Low testosterone has become an increasingly common explanation for almost any change a man notices in his thirties or forties.
Suppose your sex drive is lower. Training feels harder, recovery takes longer, or you do not have the same drive you used to.
Testosterone could be part of the reason. But none of those changes, on their own, can tell you that it is low.
Poor sleep can affect sexual function and energy. Stress can reduce libido. Thyroid problems, medication, low iron, and changes in metabolic health can create a very similar picture.
Low testosterone is a specific medical condition. It is diagnosed when relevant symptoms appear alongside consistently low testosterone levels measured under the right conditions. One online checklist or one unexpected blood result is not enough.
So, how do you know?
You start by understanding what testosterone does, then look at the most important symptoms, what may be lowering it, and how to test it properly.
Testosterone is the main male sex hormone. Most men associate it with sex drive and muscle, and it does influence both, but its reach is considerably wider.
Almost all testosterone is produced in the testes, in cells called Leydig cells. A much smaller amount comes from the adrenal glands, which sit above the kidneys.
But production starts in the brain:
It works more like a thermostat than a tap, constantly adjusting rather than producing the same amount all day.
Testosterone is also released in pulses and follows a daily rhythm. Levels rise during sleep and usually reach their highest point in the morning.
Testosterone tends to decline gradually with age. Population studies have estimated an average annual decline in total testosterone of roughly 0.4%, while free testosterone may decline faster as SHBG rises.
That is a slow drift, not a cliff. When something changes sharply in your late thirties or forties, ageing alone is rarely the whole explanation.
The difficulty is that most of these effects do not belong only to testosterone. Some symptoms are much stronger clues than others.
The symptoms that track low testosterone most reliably are sexual:
That does not mean every change in sexual function is hormonal. Erections also depend on blood flow, nerve function, medication, psychological health and several other factors. But sexual symptoms are more specific to testosterone deficiency than fatigue, weight gain or low mood.
This became clear in the European Male Ageing Study, which examined 3,369 men aged 40 to 79. Researchers compared measured testosterone levels with more than 30 possible symptoms.
Only three sexual symptoms showed a consistent relationship with low testosterone: reduced sexual thoughts, fewer morning erections and erectile difficulties. The estimated prevalence of late-onset hypogonadism in the study was 2.1%, far lower than symptom checklists might suggest.
Occasionally there is a medical problem affecting the testes or the hormone signals coming from the brain. But testosterone can also fall when poor health, medication, or a sustained strain on the body suppress an otherwise functioning system.
That distinction matters because you can improve several influences.
The problem can also begin in the testes themselves. Causes include previous testicular injury, certain infections, cancer treatment, an undescended testicle or genetic conditions such as Klinefelter syndrome.
It can also begin higher up, when the hypothalamus or pituitary gland does not provide enough stimulation. Raised prolactin, iron overload and pituitary disease are among the possible causes.
This is why a low testosterone result is the beginning of the investigation, not the end.
The first number on your report will usually be total testosterone. In Europe it’s often given in nmol/L; in the U.S., ng/dL.
Total testosterone counts all the testosterone in your blood. But most of it is not available to your tissues.
Testosterone travels around your body bound to two proteins made by your liver:
Only a small amount is not stuck to anything at all. That’s called free testosterone, and along with the loosely bound portion, it’s the part your body can actually use.
Think of SHBG as a gatekeeper. Total testosterone tells you how much testosterone is in your blood. It does not tell you how much is getting through to your muscles, bones, and brain.
This is why two men can have exactly the same total testosterone, different amounts of SHBG, and feel entirel different. One has plenty to get through. The other has most of it locked away.
If your total testosterone doesn’t match how you feel, measuring SHBG and calculating free testosterone will usually explain why.
You now know that three numbers matter (total T, free T, SHBG) and around a dozen things that move them.
Here’s what happens when all of that gets reduced to one word on a report.
A reference range shows where your result sits compared with a group of people who were tested when the range was created. That’s all it does.
For men, the widely used range for total testosterone is roughly 9–32 nmol/L (about 260–920 ng/dL). That range was built by measuring healthy men aged 19–39 who were not overweight.
Now compare two men:
One sits near the top of the range. The other is just above the bottom of it. They’re given the same word. Neither has been told anything about their SHBG, so neither knows how much testosterone is actually reaching them.
There’s a second problem. Laboratories do not agree on what the range should be.
One survey of 25 laboratories found 17 different sets of reference values in use, with the lowest acceptable value varying by more than three times between them.
The Endocrine Society, the main international body for hormone specialists, has said that because testing methods are not standardised, the same sample of blood can be reported as low at one laboratory and normal at another. They noted this leads to some men being wrongly diagnosed and others being wrongly reassured.
A reference range cannot tell you:
So the question worth asking is not whether your testosterone is "normal". It’s what your result means for your body, at your age, given everything else going on.
The first test is total testosterone, but the conditions under which it is taken matter.
European guidance recommends:
Testosterone is normally higher in the morning, and food can temporarily reduce the measured concentration. A sample taken late in the afternoon after eating may therefore appear low in a man whose morning result would be normal.
One test gives you a number. The wider panel helps explain it.
Testosterone shows the level. SHBG and free testosterone help explain how much may be available. LH and FSH help identify where the problem may begin. The remaining biomarkers help show whether testosterone is the main story at all.
This is also why testing one hormone in isolation can leave you with more questions than answers.
With Axo Longevity, testosterone can be assessed alongside the hormonal, metabolic, thyroid and nutritional biomarkers needed to build a broader picture of what may be influencing how you feel.
See every biomarker included in the Axo Longevity panel →
A testosterone result only becomes useful when you understand what may be influencing it.
That is why Axo Longevity looks beyond testosterone alone.
We measure more than 100+ biomarkers across 36+ health areas. Your testosterone result is then read alongside this wider picture, rather than treated as an isolated number on a laboratory report.
You receive:
The aim is not to push testosterone as high as possible.
The aim is to understand what your level means for your body, what may be driving it, and which actions are most relevant for you.
General information about health and wellness, not medical advice. Reference ranges vary between laboratories and assays. Discuss your own results with a qualified clinician before changing medication or supplementation.
