HORMONES
What Causes Low Testosterone in Men?
QUICK ANSWER
Low testosterone is usually driven by something else: carrying excess body fat, untreated sleep apnoea, type 2 diabetes, certain medicines, heavy alcohol, anabolic steroid use, or a problem in the testes or pituitary gland. LH and FSH on a blood test show a clinician whether the fault starts in the testes or higher up, which decides what happens next.
A low testosterone result is rarely the beginning of the story. It is usually the end of a chain that started somewhere else, and the number on the page is the visible part of a problem with a cause you can often name and sometimes reverse. Treating the reading as the disease, rather than asking what produced it, is the most common mistake men make when they see a low figure.
This guide walks through the real causes of low testosterone, from the everyday and reversible to the medical and rare, and explains how the right blood test points a clinician to the right one. If you want the numbers themselves, our testosterone levels by age guide covers what counts as normal; if you arrived here from symptoms, the symptoms of low testosterone guide is the better starting point.
The fault sits in one of two places
Testosterone production runs on a feedback loop. The brain's hypothalamus signals the pituitary gland, the pituitary releases two hormones called LH and FSH, and those travel to the testes and tell them to make testosterone and sperm. When the level falls, the problem is almost always in one of two places: the testes themselves, or the control system above them.
Clinicians call these primary hypogonadism (the testes cannot respond, even when told to) and secondary hypogonadism (the testes are fine but the signal from above is weak). It is not a technicality. The two have different causes, different consequences for fertility, and different treatment, so working out which one you have is the whole point of testing properly rather than acting on a single low number.
The elegant part is that the same blood draw that measures testosterone can usually tell the two apart, because LH and FSH behave in opposite directions depending on where the fault lies. We come back to how that works once the causes are clear.
The common, reversible causes
For most men under retirement age, low testosterone is not a disease of the testes at all. It is the body responding to something it can change. These causes act mainly on the signal from above, which is why they so often come with normal or low LH and FSH rather than high ones.
- Excess body fat.This is the single biggest modifiable cause. Fat tissue converts testosterone into oestrogen and disrupts the brain's signalling, so the more abdominal fat a man carries, the lower his testosterone tends to run. The relationship is two-way and self-reinforcing, and weight loss is one of the few things shown to lift the level without medication.
- Type 2 diabetes and metabolic syndrome. Insulin resistance and low testosterone travel together so reliably that UK guidance suggests checking testosterone in men with type 2 diabetes. Our insulin resistance guide explains the metabolic side of the same picture.
- Obstructive sleep apnoea. Broken, oxygen-starved sleep suppresses the overnight testosterone surge that normally tops the level up. Many men with unexplained low testosterone turn out to have undiagnosed sleep apnoea, and treating it can help.
- Poor and short sleep. Even without apnoea, testosterone is made largely during sleep, and restricting it to five hours a night for a week measurably lowers the daytime level in healthy young men.
- Heavy alcohol. Regular heavy drinking damages both the testes and the signalling above them, and is one of the more common reversible causes in otherwise well men.
- Chronic stress. Sustained high cortisol suppresses the hormonal axis that drives testosterone, which is one reason a useful male panel measures cortisol alongside it.
The reason these matter is hopeful: a low level driven by any of them is, in principle, a level that can recover. Our guide to raising testosterone naturally sets out what actually moves the number and what does not.
Medical and hormonal causes
A second group of causes is more clearly medical, and several are things a blood test is specifically designed to catch.
- Medications. Opioid painkillers are a major and under-recognised cause; long-term opioid use suppresses testosterone in most men who take it. Corticosteroids, some antidepressants, and a few other drugs do the same. None of this shows up unless someone thinks to look.
- Anabolic steroids, past or present.Taking testosterone or its derivatives shuts down the body's own production, and after a cycle ends the natural level can stay suppressed for months or, sometimes, fail to recover. This is a common cause of low testosterone in younger, gym-going men.
- High prolactin. Prolactin, when raised, directly switches off the signal that drives testosterone. A markedly high level can point to a small, benign pituitary tumour, which is why prolactin is checked when a low testosterone result does not have an obvious cause.
- Raised oestradiol. The male body makes a small amount of oestrogen, and when it runs high — often alongside excess body fat — it can suppress testosterone and cause symptoms of its own. Our oestradiol guide covers why men measure it.
- Testicular problems. An undescended testicle in childhood, a past injury, mumps that involved the testes, a varicocele, or treatment for testicular cancer can all leave the testes unable to produce enough testosterone. These tend to cause primary hypogonadism, with high LH and FSH.
- Genetic and pituitary conditions.Klinefelter syndrome is the most common genetic cause and is often diagnosed only in adulthood, while pituitary disease and iron overload (haemochromatosis) are rarer causes worth a clinician's attention when the picture is unusual.
This is exactly the territory where guessing fails and measuring wins. Several of these causes are invisible without the right markers, and a few of them matter well beyond testosterone.
What about age?
Testosterone does fall gradually with age, by roughly one to two per cent a year from the late thirties onward. But age alone is a slow, modest decline, and it is a poor explanation for a level that has dropped sharply or sits well below the range for a man's years. When testing finds a clearly low level, ageing is usually the least of it; a reversible cause is hiding underneath far more often than men expect.
This matters most in younger men, where age explains almost nothing and a low result nearly always has a findable cause. Our guide to low testosterone in young men goes into that age group in detail, and the male menopause guide covers the genuine age-related end of the spectrum.
How a blood test finds the cause
A single testosterone number tells you the level but not the reason. To find the cause, a clinician reads it alongside a small group of supporting hormones, and the combination is what points to the answer:
- LH and FSH are the deciding pair. Low testosterone with high LH and FSH means the brain is shouting and the testes are not answering — primary hypogonadism. Low testosterone with low or normal LH and FSH means the signal itself is weak — secondary hypogonadism, the pattern that goes with body fat, medication and sleep apnoea. Our LH and FSH guide explains this in full.
- SHBG and free testosterone. Much of your testosterone is bound to a carrier protein and unavailable, so a total that looks borderline can hide a genuinely low usable level. SHBG and free testosterone are what separate the two.
- Prolactin and oestradiol catch the hormonal causes above — a pituitary problem, or oestrogen running high — that a testosterone reading alone would miss entirely.
This is the reasoning behind the Complete Male Hormones panel, which measures total and free testosterone, SHBG, LH, FSH, prolactin, oestradiol, DHEA-S and cortisol in one draw so a clinician can read the cause, not just the level. How and when you take that sample matters too: testosterone should be measured in the morning and fasted, as our timing guide explains, and the practical mechanics are in the testosterone blood test guide.
One reading is never enough on its own. UK clinical practice confirms a low result with a second morning fasting sample before treating it as real, and symptoms — not numbers in isolation — lead the investigation throughout.
Frequently asked questions
What is the most common cause of low testosterone?
In men under retirement age, excess body fat is the most common and most reversible cause. Fat tissue converts testosterone to oestrogen and disrupts the brain's hormonal signalling, and the relationship is self-reinforcing, which is why losing weight is one of the few things shown to raise the level without medication.
Can low testosterone be reversed?
Often, yes, when the cause is reversible. Low testosterone driven by body fat, poor sleep, untreated sleep apnoea, heavy alcohol or certain medicines can recover when the underlying cause is addressed. Causes that sit in the testes themselves are less reversible, which is why identifying the cause first matters.
Does age cause low testosterone?
Age causes a slow decline of roughly one to two per cent a year from the late thirties, but that alone rarely explains a level that is clearly low. When testing finds a low result, a reversible cause is hiding underneath far more often than ageing accounts for, especially in younger men.
How does a blood test show why testosterone is low?
LH and FSH are the key. Low testosterone with high LH and FSH points to a problem in the testes; low testosterone with low or normal LH and FSH points to a weak signal from the pituitary or hypothalamus, the pattern seen with body fat, medication and sleep apnoea. Prolactin and oestradiol catch the hormonal causes a testosterone reading alone would miss.
Can medication cause low testosterone?
Yes. Opioid painkillers are a major and under-recognised cause, and long-term use suppresses testosterone in most men who take them. Corticosteroids and some other drugs do the same, and past or present anabolic steroid use can suppress natural production for months. Note anything you take so a reviewing clinician can account for it.
FIND THE CAUSE, NOT JUST THE NUMBER
Helvy measures testosterone alongside the hormones that explain it — LH, FSH, SHBG, prolactin and oestradiol — so a clinician can read the cause. Two minutes to find the right test.