A testosterone blood test result is more than a single number. Understanding your results means knowing which type of testosterone was measured, how your level compares to established reference ranges, and what factors like timing, age, and binding proteins can shift the picture. The most widely used threshold for low testosterone in men is a total testosterone below 300 ng/dL, but context matters enormously.
Total, Free, and Bioavailable Testosterone
Most testosterone in your blood is bound to proteins, primarily one called sex hormone-binding globulin (SHBG) and another called albumin. Only a small fraction circulates unbound. Your lab report may include one, two, or all three of these measurements, and each tells you something different.
Total testosterone is the most commonly ordered test. It measures everything: bound and unbound testosterone combined. For men aged 19 to 39 who are not obese, the harmonized normal range established by the Endocrine Society is 264 to 916 ng/dL. For women aged 19 and older, the typical reference range is much lower, around 8 to 60 ng/dL.
Free testosterone measures only the unbound fraction, the testosterone your body can readily use for building muscle, maintaining bone density, and supporting sexual function. In women, a normal free testosterone range is roughly 0.06 to 1.08 ng/dL. Free testosterone is ordered less often, but it becomes important when total testosterone looks normal yet symptoms persist.
Bioavailable testosterone captures free testosterone plus the portion loosely bound to albumin, which your body can still access relatively easily. This test is uncommon but can help clarify situations where SHBG levels are distorting the total number.
Why SHBG Can Make Your Results Misleading
SHBG is the protein that binds most of your testosterone and locks it away from use. The total testosterone test cannot distinguish between testosterone that’s tightly bound to SHBG and testosterone that’s actually available to your tissues. This creates a gap: you can have a normal total testosterone level while your free, usable testosterone is too low, or too high.
When SHBG is elevated, more testosterone gets bound, leaving less for your body to use. You might feel symptoms of low testosterone (fatigue, low libido, difficulty building muscle) even though your total number looks fine. Conditions that raise SHBG include aging, liver disease, hyperthyroidism, and certain medications. Conversely, when SHBG is low, more of your testosterone is free and active, which can cause symptoms of excess even with a normal-looking total. Obesity, insulin resistance, and hypothyroidism tend to lower SHBG.
If your symptoms don’t match your total testosterone result, asking about SHBG and free testosterone testing can fill in the missing piece. Doctors can calculate free testosterone using your total testosterone, SHBG, and albumin levels through validated formulas rather than running a separate test.
What Counts as Low Testosterone in Men
The American Urological Association uses a total testosterone level below 300 ng/dL as the cutoff supporting a diagnosis of low testosterone. But a single low reading isn’t enough. The guidelines require two separate blood draws, both taken in the early morning, before a diagnosis is made. This matters because testosterone levels fluctuate throughout the day and from one day to the next.
Equally important: the number alone doesn’t make the diagnosis. Low testosterone is only diagnosed when low levels appear alongside symptoms. Those symptoms fall into three broad categories:
- Physical: reduced energy, fatigue, loss of lean muscle mass, reduced endurance, loss of body hair, reduced beard growth, weight gain
- Cognitive: depressive symptoms, poor concentration, reduced motivation, irritability, memory problems
- Sexual: reduced sex drive, reduced erectile function
A man with a total testosterone of 280 ng/dL who feels fine and has no symptoms would not necessarily receive a diagnosis. Meanwhile, someone at 310 ng/dL with significant fatigue, low libido, and depressive symptoms might warrant further investigation with free testosterone or SHBG testing.
Why Morning Testing Matters
Testosterone levels peak in the early morning and decline as the day progresses. A blood draw at 2 p.m. could give you a reading meaningfully lower than one taken at 8 a.m., creating a falsely low result. This is why labs and clinicians recommend morning blood draws for testosterone testing. Your provider may also ask you to fast for several hours beforehand, since food intake can influence the result.
If you got tested in the afternoon and your level came back borderline low, it’s worth repeating the test in the morning before drawing conclusions.
Age and the 1% Per Year Decline
Testosterone levels naturally decline with age at an average rate of about 1% per year after age 30. This means a 50-year-old man will typically have a notably lower testosterone level than he did at 25, and that’s biologically normal. The reference range of 264 to 916 ng/dL was established in healthy men aged 19 to 39, so comparing yourself directly to that range at age 55 requires some nuance.
That said, the gradual decline associated with aging is different from clinical testosterone deficiency. A slow, modest drop usually doesn’t produce dramatic symptoms. If you’re experiencing significant changes in energy, mood, or sexual function, the age-related decline alone may not explain it, and further testing is reasonable.
What Additional Lab Values Reveal
When testosterone comes back low, the next question is why. Two hormones produced by the pituitary gland, called LH and FSH, help answer that question. Your brain sends LH and FSH to signal the testes to produce testosterone, so measuring them reveals where the problem originates.
If testosterone is low but LH and FSH are high, the signal from the brain is working fine, but the testes aren’t responding. This is called primary hypogonadism, and causes include testicular injury, genetic conditions, or damage from infections or treatments like chemotherapy.
If testosterone is low and LH and FSH are also low (or inappropriately normal), the problem is upstream. The brain isn’t sending enough signal. This pattern, called secondary hypogonadism, can result from pituitary tumors, head injuries, chronic opioid use, or severe obesity. The distinction matters because the treatment approach and underlying cause differ significantly.
There’s also a third pattern: FSH is elevated while testosterone and LH remain normal. This suggests sperm production is impaired even though testosterone output is fine, which is relevant for fertility evaluations but wouldn’t show up on a testosterone test alone.
Supplements That Skew Results
Biotin, a common supplement taken for hair, skin, and nail health, can cause dramatically false results on standard testosterone blood tests. In one documented case, a single 10 mg biotin tablet taken 33 hours before a blood draw produced alarmingly high testosterone readings that initially suggested a hormone-producing tumor. When the same patient was retested after a two-week break from biotin, using a more precise testing method called liquid chromatography-mass spectrometry, all hormone levels came back completely normal.
The interference occurs because many common lab tests (called immunoassays) use biotin-based chemistry in their testing kits. High levels of biotin in your blood can throw off the results in either direction, depending on the specific test. If you take biotin supplements, even at doses found in many multivitamins, mention this to your provider before getting tested. Stopping biotin at least two days before a blood draw is a common precaution, though some experts recommend a longer washout period.
Reading Your Lab Report
Your lab report will list your result alongside a reference range specific to that laboratory. These ranges vary between labs because different testing methods and equipment produce slightly different numbers. A result of 350 ng/dL might fall within the normal range at one lab and sit at the low end at another. Always compare your number to the range printed on your specific report rather than to ranges you find online.
Pay attention to the units. Most U.S. labs report testosterone in ng/dL (nanograms per deciliter), but some use nmol/L (nanomoles per liter). To convert from nmol/L to ng/dL, multiply by 28.84. A result of 10 nmol/L, for example, equals roughly 288 ng/dL.
If your result is borderline, falling just above or below the reference range, a single test is not definitive. Natural day-to-day variation, recent illness, poor sleep, acute stress, or intense exercise can all temporarily shift your levels. A repeat test on a different morning gives a much clearer picture than one data point alone.