How low testosterone is actually diagnosed

Somewhere in this process a man gets handed one figure and told what it means about him. That is where most of the trouble starts.

Testosterone is one of the few measurements in medicine where the result depends on the hour you gave the sample, on a carrier protein most people have never heard of, and on whether anyone bothered to ask what has actually changed for you. Read one figure without those three things and you are reading a rumor.

It is a diagnosis, not a reading

Testosterone deficiency, properly called male hypogonadism, is defined as persistently low testosterone together with symptoms that fit. Both halves are required. A low number in a man who feels excellent is a number. Loud symptoms with a genuinely healthy hormonal picture are a reason to look elsewhere, not a reason to treat the hormone.

The symptoms usually blamed on testosterone, flat mood, poor concentration, low libido, lost muscle, gained fat, all have several possible causes. The diagnosis is made when the biochemistry and the man point the same way.

The morning rule, and the second sample

Testosterone runs on a daily rhythm and peaks in the morning, so that is when it is measured, fasting. And because one low value can be a blip, it is confirmed on a second morning sample before anyone settles on a diagnosis.

That sounds pedantic until you count how many men have been labeled, or waved away, on the strength of a single draw taken late in the afternoon because that was when the appointment happened to be free.

Two official thresholds, one man sitting between them

The American Urological Association defines deficiency as a total testosterone below 300 ng/dL, about 10.4 nmol/L. The Endocrine Society uses a harmonized lower limit of 264 ng/dL.

Both are serious bodies reading serious data, and they do not agree. So a man can sit between the two lines, unchanged in every respect, and be deficient or normal depending on which document is open on the desk that morning. This is not a scandal. It is what happens when a continuous biological variable meets a committee obliged to draw a line somewhere. It is also the best argument there is against treating any threshold as the answer.

What normal was built from

The reference range beside your result is not arbitrary. A harmonised range was calculated from over 9,000 healthy, non-obese young men across Europe and North America, and it runs from 9.2 to 31.8 nmol per liter.

That is a wide span, and it is the reason two men can both be told they are normal while sitting at opposite ends of it. It is also why the guidelines set decision thresholds well below the top of the range rather than treating the printed range as the answer.

The fraction that does the work

Most of the testosterone in your blood is bound to a carrier protein called SHBG and is inactive. Only the small unbound portion, free testosterone, is available to your tissues. When the total sits near the lower limit, or when SHBG runs unusually high or low, an estimate of free testosterone by a validated method tells you considerably more than the total alone.

This is where a routine workup and a careful one part company. A comfortable looking total sitting on top of a low free testosterone, in a man with clear symptoms, is exactly the picture a single number misses.

Then the more useful question: why

Once low testosterone is confirmed, the next step is not treatment. It is establishing where the failure sits, and two pituitary hormones, LH and FSH, do that job.

• Primary. The testes themselves are underproducing, and the brain pushes LH and FSH high trying to compensate.
• Secondary. The signal from the brain and pituitary is the problem, so LH and FSH sit low or normal despite the low testosterone. When they are low, further evaluation, a prolactin level among others, looks for a treatable cause.

The distinction is not academic. It changes what the right answer is, it decides whether fertility can be protected, and it can reveal something upstream that needs attention long before any hormone is discussed.

What gets established before treatment is even a conversation

A serious assessment sets a baseline first. Hematocrit, meaning how thick your blood is, because a baseline above 48% is a reason not to start. PSA in men 40 and older, because the prostate has to be accounted for. And a short list of circumstances in which treatment is generally held back: plans to conceive in the near future, recent heart attack or stroke, untreated severe obstructive sleep apnea, or known prostate or breast cancer.

What proper care looks like after that

Where testosterone therapy is used properly, the supervision is described in the guidelines rather than improvised. Testosterone and hematocrit are rechecked at around 3 to 6 months and then at least yearly. PSA is reassessed after starting and at least annually in older men. A hematocrit above 54% is treated as a signal to stop and involve a blood specialist. All of that is written down in the guidelines, which is the point. It is checkable, and it is the part that quietly disappears when the prescription is the product.

None of that is exotic. It is simply the part that quietly disappears when the prescription is the product.

Why this matters to you specifically

If you have been told you are fine and you do not feel fine, the useful question is no longer "what was my number". It is what time the blood was taken, whether it was confirmed on a second morning sample, whether free testosterone was estimated, whether LH and FSH were read, and whether anyone asked what had changed for you.

If the answer to most of those is no, you have not been assessed. You have been sampled. What the rest of the bloodwork can tell somebody who knows how to read it is set out in what a blood panel actually measures.

Jayasena CN, et al. "Society for Endocrinology guidelines for testosterone replacement therapy in male hypogonadism." Clinical Endocrinology, 2022. doi:10.1111/cen.14633