What Causes Low Testosterone? The Two Types, and Why One Is Often Reversible

Ask what causes low testosterone and you will get the same list everywhere: ageing, obesity, stress, medications, poor sleep. All true, and all missing the thing that actually matters.

Because low testosterone is not one condition. It is two, and they are as different as a factory that has broken down and a factory that never received the order.

In one version, your testicles themselves have stopped producing properly. In the other, your testicles are perfectly capable, but the brain has stopped sending the signal telling them to work.

Doctors call these primary and secondary hypogonadism, and knowing which one you have changes the diagnosis, the treatment, and, most importantly, whether it can be reversed at all.

Here is the encouraging part, and it is the reason this page exists. The second type, the signalling problem, is the more common one, and its causes are far more often fixable.

Obesity, sleep apnoea, chronic stress, certain medications, and alcohol all belong to that group. Many men who assume their testosterone has permanently declined are actually carrying a cause that can be reversed.

First, how testosterone is actually supposed to work

You need this to make sense of everything below, and it takes thirty seconds.

Testosterone production runs on a chain of command with three links:

1. Your hypothalamus (a region of the brain) releases a hormone that signals the pituitary.
2. Your pituitary gland responds by releasing two messenger hormones, LH (luteinising hormone) and FSH.
3. Your testicles receive LH and produce testosterone in response.

That chain is the whole system. Break it anywhere and testosterone falls, but where it breaks determines everything.

• Break it at step 3, and you have primary hypogonadism. The testicles cannot do the job.
• Break it at steps 1 or 2, and you have secondary hypogonadism. The order never arrives.

Same low number on the blood test. Completely different problems.

Type one: primary hypogonadism (the testicles)

Here the testicles themselves are damaged or not functioning, so no amount of signalling from the brain gets the result. This is why doctors also call it primary testicular failure.

What causes it:

• Klinefelter syndrome, a genetic condition involving an extra X chromosome, and one of the more common genetic causes.
• Undescended testicles that were not corrected in childhood.
• Mumps orchitis, where a mumps infection in adolescence or adulthood damages the testicles.
• Testicular injury, since the testicles sit outside the body and are vulnerable to trauma.
• Cancer treatment, particularly chemotherapy and radiation, which can damage testosterone-producing cells.
• Haemochromatosis, an iron overload disorder.
• Testicular surgery or cancer, including its treatment.
• Other genetic conditions such as Noonan syndrome and myotonic dystrophy.
• Normal ageing, which gradually reduces testicular function over decades.

The honest picture: most primary causes are structural or genetic, which means they are usually permanent. Testosterone replacement therapy under a doctor is the typical treatment, because the production capacity itself is gone.

Type two: secondary hypogonadism (the brain signal)

Here the testicles are fine. The problem is upstream, in the hypothalamus or pituitary, so the instruction to produce testosterone is weak or absent. This type is generally more common than primary, and this is where nearly all of the reversible causes live.

What causes it:

Obesity. One of the biggest and most reversible causes. Body fat produces aromatase, which converts testosterone into oestrogen, and the resulting rise in oestrogen tells your brain to reduce its signalling further. It is a self-reinforcing loop, and losing the fat can genuinely break it. We cover this cycle in detail in our guide to testosterone and weight loss.

Obstructive sleep apnoea. Badly underdiagnosed in middle-aged men, and it suppresses testosterone directly through fragmented sleep and low oxygen. It also causes exactly the fatigue and low mood men blame on their hormones. Treating it often improves testosterone.

Opioid medications. A major and under-recognised cause. The suppression is dose-dependent, can begin within hours of starting, and can persist for as long as use continues. It is also typically reversible once the medication stops, which is why this is worth raising with your doctor rather than ignoring.

Anabolic steroid use, past or present. Taking external testosterone shuts down your own production, because the brain detects plenty circulating and stops signalling. After a cycle ends, that shutdown can persist for months or longer, and in some cases does not fully recover. This is one of the most common causes of low testosterone in younger men, and it is rarely mentioned on pages like this.

Chronic stress. Sustained high cortisol suppresses the signalling chain. This is a real mechanism, not wellness talk.

Type 2 diabetes and metabolic syndrome. Strongly associated with low testosterone through insulin resistance and inflammation.

Pituitary disorders. Tumours (usually benign), pituitary damage, or surgery in that area. Hyperprolactinaemia, where excess prolactin suppresses testosterone signalling, belongs here too and is important because it is treatable once identified.

Certain other medications, including glucocorticoids (steroids like prednisone), some antidepressants, and others.

Heavy alcohol use, which suppresses testosterone at multiple points.

Severe illness, chronic organ disease, HIV, and significant weight loss or malnutrition.

Kallmann syndrome and other congenital conditions affecting the hypothalamus, which are present from birth.

The framing that actually helps: reversible or not

Forget the alphabetical list of causes. This is the split that matters to you.

Often reversible, and worth chasing hard:

• Obesity and excess body fat
• Obstructive sleep apnoea
• Chronic stress and poor sleep
• Heavy alcohol use
• Opioids and certain other medications
• Poorly controlled type 2 diabetes
• Hyperprolactinaemia and some pituitary issues
• Recovery after anabolic steroid use, in many though not all cases
• Vitamin D or zinc deficiency contributing at the margins

Usually permanent, and managed rather than cured:

• Klinefelter syndrome and other genetic conditions
• Testicular damage from injury, mumps, or cancer treatment
• Undescended testicles left uncorrected
• Age-related decline, though its speed is influenced by lifestyle

Look at the first list again. Almost everything on it is either a lifestyle factor or a treatable medical condition. That is genuinely good news, and it is the reason a diagnosis is worth far more than a supplement.

How doctors tell the two apart

This part is simple, actionable, and almost never explained on affiliate pages.

The distinction is made by measuring LH and FSH alongside your testosterone.

• Low testosterone with HIGH LH and FSH points to primary hypogonadism. Your brain is shouting the order and the testicles are not responding, so the brain shouts louder.
• Low testosterone with LOW or NORMAL LH and FSH points to secondary hypogonadism. The order was never properly sent, which is why the messengers are not elevated.

That is why asking only for “a testosterone test” is not enough. Ask for total testosterone, free testosterone, SHBG, LH, and FSH, and ideally prolactin, thyroid function, HbA1c, full blood count, and vitamin D to catch the conditions that mimic or contribute to low testosterone.

One more practical detail: have the blood drawn in the morning, ideally before 10am, because testosterone peaks early in the day and an afternoon sample can read misleadingly low.

Our guide to the signs of low testosterone in men over 45 covers exactly what to ask for and why.

The pain points, answered honestly

Is it just my age?

Partly, but far less than most men assume. Testosterone declines gradually by around 1 to 2 percent a year after your thirties. A level that has fallen below the clinical range, or symptoms that leave you feeling unlike yourself, is not simple ageing and deserves investigation.

I am young, this cannot be me.

It can. Secondary causes like obesity, sleep apnoea, opioids, chronic stress, and prior anabolic steroid use affect men in their twenties and thirties regularly.

My test came back normal but I feel terrible.

Ask about free testosterone and SHBG. SHBG rises with age and binds testosterone, so your free level can be low while the total looks fine. If those are normal too, look at thyroid, blood sugar, sleep apnoea, and depression, all of which mimic low testosterone.

Can I fix this myself?

If your cause is on the reversible list, lifestyle changes genuinely help, sometimes dramatically. If it is primary hypogonadism, no amount of effort restores production, and you need medical treatment. That is exactly why identifying the cause comes first.

Will a supplement fix it?

No. Supplements do not treat clinically low testosterone of either type. They can correct deficiencies like vitamin D or zinc that contribute at the margins, and that is the honest limit of what they do.

What to do next

1. Book a morning blood test and ask for the full panel, not just total testosterone. LH and FSH are the ones that reveal which type you have.
2. Answer the sleep apnoea question honestly. If you snore heavily, wake unrefreshed, or your partner has noticed you stop breathing, get assessed. It is common, damaging, and treatable.
3. Review your medications with your doctor, especially opioids and steroids. Never stop a prescription on your own.
4. Be honest about alcohol, body fat, and stress, which sit behind a large share of reversible cases.
5. Get the diagnosis before you buy anything. Knowing whether your problem is the factory or the order changes everything that follows.

The most useful thing on this page is not a product. It is the knowledge that most causes of low testosterone are the reversible kind, and that a single blood panel can tell you which kind you are dealing with.

FAQs

What causes low testosterone in men?

There are two distinct types. Primary hypogonadism comes from the testicles themselves failing, caused by genetic conditions like Klinefelter syndrome, undescended testicles, mumps, injury, or cancer treatment. Secondary hypogonadism comes from the brain not signalling properly, and its causes include obesity, sleep apnoea, opioid medications, prior anabolic steroid use, chronic stress, diabetes, pituitary disorders, and heavy alcohol use. Secondary is more common and far more often reversible.

What is the difference between primary and secondary hypogonadism?

In primary hypogonadism the testicles cannot produce testosterone despite normal signalling from the brain, and LH and FSH levels are high. In secondary hypogonadism the testicles are capable but the brain's signal is weak or absent, so LH and FSH are low or normal. A blood test measuring LH and FSH alongside testosterone distinguishes them.

Can low testosterone be reversed?

Yes, depending on the cause. Reversible causes include obesity, sleep apnoea, chronic stress, heavy alcohol use, opioid medications, and poorly controlled diabetes. Causes like genetic conditions or testicular damage are usually permanent and managed with medical treatment rather than cured.

What is the most common cause of low testosterone?

Secondary causes are more common than primary ones, and obesity is among the most significant and reversible. Excess body fat converts testosterone to oestrogen and suppresses the brain's signalling, creating a self-reinforcing cycle that weight loss can break.

Can low testosterone happen to young men?

Yes. Obesity, obstructive sleep apnoea, opioid use, chronic stress, and previous anabolic steroid use all cause low testosterone in men in their twenties and thirties. Age is not a requirement.

Do steroids cause low testosterone?

Yes. Taking external testosterone or anabolic steroids suppresses your own production because the brain stops signalling. This shutdown can persist for months after stopping and does not always fully recover, making it a significant cause of low testosterone in younger men.

What blood tests diagnose the cause of low testosterone?

Ask for total testosterone, free testosterone, SHBG, LH, and FSH, taken in the morning. LH and FSH reveal whether the problem is in the testicles or in the brain. Prolactin, thyroid function, HbA1c, full blood count, and vitamin D help rule out contributing or mimicking conditions.

Can sleep apnoea cause low testosterone?

Yes, and it is commonly missed. Obstructive sleep apnoea fragments sleep and lowers oxygen levels, suppressing testosterone, while producing fatigue and low mood that men often blame on hormones. Treating the apnoea frequently improves testosterone.