What Causes Low Testosterone? The Full List, Most Common First

A man gets a result back. Total testosterone 9.1 nmol/L. Underneath it, two words: low testosterone.

That is not a diagnosis. It is a finding, in the same way a temperature of 38.5 is a finding. The useful question, the one that decides everything that follows, is why.

Sometimes the answer is a medication he started eighteen months ago. Sometimes it is untreated sleep apnoea. Sometimes it is thirty kilograms of visceral fat driving a loop that feeds itself. Occasionally it is something in the pituitary that needs imaging this month rather than next year. Each leads somewhere different, and only one of them is fixed by an injection.

The first fork: where the problem sits

Everything downstream depends on one distinction, and it is settled by two hormones rather than by symptoms.

Your pituitary sends luteinising hormone to the testes, telling them to produce testosterone, and follicle-stimulating hormone, driving sperm production. Measure those alongside testosterone and the picture separates.

Primary hypogonadism means the testes themselves are not delivering. The brain notices, shouts louder, and LH and FSH come back high. The signal is fine; the factory is not.

Secondary hypogonadism means the signal itself is weak. Testosterone is low and LH and FSH are low or unremarkable, which is the giveaway, because they should be climbing in response. The factory is capable; nobody is placing the order.

Most men reading this fall into the second group, and most of that group have a reason that can be addressed. That is genuinely good news, and it is why ordering LH and FSH matters rather than testing testosterone alone. How to read your blood test results covers reading a panel properly.

The common causes, most common first

Visceral fat

The largest single contributor in middle-aged men, and the most reliably reversible.

Fat tissue contains aromatase, the enzyme converting testosterone to oestradiol. More fat, more conversion, less testosterone and more oestrogen. Higher oestrogen then feeds back to the pituitary and reduces the signal further, while low testosterone makes fat easier to store and muscle harder to keep. The loop closes on itself.

Weight loss reverses a meaningful part of it. Men who lose substantial weight through diet or surgery show testosterone climbing in proportion to the fat lost, with no hormone prescribed. Why your belly fat will not budge covers the loop, and how to lower oestrogen in men covers the conversion side.

Sleep, and sleep apnoea in particular

Most of your daily testosterone is produced while you sleep, in pulses tied to deep sleep stages. Restrict healthy young men to five hours a night for one week and daytime testosterone falls by roughly ten to fifteen per cent. That is one week, in men with nothing wrong with them.

Obstructive sleep apnoea does something worse, because it fragments sleep every night for years without the man knowing. It is common and strongly associated with low testosterone, and it is missed constantly, because the symptom people report is tiredness rather than breathing. What it does not do is respond to a machine: pooled trial data show that treating apnoea with CPAP does not by itself raise testosterone, most likely because excess weight drives both problems. Sleep apnoea and testosterone covers what that means in practice.

If you snore heavily, wake unrefreshed, or have been told you stop breathing at night, that is worth investigating before anything hormonal. Why you wake up tired after eight hours covers the pattern, and the 90-minute rule covers timing.

Medications

This is the cause most likely to be sitting in plain sight, and the one least often asked about.

• Opioid painkillers are the strongest offender. Sustained use suppresses the pituitary signal profoundly, and the effect is dose-related and often complete.
• Glucocorticoids such as prednisolone, taken regularly, suppress the same axis.
• Some antidepressants and antipsychotics raise prolactin, which lowers testosterone indirectly.
• Certain drugs for prostate conditions alter androgen signalling by design.
• Older acid reflux and antifungal drugs at high doses interfere with testosterone production.

The tell is timing. Symptoms beginning within a few months of a new prescription are a lead, not a coincidence. Nothing here means stopping a medication on your own. It means raising it. Medications that lower libido covers the overlap.

Alcohol

Alcohol suppresses testosterone production directly, burdens the liver that clears oestrogen, and wrecks sleep quality even when it helps you fall asleep. The dose matters more than its presence, and the pattern matters as much as the total. Several drinks on consecutive nights does more damage than the same number spread across a fortnight. The alcohol and testosterone truth covers where the line actually sits.

Insulin resistance and type 2 diabetes

The association is strong and runs both directions. Low testosterone worsens insulin sensitivity and insulin resistance lowers testosterone, which is why the two are found together often enough that some guidance suggests testing testosterone in men with type 2 diabetes and suggestive symptoms.

Under-eating and over-training

This one catches conscientious men rather than lazy ones. Sustained low energy availability, meaning not eating enough for the training you are doing, suppresses the signal from brain to testes. It is well documented in endurance athletes and in prolonged calorie deficits, and it is a relatively recent recognition that it happens in men at all.

The signs read like a reason to train harder: performance stalling, sleep worsening despite exhaustion, libido gone, minor illnesses arriving repeatedly. Why more cardio is tanking your testosterone covers the endurance version.

Chronic illness and chronic stress

Kidney disease, liver disease, inflammatory conditions and any serious illness lower testosterone as part of a general shutdown of non-essential systems. Sustained psychological stress does a milder version of the same thing. Neither responds to treating the testosterone in isolation.

The causes that get missed

These are less common, but each has been the answer for men who spent a year on the wrong track.

Thyroid disease. An underactive thyroid produces almost the same symptom list as low testosterone, and can lower testosterone as well. It is a cheap test and it is not always ordered.

Raised prolactin. Prolactin suppresses the pituitary signal. It can be raised by medication, by an underactive thyroid, or by a benign pituitary tumour called a prolactinoma. A markedly low LH with low testosterone, particularly alongside headaches or any change in vision, needs prolactin measured and often the pituitary imaged. This is the single most important rare cause to exclude, because it is treatable and because the alternative is treating the symptom while the cause continues.

Haemochromatosis. An inherited condition causing iron overload, more common in men of northern European ancestry than most people expect. Iron deposits in the pituitary and the testes, often alongside joint pain, fatigue and abnormal liver tests. A ferritin and transferrin saturation will flag it.

Previous anabolic steroid use. Sometimes years earlier, sometimes a single course in a man's twenties. Recovery of the axis is usually complete but not always, and men do not always volunteer the history. How testosterone recovery works covers the physiology.

Head injury. Significant head trauma can damage pituitary function, sometimes presenting years later. Rarely asked about.

When the testes themselves are the problem

Primary hypogonadism, indicated by high LH and FSH with low testosterone, has its own list.

• Mumps orchitis in adolescence or adulthood, which can damage the testes permanently
• Trauma or testicular torsion, particularly if treatment was delayed
• Undescended testicles in childhood, even when surgically corrected
• Chemotherapy or radiotherapy, which affect sperm production more than testosterone but can affect both
• Klinefelter syndrome, an extra X chromosome, and the most common genetic cause. Frequently diagnosed late, often during a fertility investigation, and worth considering in a man with small firm testes, tall stature and lifelong low levels
• Varicocele, where the association with testosterone is real but weaker and more debated than the association with fertility

Primary hypogonadism generally does not reverse with lifestyle change, which is precisely why establishing which type you have comes before deciding what to do.

What about age?

Testosterone does decline with age, at roughly one to two per cent a year from somewhere in the thirties. Over decades that adds up, and it is real.

What the large population studies found, though, is that a substantial part of the decline attributed to age tracks health rather than years. Men who stay lean, sleep properly, keep training and avoid chronic illness decline far more slowly than the average, and a good number of men in their sixties have levels a sedentary man of forty would envy.

Age explains a gradual drift. It does not explain a man who felt fine at thirty-eight and dreadful at forty-one. That is a change, and changes have causes. Testosterone levels by age covers what is typical, and am I in andropause covers the wider picture.

What does not cause it

Worth stating plainly, because these absorb attention that belongs elsewhere.

Masturbation. It does not lower your testosterone. The evidence is clearer than the forums suggest.

Soy, in ordinary amounts. Reviews of the human evidence have not found meaningful effects on testosterone at normal dietary intakes.

Tight underwear and a phone in your pocket. Sustained scrotal heat does affect sperm quality. Neither is an established cause of low testosterone.

A single low reading. Testosterone varies substantially through the day and between days, and one result taken in the afternoon after a bad night proves very little. Diagnosis requires two morning samples on separate days.

How it is actually worked out

A proper assessment is not expensive and is mostly a matter of ordering the right things together.

• Total testosterone, twice, before eleven in the morning, on separate days
• SHBG and free or calculated free testosterone, because a normal total with a high SHBG can leave very little free hormone available. This is the most common reason a man is told he is fine while feeling otherwise
• LH and FSH, which decide primary versus secondary and therefore everything else
• Prolactin, particularly if LH is low
• Thyroid function, full blood count, ferritin and transferrin saturation, HbA1c, liver and kidney function
• Oestradiol, ideally by a sensitive assay, interpreted against testosterone rather than alone

If that panel is normal apart from testosterone, and LH is low or normal, the conversation moves to weight, sleep, alcohol, medications and training load before it moves to prescriptions. When normal blood tests still mean low testosterone covers the cases missed in the other direction, and the blood test guide covers what to ask for.

What to do with all this

The order that makes sense is unglamorous and works.

Establish the number properly before accepting any explanation, including this one. Two morning samples, with SHBG and LH.

Look for a single dominant cause before assuming it is multifactorial. A man on long-term opioids, or with untreated apnoea, or thirty kilograms overweight, usually has one thing doing most of the work.

Give the reversible causes a real attempt, meaning three to six months rather than three weeks, and then retest. The 30-day reset is the structured version of the first month.

Escalate rather than wait if LH is low with markedly low testosterone, if there are visual symptoms or headaches, or if levels are very low with no obvious explanation.

If you are not sure where you sit, the free two-minute testosterone test gives you a structured symptom picture to take to an appointment, and the home hormone audit works through the environmental and lifestyle side.

Common questions

What is the most common cause of low testosterone in men?

In middle-aged men, excess visceral fat, usually alongside poor sleep and low activity. It works through increased conversion of testosterone to oestrogen in fat tissue and a self-reinforcing loop, and it is substantially reversible.

Can low testosterone be reversed?

Often, when the cause is secondary and functional. Weight loss, reducing alcohol, changing a suppressing medication and correcting under-eating all raise levels. Primary hypogonadism, where the testes are damaged, generally does not reverse.

What blood tests should I ask for?

Two morning total testosterone samples, plus SHBG, free testosterone, LH, FSH, prolactin, thyroid function, ferritin and HbA1c. LH and FSH are the ones most often omitted and the ones that decide the direction.

Does stress cause low testosterone?

Sustained stress lowers it modestly, mostly through cortisol and disrupted sleep. It is rarely the whole explanation on its own, and it is worth looking for a physical cause alongside it.

At what age does testosterone start to drop?

Gradually from around the thirties, at roughly one to two per cent a year. A sudden change over months is not ageing and deserves investigating.

Sources: Bhasin S et al., Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline, J Clin Endocrinol Metab 2018;103(5):1715-44 · Wu FCW et al., Identification of late-onset hypogonadism in middle-aged and elderly men, N Engl J Med 2010;363:123-35 · Leproult R, Van Cauter E, Effect of 1 week of sleep restriction on testosterone levels in young healthy men, JAMA 2011;305(21):2173-4 · Corona G et al., Body weight loss reverts obesity-associated hypogonadotropic hypogonadism, Eur J Endocrinol 2013;168(6):829-43 · Cignarelli A et al., Effects of CPAP on testosterone levels in patients with obstructive sleep apnea: a meta-analysis study, Front Endocrinol 2019;10:551 · Daniell HW, Hypogonadism in men consuming sustained-action oral opioids, J Pain 2002;3(5):377-84 · Educational only, not medical advice.

Keep reading: Sleep apnoea and testosterone · High testosterone symptoms in men · Normal blood test, still low testosterone · Doctor said normal but I still feel tired · Blood test: what to check · Free 2-minute testosterone test