Testosterone: What It Does, Normal Levels and What Men Should Know

Many men first hear the word "testosterone" from fitness videos or supplement adverts, and come to think of it as just the hormone of muscle and sex. When a patient asks me, "Should I top up my testosterone?", I usually suggest we take a step back first: what does this hormone do, what is a normal level, and how reliable is the blood result in your hand? If you start from the wrong place, the treatment usually goes wrong too.

What is testosterone and where is it made?

Testosterone is the main male sex hormone. In men, about 95% is made in the testicles and the rest comes from the adrenal glands. Its production is controlled by the brain in a chain of signals:

1. The hypothalamus releases GnRH.
2. The pituitary gland releases LH, which stimulates the testicles to make testosterone, and FSH, which supports sperm production.
3. When hormone levels are high enough, the brain turns down its signals by itself, like a thermostat in an air-conditioned room.

This system is called the HPG (hypothalamic–pituitary–gonadal) axis, and it explains why hormones from outside the body, both TRT and steroids, shrink the testicles and reduce sperm. Read the details in TRT side effects.

Hormone levels also go up and down during a single day, peaking in the morning and falling in the afternoon and evening. This is especially noticeable in young men.

What this hormone does

Some testosterone is converted to DHT, which is involved in the prostate, acne and thinning hair, and some is converted to oestrogen, which men need as well. The details are in Hormone imbalance in men.

What is normal, and how to read your blood result

Total testosterone results are usually reported in one of two units. A simple conversion is 1 nmol/L ≈ 28.8 ng/dL.

Different organisations use slightly different cut-offs. The American Urological Association (AUA) uses about 300 ng/dL, the European Association of Urology (EAU) still uses 12 nmol/L as its cut-off in its 2025–2026 edition, and the Endocrine Society uses the lower limit of a harmonised reference range, about 264 ng/dL. All of them agree on the key point: the number alone is not enough; there must also be symptoms.

Normal ranges also differ between laboratories depending on the test method, so I always read a result alongside that lab's own reference range.

Does testosterone really fall with age?

Yes, but it doesn't plunge like oestrogen does at menopause in women. On average it falls gradually by about 1% a year after the age of 30–40. Meanwhile SHBG, the protein that binds the hormone, tends to rise with age, so the hormone actually available to the body falls more than the total figure suggests (read What is SHBG?).

However, a large European study (EMAS) found that only about 2% of men aged 40–79 had both clear symptoms and genuinely low testosterone, and that weight gain, chronic illness and lack of sleep pull testosterone down more than age does. So the idea that "once you're older you need to top up testosterone" isn't really right. A better rule is: "If you have symptoms, look for the cause."

What can suppress testosterone

• Belly fat, poorly controlled diabetes, fatty liver
• Too little sleep, or obstructive sleep apnoea
• Acute illness, major surgery, severe stress
• Some medicines, such as opioid painkillers and long-term oral or injected steroids
• Heavy drinking, very intense exercise combined with eating too little
• Previous use of steroids for muscle building
• Diseases of the testicles or pituitary gland, such as a prolactin-secreting tumour, genetic conditions like Klinefelter syndrome, testicular injury or previous chemotherapy

Many items on this list can be fixed, and once they are, testosterone often rises again by itself. Practical guidance is in Supporting testosterone with sleep, food and exercise.

How to test so the result is reliable

1. Have your blood taken in the morning, between 07:00 and 10:00.
2. Fast before the test, because high-sugar food can temporarily lower the result.
3. Avoid testing while ill, sleep-deprived or after an overnight journey.
4. If the result is low, repeat it on another day.
5. Test LH and FSH to tell whether the cause is in the testicles or the brain, and SHBG in men with borderline results.

A more complete list of tests, especially if you are thinking about treatment, is in Blood tests before starting TRT.

When to see a doctor

If you have several symptoms at once, such as a clear drop in sex drive, no morning erections, tiring easily without an obvious reason, losing muscle, gaining belly fat or breast enlargement, read Symptoms of low testosterone and then get checked properly.

At Dr. Kris Clinic I always start with a history, physical examination and blood tests at the right time of day to find the cause first. Only if you genuinely meet the criteria for testosterone deficiency do we discuss treatment options such as TRT, together with a safety monitoring plan. You can consult at the clinic in Surat Thani or online, and consultations in English are available.