The Loaded Joint

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Is 400 testosterone low? What the number means at your age

A total testosterone of 400 ng/dL (about 13.9 nmol/L) is not low by any major guideline. The harmonised reference range for healthy non-obese men aged 19 to 39 runs from about 264 to 916 ng/dL, and the American Urological Association uses under 300 ng/dL, confirmed on two separate early-morning tests, as the cut-off for low testosterone. A 400 sits in the lower part of normal. Whether it matters depends on when the blood was drawn, your SHBG and free testosterone, and whether you have symptoms.

By The Loaded Joint · Last updated

You got your results back, you saw 400, and then you searched it, which is how you ended up here. The short version is that 400 is a normal number. The longer version is that a single number, on its own, says less than people think, and there are a few things worth checking before you decide whether it is good news or a reason to dig further.

What the reference range actually says

For years every lab used its own testosterone range, which is why the same result could be flagged at one clinic and not another. In 2017 Travison and colleagues published a harmonised range in the Journal of Clinical Endocrinology and Metabolism, built from four large cohorts and calibrated against a CDC standard. For healthy, non-obese men aged 19 to 39, it runs from roughly 264 to 916 ng/dL.

So 400 is comfortably inside it. It is in the lower part of the range, not the bottom of it.

The guideline thresholds point the same way:

If you prefer the units used in the UK, Canada and Australia, divide by 28.84. 400 ng/dL is about 13.9 nmol/L, and 300 ng/dL is about 10.4 nmol/L.

Why the same man can test 400 and 550

Testosterone is not a fixed trait you measure once. It moves, and three things move it more than most men realise.

Time of day. Levels peak in the early morning and fall through the day. A sample taken at 4pm can read noticeably lower than one taken at 8am. This is why both guidelines insist on morning testing.

Whether you ate first. In a 2013 study, drinking a glucose load lowered testosterone by around 25% for up to two hours afterwards. A fasting sample gives you the cleanest read.

How you slept. In a 2011 study in JAMA, one week of sleeping five hours a night lowered daytime testosterone in healthy young men by 10 to 15%. That is roughly the effect of ten to fifteen years of ageing, from one bad week.

So if your 400 came from an afternoon draw, after lunch, in a week of short nights, it may be an underestimate of where you usually sit.

Total versus free: the number underneath

Total testosterone includes hormone that is tightly bound to a carrier protein called sex hormone binding globulin, or SHBG. Bound testosterone is largely unavailable to tissues. What your body can actually use is the free and loosely bound fraction.

That matters because two men with the same total can be in very different places:

Total T SHBG What it tends to mean
400 ng/dL Normal Usually unremarkable
400 ng/dL High Free T may be on the low side
400 ng/dL Low Free T often fine

SHBG rises with age and with some conditions, and falls with excess weight and insulin resistance. The Endocrine Society suggests measuring free testosterone when total is near the lower limit or SHBG is likely to be abnormal, which describes a 400 result reasonably well. If your report only shows total, it is fair to ask for SHBG and free testosterone at the next draw.

What to do with a 400

If you feel fine, there is nothing to fix. A normal number in a man without symptoms is a normal number.

If you feel flat, tired or unmotivated, the useful next step is a better test, not a treatment decision:

  1. Repeat it on a weekday morning, before 10am, fasted.
  2. Ask for SHBG and free testosterone alongside total.
  3. Ask for ferritin, vitamin D, thyroid (TSH with free T4) and HbA1c in the same draw, because each of those causes the same symptoms.
  4. If you snore or wake unrefreshed, ask about a sleep study first.

That last point is the one most men skip. Undiagnosed sleep apnoea is common, badly underdiagnosed in men, and lowers testosterone on its own. Treating the sleep problem often moves the number more than anything else would. We go through the full list in why "your labs are fine" is not a result, and the sleep side in tired, not low testosterone.

If a repeat morning test comes back under 300 twice and you have symptoms, that is a conversation for your doctor. Testosterone therapy is a prescription decision with real trade-offs, including fertility. It is not something to start from one afternoon blood test.

The honest bottom line

400 is a normal testosterone level for a man of almost any age. It sits in the lower part of the range, which makes the testing conditions and the free testosterone number worth getting right. In most men who feel off with a 400, the cause turns out to be sleep, weight, alcohol, training load or one of the other blood markers, and all of those are cheaper and safer to fix than the hormone itself.

If your results came back normal and you still feel flat, here is what a reference range does and does not tell you.

Frequently asked questions

Is 400 ng/dL testosterone low for a man?

No. The harmonised range for healthy men aged 19 to 39 is about 264 to 916 ng/dL, and the AUA cut-off for low testosterone is under 300 ng/dL on two morning tests.

Is 400 testosterone enough to build muscle?

Yes. Men across the normal range build muscle with consistent training and enough protein and calories. Training, food and sleep move results far more than the difference between 400 and 600.

What time of day should testosterone be tested?

In the morning, ideally before 10am and fasted. Levels peak early and fall through the day, and eating can lower them temporarily.

What is 400 ng/dL in nmol/L?

About 13.9 nmol/L. Divide ng/dL by 28.84 to convert.

Should I ask for free testosterone if my total is 400?

It is reasonable. A total near the lower end is exactly when free testosterone and SHBG add useful information.

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