How is low T diagnosed?
You are diagnosed with low T when you have symptoms that fit and your blood tests show clearly low testosterone, more than once. The Endocrine Society says to diagnose it only in men with symptoms and signs of low testosterone plus testosterone that is “unequivocally and consistently low.” The American Urological Association (AUA) agrees.
A man is a candidate for treatment only when he meets both tests. If you are heading toward treatment, our guide to starting TRT picks up where this one ends.
In practice, the path usually looks like this:
- Symptoms that suggest low testosterone, such as low sex drive or fewer morning erections.
- A fasting morning blood test for total testosterone, using an accurate lab method.
- A repeat test on another morning to confirm the result.
- LH and FSH tests to find out where the problem starts, plus other tests to look for a cause.
The rest of this guide walks through each step and explains why the details matter.
It also covers where experts still disagree. It can’t tell you whether you have low T, because only your clinician can do that.
Which symptoms count toward a low T diagnosis?
Symptoms are half of the diagnosis, but most of them don’t point only to low testosterone. The AUA calls them “very non-specific.”
Chronic fatigue, ongoing stress or depression can cause the same problems. In a European study of 3,369 men aged 40 to 79, only three sexual symptoms went along with truly low testosterone: poor morning erections, low sex drive and erectile dysfunction. Our guide to low testosterone symptoms covers the full list.
Questionnaires can help you decide whether to get tested. They don’t make the diagnosis.
The ADAM questionnaire caught 88% of men with low levels when it was tested. But it was only 60% specific, which means many men who score positive have normal levels. The Endocrine Society also advises against routine screening of all men.
Why does the testosterone test have to be in the morning?
Your testosterone follows a daily rhythm. It is highest in the morning and drops through the day. In one study, men aged 30 to 40 had levels 20 to 25% lower at 4 pm than at 8 am.
The gap shrank with age, to about 10% at age 70.
That difference is big enough to change a diagnosis. In the same study, 17 men had at least one result under 300 ng/dL after noon.
All 17 had normal levels at all three morning visits. The researchers concluded that testosterone should be measured in the morning, in younger and older men alike.
A laboratory medicine review puts the window at roughly 7 to 11 am. It matters most for men under 40. If your test was drawn in the afternoon and came back low, ask whether it should be repeated in the morning.
Do you need to fast before a testosterone test?
The Endocrine Society recommends a fasting morning test, because food can lower testosterone for a few hours. Here is what the studies found:
- A sugary drink: after a 75 g glucose drink, testosterone fell by 25% on average. 10 of 66 men (15%) with normal levels dropped into the low range at one or more time points.
- A mixed meal: testosterone fell by about 30% one to two hours after eating, compared with fasting samples.
- Overnight fasting: in men over 40, fasting raised morning testosterone and related hormones by 9 to 16%. It also made results less variable.
Not every group insists on fasting. One laboratory review says the evidence for fasting is still limited. British guidance prefers a fasting first test but accepts a non-fasting early morning sample.
The simple answer is to follow your lab’s and clinician’s instructions. If a non-fasting result comes back low, the researchers behind the sugary drink study suggest re-checking it fasting.
Why do you need two testosterone tests?
One result can mislead you. Testosterone changes from day to day as well as across the day.
A study of repeated samples found that one sample “is generally not sufficient” to describe a man’s hormone levels. More than three is probably unnecessary.
That is why the Endocrine Society recommends confirming a low result by repeating the fasting morning test. Large treatment trials use the same rule. The TRAVERSE trial, for example, enrolled only men with symptoms and two fasting testosterone levels under 300 ng/dL.
Guidelines don’t set a fixed gap between the two tests. One laboratory review of British guidance says borderline or low results should be repeated, ideally after about four weeks. Ask your clinician what timing they prefer.
What testosterone level counts as low?
In the US, the number you’ll hear most is 300 ng/dL. The AUA guideline says clinicians should use a total testosterone below 300 ng/dL as a reasonable cut-off. It also says to make the diagnosis only after two early-morning measurements on separate days.
The Endocrine Society sets no single cut-off. It tells clinicians to compare results against carefully built reference ranges, meaning the span of levels seen in healthy men. It gives 264 ng/dL as the lower limit of normal in healthy, nonobese young men for CDC-certified lab methods.
A reference range is a separate idea from a cut-off. A study pooled results from 9,054 men in four US and European groups. It found a normal range of 264 to 916 ng/dL for healthy, nonobese men aged 19 to 39.
It also found that much of the difference between studies came from lab methods, not the men.
Other definitions exist, which is one reason two clinicians can read the same number differently:
| Source | What it uses | Notes |
|---|---|---|
| AUA guideline (2018) | Total testosterone below 300 ng/dL on two early-morning tests | Plus symptoms and/or signs |
| Endocrine Society guideline (2018) | No single cut-off; 264 ng/dL is the lower limit of normal for healthy nonobese young men on CDC-certified assays | Plus symptoms and signs |
| Pooled analysis of four cohorts | 264 to 916 ng/dL | Healthy nonobese men aged 19 to 39 |
| European Male Ageing Study | Total below 3.2 ng/mL (320 ng/dL) plus low free testosterone | Plus at least three sexual symptoms |
| Your lab report | The lab’s own reference range | Depends on the method; check the units |
Accuracy matters too. Testosterone tests haven’t always been accurate or consistent.
The CDC runs a Hormone Standardization Program. From 2007 to 2011 it cut the average error of mass spectrometry tests in half. Our page on total testosterone has the reference ranges by age and explains how to read the number on your report.
When is free testosterone or SHBG tested?
Most testosterone in your blood is attached to proteins, mainly SHBG (sex hormone-binding globulin). Free testosterone is the small share that isn’t attached. If your SHBG is unusually high or low, total testosterone can give a misleading picture.
Obesity, aging and some illnesses can change SHBG.
The Endocrine Society recommends checking free testosterone when total testosterone is near the lower limit of normal, or when a condition changes SHBG. Labs can measure it by a method called equilibrium dialysis.
They can also calculate it from total testosterone, SHBG and albumin, a blood protein. A classic study found that calculated free testosterone was reliable for routine use. The free androgen index and direct “analog” free testosterone tests were not.
Experts don’t fully agree here. The Endocrine Society of Australia, for example, says calculated free testosterone shouldn’t drive clinical decisions. Free testosterone and SHBG each have their own page in our blood work guide.
What do LH and FSH tests show?
Once low testosterone is confirmed, the next question is where the problem starts. LH (luteinizing hormone) and FSH (follicle-stimulating hormone) are signals from the pituitary gland, at the base of the brain.
They tell the testes to make testosterone and sperm. The Endocrine Society recommends measuring them to tell primary from secondary hypogonadism. Hypogonadism is the medical word for low testosterone from the testes or the brain.
| Pattern | Testosterone | LH and FSH | What it suggests |
|---|---|---|---|
| Primary hypogonadism | Low | High | The testes aren’t responding |
| Secondary hypogonadism | Low | Low or normal | The brain isn’t sending enough signal |
| Compensated (a research category) | Normal | High | A pattern described in the European Male Ageing Study |
These definitions come from the European Male Ageing Study. “Compensated” is a research category.
The guidelines above don’t name it as a diagnosis. The study found secondary hypogonadism in 11.8% of men, primary in 2.0% and the compensated pattern in 9.5%.
Secondary was strongly linked to obesity, and primary mainly to age.
The type matters for what comes next, including fertility. In secondary hypogonadism, hormone treatment can sometimes restore fertility.
That isn’t possible when the testes themselves have failed. If you might want children, read TRT and fertility before any treatment decision. Our blood work guide has a page on LH and FSH that explains the tests themselves.
What else can lower testosterone?
The Endocrine Society recommends looking for the cause once low testosterone is confirmed. Some causes can be reversed, which can change the plan.
- Weight and metabolic health: obesity, metabolic syndrome and type 2 diabetes lower testosterone without raising LH and FSH. In one study, each extra BMI point went with lower testosterone.
- Medicines: opioids and glucocorticoids (steroid medicines such as prednisone) are linked to drops in testosterone that may be reversible.
- Acute illness: testing during an illness can give a falsely low result.
- Pituitary problems: your clinician may check prolactin, another pituitary hormone, alongside LH and FSH.
Bring a full list of your medicines and supplements to the visit. Our blood work guide and appointment guide have a checklist for what to bring.
Is low T just part of getting older?
Not as much as people think. In a study of men over 40 who reported very good or excellent health, testosterone didn’t fall with age. Obesity and having smoked were linked to lower levels instead.
The authors suggest the drop often blamed on age may come from health problems that build up over time.
That doesn’t mean age never matters. In the European Male Ageing Study, primary hypogonadism (where the testes stop responding) was linked mainly to age.
Secondary hypogonadism was linked mainly to obesity. The takeaway is simple: low testosterone in an older man is worth investigating, not just accepting.
What happens after a low T diagnosis?
A confirmed diagnosis is the start of a conversation, not an automatic prescription. You and your clinician will talk about the likely cause and whether anything reversible should be tackled first.
You will also discuss your plans for children, and the benefits and risks of treatment. Your clinician can explain the risks in plain language.
If you decide to go ahead, starting TRT means baseline labs, supplies and the first months.
That guide ends with a first 90 days checklist. Once you’re on treatment, the timing of blood tests changes again. Our blood test timing tool helps you plan draws around your injections.
Questions to ask your clinician
- Were both of my tests done in the morning, and was I fasting?
- Which lab method was used, and what is this lab’s reference range?
- Do I need free testosterone or SHBG, given my weight and health?
- What do my LH and FSH results suggest about the cause?
- Is there anything reversible we should address first?
- How would treatment affect my fertility?
How can you prepare for a testosterone blood test?
A few simple habits make your results easier to interpret, now and later:
- Book an early morning draw, ideally before about 10 to 11 am.
- Follow the fasting instructions you’re given, and say if you ate.
- Avoid testing while you’re ill, or tell the lab if you are.
- Ask for a copy of every report, including the reference range and units.
- Keep a record of each draw: date, time, fasting or not, the lab, and how you’d been feeling.
That record is useful long after the diagnosis. TRTracker keeps your lab results, symptom check-ins and draw times in one place. You can show your clinician how things have changed over time.