unbound (“free”) portion is biologically active. If your SHBG levels are unusually high or low, your total testosterone number can be misleading.
This matters most in the borderline zone. The Endocrine Society recommends checking free testosterone when total testosterone falls between roughly 200 and 400 ng/dL. Several international guidelines place the lower limit of normal free testosterone between 220 and 243 pmol/L (about 64 to 65 pg/mL). If your total testosterone is 310 ng/dL but your free testosterone is below that threshold and you have clear symptoms, many clinicians will still diagnose hypogonadism and consider treatment.
Conditions that raise SHBG include aging, liver disease, and hyperthyroidism. Obesity, type 2 diabetes, and hypothyroidism tend to lower it. If any of these apply to you, free testosterone becomes an especially important part of the evaluation.
What Additional Blood Work Reveals
Once low testosterone is confirmed, your doctor will typically measure two hormones produced by the pituitary gland: LH and FSH. These determine why your testosterone is low, which influences treatment decisions.
If LH and FSH are elevated, it means your brain is sending strong signals to your testes to produce testosterone, but the testes can’t keep up. This is primary hypogonadism, a problem at the testicular level, often caused by genetic conditions, injury, or prior chemotherapy. If LH and FSH are low or normal despite low testosterone, the problem originates higher up, in the pituitary gland or hypothalamus. This is secondary hypogonadism, and it can be caused by pituitary tumors, obesity, opioid use, or other hormonal disruptions. Secondary hypogonadism sometimes has a treatable underlying cause that resolves the testosterone issue without lifelong TRT.
Does Testosterone Decline Enough With Age to Qualify?
A common assumption is that testosterone drops steadily as men age, eventually falling below the treatment threshold for most men. The data tells a more complicated story. A large normative study found that average total testosterone peaks around age 19 and declines to about 375 ng/dL (13.0 nmol/L) by age 40, but found no evidence of further decline in the average case after that point through old age.
What does change with age is the range of variation. The spread between the lowest and highest testosterone levels in healthy men widens considerably after 40, meaning more individual men end up at the extremes. The 2.5th percentile (the low end of normal) drops modestly: from about 190 ng/dL at age 30 to about 170 ng/dL at age 60 and 160 ng/dL at age 90. So aging alone pushes some men below 300 ng/dL, but it’s not inevitable, and current guidelines do not use age-adjusted cutoffs for treatment decisions. The 300 ng/dL threshold applies regardless of whether you’re 35 or 75.
What Can Disqualify You From TRT
Even with confirmed low testosterone and symptoms, certain health markers can delay or prevent treatment. TRT stimulates red blood cell production, which thickens the blood. If your hematocrit (the percentage of your blood made up of red blood cells) is already above 50% before starting, you’re likely to exceed the safety ceiling of 54% on therapy. A hematocrit above 54% significantly increases the risk of blood clots and cardiovascular events.
Prostate screening is also required before starting. A PSA level above 4 ng/mL (or above 3 ng/mL for men at higher risk of prostate cancer, including Black men and those with a first-degree relative diagnosed with prostate cancer) requires a urology referral before TRT can be considered. The same applies if a digital rectal exam finds any nodules or hardened areas. Untreated severe sleep apnea is another barrier, as testosterone can worsen it.
Once on therapy, monitoring continues. PSA is rechecked at 3 to 6 months to ensure it hasn’t risen more than 1.4 ng/mL above your baseline. Hematocrit is monitored regularly, and if it crosses 54%, your dose is reduced or therapy is paused until it normalizes. The treatment goal is typically to bring testosterone into the 400 to 700 ng/dL range, well within normal but not at the upper extreme.