What Causes Low Testosterone? Medical and Lifestyle Causes Explained

Low testosterone is not a single condition with a single cause. It is a clinical pattern that can arise from many different routes — some are anatomical, some are hormonal, some are lifestyle-driven, and some are side effects of other medical treatment. This guide walks through the main categories doctors think in when they investigate low testosterone, and explains why getting to the cause matters for the treatment that follows.

For an overview of low testosterone in general, see our complete guide.

Two big categories: primary and secondary hypogonadism

Doctors split the causes of low testosterone into two anatomical categories, based on where in the body the problem originates.

Primary hypogonadism means the issue is in the testes themselves. The brain is sending normal signals (luteinizing hormone, LH, and follicle-stimulating hormone, FSH) telling the testes to produce testosterone, but the testes either cannot respond or cannot produce enough. On blood tests, this typically shows up as low testosterone with high LH and FSH — the brain is shouting louder because the response is missing.

Secondary hypogonadism means the issue is further upstream, in the hypothalamus or the pituitary gland in the brain. The testes themselves are healthy and could produce testosterone, but the signals telling them to do so are not coming through. On blood tests, this typically shows up as low testosterone with low or inappropriately normal LH and FSH — the brain is not making the request.

Distinguishing these matters because the workup differs. Primary hypogonadism might lead a doctor to look at the testes (imaging, sometimes genetic testing). Secondary hypogonadism leads them upstream — prolactin levels, pituitary imaging, thyroid function. Most men with age-related low testosterone fall into a mixed picture that leans secondary, but the categories are still the starting frame.

Specific causes within each category

Primary hypogonadism — testicular causes

• Genetic conditions, most commonly Klinefelter syndrome (an extra X chromosome). Klinefelter affects approximately 1 in 500–1,000 male births and is the single most common genetic cause of primary hypogonadism.
• Undescended testes (cryptorchidism) in childhood, particularly if uncorrected, can affect later testosterone production.
• Mumps orchitis — mumps infection in adolescence or adulthood that involves the testes can damage them.
• Testicular injury, including sports injuries and surgical complications.
• Cancer treatments, particularly chemotherapy or radiation to the pelvic region.
• Autoimmune conditions in which the body attacks the testes (rare).

Secondary hypogonadism — brain and pituitary causes

• Pituitary disorders, including tumours (usually benign), inflammation, or post-surgical changes. Prolactin-secreting pituitary tumours are a specific subset — high prolactin suppresses the testosterone axis.
• Opioid medications, particularly long-term use, suppress the hypothalamic-pituitary-gonadal axis significantly.
• Long-term corticosteroid use, for conditions like asthma, rheumatoid arthritis, or inflammatory bowel disease.
• Severe obesity, which suppresses testosterone production through several mechanisms including increased aromatase activity (which converts testosterone to oestrogen) and reduced LH pulsatility.
• Chronic medical illness, particularly when it involves significant weight loss, malnutrition, or systemic inflammation.
• Untreated obstructive sleep apnoea, which is significantly under-diagnosed in men and meaningfully suppresses testosterone.
• Anabolic steroid use — even after stopping, the body's own testosterone production can take months or years to recover.

Lifestyle and reversible factors

A meaningful proportion of low testosterone in men under 50 is driven by lifestyle factors that can be addressed — sometimes enough to restore healthy testosterone without medical treatment.

• Chronic sleep deprivation. A landmark 2011 JAMA study by Leproult and Van Cauter showed that just one week of restricted sleep (5 hours per night) reduced testosterone by 10–15% in healthy young men.
• Excess body weight, particularly visceral abdominal fat. Aromatase, an enzyme present in fat tissue, converts testosterone to oestrogen. The more fat, the more aromatase activity, the more testosterone is converted.
• Heavy alcohol use, both acutely and over time. Chronic heavy drinking damages testicular Leydig cells.
• Persistent psychological stress, which elevates cortisol and suppresses the entire HPG axis.
• Insufficient resistance training. Strength training does not dramatically raise testosterone, but the absence of it accelerates muscle and bone decline that compounds with low testosterone.
• Extreme endurance training or under-fuelling, particularly in younger men, can suppress testosterone (sometimes called "exercise-related male hypogonadism").

Late-onset hypogonadism (andropause)

In men over 40, the most common cause of low testosterone is a gradual, age-related decline — often called andropause or late-onset hypogonadism. Most men's total testosterone falls by roughly 1% per year from about age 40 onwards. Lifestyle factors usually accelerate this trajectory. Our article on andropause vs low testosterone covers this in more detail.

Why pinning down the cause matters

For most men, the practical treatment options are similar regardless of cause. But the cause shapes a few things:

• If a reversible factor is the main driver, addressing that may resolve the issue without needing prescription treatment.
• If a pituitary or testicular abnormality is identified, that may need its own management beyond just treating the testosterone level.
• If a medication is contributing, your doctor may consider alternatives.
• Fertility-related causes (TRT can suppress sperm production) shape whether testosterone replacement therapy is even appropriate.

When to investigate

A doctor will usually want to look into the cause when:

• Testosterone is confirmed low on two separate tests
• The pattern doesn't fit straightforward age-related decline (e.g. a 30-year-old, or a man with sudden onset)
• Other findings suggest a specific cause (high prolactin, low LH/FSH, abnormal thyroid)
• Treatment decisions depend on knowing the cause (e.g. fertility considerations)

For background on the diagnostic process, see our guide to testosterone testing in Singapore.

Frequently asked questions

Can low testosterone be inherited?

Klinefelter syndrome and a few other specific conditions have genetic components, but most cases of low testosterone are not directly inherited. Family history can offer clues, particularly when it points to specific endocrine conditions.

Does stress really lower testosterone?

Yes. Chronic psychological stress raises cortisol, which suppresses the hypothalamic-pituitary-gonadal axis. The effect is well-documented and is one reason that high-stress lifestyles correlate with lower testosterone, particularly when combined with poor sleep.

Can past anabolic steroid use cause permanent low testosterone?

It can. The body's own testosterone production can take months to years to recover after stopping anabolic steroid use, and in some men recovery is incomplete. If you've used steroids, mention it to your doctor — it shapes the investigation.

Can a vasectomy cause low testosterone?

No. Vasectomy only affects the sperm-carrying tubes; testosterone production is not affected.

Speak to a doctor on Noah

If you have a low testosterone result and want to discuss the likely cause and next steps, book a confidential consultation with a Singapore-licensed doctor on Noah.