A testosterone test measures the hormone in a blood sample. Because levels peak in the early morning and swing with sleep, illness and stress, a valid test is drawn before about 10 a.m., ideally fasting, and a low result is always repeated on a second morning. Total testosterone in adult men typically runs 300–1,000 ng/dL (10.4–34.7 nmol/L); persistent values below ~300 with symptoms support a diagnosis of hypogonadism. Free testosterone (with SHBG) clarifies borderline cases, and LH/FSH locate the cause. Home finger-prick kits exist but a laboratory venous sample is the standard for decisions.
What the test measures
Testosterone circulates in three forms: tightly bound to SHBG (roughly 40–70%), loosely bound to albumin, and free (1–3%). Total testosterone — the standard first test — measures all three together. Free testosterone estimates the biologically active fraction, usually calculated from total testosterone, SHBG and albumin rather than measured directly (direct "analog" free-T assays are unreliable and best ignored).
Why timing and repetition are non-negotiable
Testosterone peaks between about 7 and 10 a.m. and can be 20–30% lower by late afternoon in younger men. It also drops — sometimes steeply — with acute illness, sleep deprivation, heavy drinking, intense recent exercise, fasting extremes and opioid painkillers. Guidelines therefore require:
- a morning draw (before ~10 a.m.) after a normal night's sleep, fasting preferred
- repetition on a separate morning for any low result before it counts
- postponement during acute illness or recovery
A single low afternoon result, taken while ill or after a short night, is how healthy men end up on lifelong therapy they never needed.
The ranges
| Measure | Typical adult male reference |
|---|---|
| Total testosterone | 300–1,000 ng/dL (10.4–34.7 nmol/L) |
| Threshold commonly used for deficiency | Below ~300 ng/dL (10.4 nmol/L) on two morning tests, with symptoms |
| Free testosterone (calculated) | Roughly 5–21 ng/dL (0.17–0.73 nmol/L); lab-specific |
Points of interpretation:
- Ranges are lab-specific — assays differ; compare your result with your own report's range.
- The cut-off is a guide, not a wall. A symptomatic man at 290 and an asymptomatic man at 310 are not different species; symptoms plus the trend matter.
- Units trip people up: US labs report ng/dL, most others nmol/L. Multiply nmol/L by ~28.8 for ng/dL.
- Levels below ~150 ng/dL are unambiguously low and usually prompt a search for pituitary or testicular disease, not just lifestyle causes.
The follow-on tests
A confirmed low result is located, not just treated:
- LH and FSH — high means the testes are failing (primary hypogonadism); low or inappropriately normal means the signal from the pituitary is lacking (secondary), which redirects the work-up.
- SHBG with calculated free T — essential when obesity, diabetes, thyroid disease, liver disease or older age make total testosterone misleading.
- Prolactin and sometimes pituitary MRI for secondary cases, especially with headache, visual change or very low levels.
- Iron studies (hemochromatosis), estradiol where gynecomastia is present, semen analysis where fertility is the question.
- General health: A1C or glucose, lipids, blood count — low testosterone keeps metabolic company.
What can skew the result
Lowers it: illness, sleep loss and sleep apnea, obesity, crash dieting, alcohol, opioids, corticosteroids, recent anabolic steroid use (suppression can persist months after stopping — tell your doctor, it changes the interpretation).
Raises it or muddies it: testosterone gels applied near the draw site, biotin supplements interfering with some assays, and the normal 15% day-to-day variation.
After the result
- Normal on a proper morning test: testosterone is not the explanation for the symptoms; the search moves to sleep, mood, thyroid, medication and life circumstances — see signs of low testosterone for the look-alikes.
- Borderline: repeat with SHBG and free T; fix sleep, weight and alcohol and retest in three months.
- Confirmed low: LH/FSH and the cause-directed work-up above, then a considered decision about treatment — the options, benefits and obligations are covered in low testosterone.
The test is cheap and the condition is treatable; the mistakes worth avoiding are testing casually, treating a number without symptoms, and starting lifelong therapy on a single bad morning.
Frequently asked questions
Do I need to fast before a testosterone test?
Fasting is preferred — a glucose load can transiently lower testosterone — and morning timing is essential. Book the draw for before 10 a.m., after a normal night's sleep.
My result was low once. Do I have low T?
Not yet proven. Up to a third of men with one low reading are normal on repeat. Illness, short sleep, alcohol the night before and afternoon timing all depress a single value. Diagnosis requires two low morning results plus symptoms.
What is SHBG and why does it matter?
Sex hormone-binding globulin is the protein that carries most testosterone in an inactive form; only 1–3% is free. Obesity and diabetes lower SHBG (total looks worse than the free reality); age and liver disease raise it (total looks better). When SHBG is likely abnormal, free testosterone tells the truer story.
Are home testosterone tests accurate?
Saliva tests correlate poorly and are not used for diagnosis. Finger-prick blood kits from reputable labs are reasonable screens, but any low or borderline result should be confirmed with a morning venous sample before conclusions or treatment.
References
- MedlinePlus, U.S. National Library of Medicine. Testosterone Levels Test (2024). https://medlineplus.gov/lab-tests/testosterone-levels-test/
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