What is testosterone: an in-depth guide

• What is testosterone?
• What does testosterone do in the body?
• What are normal testosterone levels and why do they matter?
• Symptoms of low testosterone (hypogonadism)
• Causes of low testosterone
• Diagnosing low testosterone
• Treatment options for low testosterone
• Is testosterone therapy safe? Risks and side effects
• Testosterone and men under 40
• Testosterone and men over 40
• Testosterone and athletes
• Testosterone myths busted
• FAQs about testosterone
• Conclusion: why testosterone is worth your attention

Testosterone is one of those words that gets thrown around in gyms, online forums, and even at the pub. But what does it really do, who needs to care, and why is it suddenly everyone’s favourite hormone? Whether you’re in your 20s, over 40, or an athlete chasing peak performance, understanding testosterone could be the key to unlocking your best health, energy, and confidence. Let’s break down the science, debunk the myths, and give you practical tips you’ll actually want to use.

What is testosterone?

Testosterone is a hormone, think of it as a tiny chemical messenger, that plays a starring role in the male body. Produced mainly in the testicles (and in smaller amounts by women’s ovaries and adrenal glands), testosterone is responsible for everything from muscle growth and bone density to sex drive and mood. It’s the reason voices deepen during puberty, beards sprout, and, yes, why some of us can’t resist flexing in the mirror.

But testosterone isn’t just about muscles and bravado. It’s vital for red blood cell production, fat distribution, and even memory. When levels are just right, you feel energised, motivated, and strong. When they’re off? Well, things can get a bit wobbly, physically and emotionally.

The basic chemical composition of testosterone is steroidal; and steroids are known to have significant physiological, as well as psychological, effects in males, especially adults.

What does testosterone do in the body?

Testosterone’s effects are wide-ranging and surprisingly subtle. Here’s a snapshot:

• Physical development: Drives puberty changes (deeper voice, facial/body hair, growth spurts).
• Muscle and bone health: Builds and maintains muscle mass and bone strength.
• Sexual function: Fuels libido, sperm production, and erectile function.
• Mood and cognition: Influences confidence, motivation, and even memory.
• Red blood cell production: Supports oxygen delivery throughout the body.

It’s a bit like the conductor of an orchestra, when it’s in sync, everything works beautifully.

What are normal testosterone levels and why do they matter?

This is a tough question to answer, and unfortunately, it doesn’t really have an answer in my opinion.

Normal values are considered to be the average levels of testosterone in studies across all-male populations found in studies of men ranging from the age of 20-80. This may seem ridiculous, but it’s very close to the truth. This will include men who are physically fit, men who are obese, men who exercise, men who take drugs or drink excessively as well as men who may be ill and be under physical, emotional or environmental stress.

To calculate normal testosterone levels, the top and bottom values are ignored, and the median average of normal levels is created – so you then have normal levels of testosterone. Or do you?

• According to the NHS, the figure is 10-30nmol/l.
• According to the British Society of Sexual Medicine, the values are:
  • A Total Testosterone (TT) level lower than 8 nmol/l or Free Testosterone (FT) level lower than 180 pmol/l (<0.180 nmol/l; based on 2 separate levels from 8 to 11 AM) usually requires testosterone therapy.
  • TT level higher than 12 nmol/l or FT level higher than 225 pmol/l (>0.225 nmol/l) does not require testosterone therapy.
  • Levels from 8 to 12 nmol/l might require a trial of testosterone therapy for a minimum of 6 months based on symptoms.

It must also be noted that these are guidelines and up to the actual individual practitioner.

In recent years, Labcorp, one of the worlds leading diagnostic companies, changed its guidelines from 12-42 nmol/l (348-1197 ng/dL) to 9-32 nmol/l(264-916 ng/dl).

To further complicate matters different Endocrine bodies in Europe and the West have different opinions as to what are low or normal levels of testosterone. Doctors have widely different opinions on the importance of testosterone in older men or whether testosterone therapy is even useful.

For the sake of argument, we’ll use 8.64 nmol/l to 32 nmol/l as the figures used by the NHS. So if testosterone levels are below 8.64 mmol/l a male patient is considered to have low testosterone and should be entitled to testosterone replacement therapy (TRT).

However, what happens when the patient has levels of 9.5 nmol/l and is feeling the traditional symptoms of testosterone deficiency. What does a doctor do? Usually, nothing. If they follow guidelines, they may issue antidepressants, or as one doctor said he would reassure the patient and send him home.

Testosterone levels are measured via a simple blood test. Levels naturally fluctuate throughout the day (highest in the morning), and tend to decline with age.

• Under 40: Usually at their peak.
• Over 40: Gradual decline—about 1% per year after age 30.
• Athletes: May be higher due to genetics and training, but can also dip with overtraining or extreme dieting[6].

Symptoms of low testosterone (hypogonadism)

Low testosterone isn’t just about struggling at the gym. Signs can be subtle and easily mistaken for “just getting older” or being stressed:

• Reduced sex drive
• Erectile dysfunction
• Fatigue and low energy
• Loss of muscle mass or strength
• Increased body fat (especially around the belly)
• Mood changes—irritability, low mood, or brain fog
• Poor sleep
• Reduced bone density (risk of osteoporosis)
• Difficulty concentrating

If you’re ticking several of these boxes, it’s worth chatting to your GP or getting touch with our team.

Causes of low testosterone

Low testosterone (hypogonadism) can be caused by:

• Ageing: The most common cause, levels slowly decline from your 30s onward.
• Medical conditions: Diabetes, obesity, chronic illness, pituitary or testicular disorders.
• Medications: Some steroids, opiates, and cancer treatments.
• Lifestyle: Poor diet, lack of exercise, excessive alcohol, chronic stress, or sleep deprivation.
• Genetics: Some men are simply more prone due to their DNA.

Diagnosing low testosterone

Diagnosis starts with a good chat, your symptoms matter as much as your blood test results. Here’s how it usually works:

1. Symptom review: Your GP or pharmacist will ask about your health, lifestyle, and any symptoms.
2. Blood tests: Early morning blood sample (when testosterone peaks).
3. Repeat testing: If low, it’s usually confirmed with a second test.
4. Other checks: To rule out underlying health issues (e.g. thyroid, diabetes).

Treatment options for low testosterone

If your levels are genuinely low and you’re experiencing symptoms, there are several treatment options:

1. Testosterone replacement therapy (TRT)

• Gels/creams: Rubbed onto the skin daily. Discreet and easy to adjust.
• Injections: Given every few weeks or months. Good for those who don’t fancy daily routines.
• Patches: Worn on the skin, changed regularly.
• Oral medications: Less common in the UK, but available in some cases.

TRT can improve mood, energy, muscle mass, bone strength, and sexual function. However, it’s not for everyone, it requires regular monitoring and may have side effects.

2. Increase testosterone naturally

• Exercise: Strength training and regular physical activity naturally boost testosterone levels.
• Nutrition: A balanced diet with enough protein, healthy fats, and micronutrients.
• Sleep: Aim for 7–9 hours per night—poor sleep can tank your levels.
• Stress reduction: Chronic stress raises cortisol, which can suppress testosterone.

3. Supplements

When looking at “testosterone boosters” look for something backed by research. Vitamin D, zinc, and ashwagandha may help if you’re deficient, but always check with a healthcare professional first.

Is testosterone therapy safe? Risks and side effects

Testosterone therapy is generally safe when prescribed and monitored by a healthcare professional, but it’s not risk-free. Possible side effects include:

• Acne or oily skin
• Increased risk of blood clots
• Enlarged prostate or prostate cancer risk (still debated)
• Sleep apnoea
• Breast enlargement (yes, really)
• Fertility issues (TRT can reduce sperm production)

Regular check-ups are essential to ensure safety.

Testosterone and men under 40

Why should younger men care?

While low testosterone is less common under 40, it does happen—often due to genetics, weight, stress, or medical conditions. Symptoms can affect confidence, fertility, and wellbeing. For younger men, lifestyle factors (diet, exercise, sleep) are usually the first port of call.

Practical tips:

• Prioritise sleep and stress management—late-night gaming marathons are fun, but your hormones disagree.
• Don’t overdo it at the gym—overtraining can actually lower testosterone.
• Watch out for crash diets or extreme weight loss.

Testosterone and men over 40

What changes after 40?

Testosterone naturally declines with age, but not everyone needs treatment. The focus should be on maintaining a healthy lifestyle, regular check-ups, and addressing symptoms if they arise.

Practical tips:

• Stay active – strength training is especially important for muscle and bone health.
• Keep an eye on weight and waistline – abdominal fat is linked to lower testosterone.
• Regular health checks – catch issues early and keep everything ticking along.

Testosterone and athletes

The performance puzzle

Athletes often have higher testosterone due to training, but intense exercise or extreme dieting can cause levels to drop. Low testosterone can mean fatigue, poor recovery, and increased injury risk.

Practical tips:

• Balance is key – mix training with adequate rest and nutrition.
• Avoid banned substances – testosterone doping is illegal and risky.
• If you suspect low levels, get tested by a professional, not a supplement salesperson.

Testosterone myths busted

• “Only older men need to worry about testosterone.” False! It can affect all ages.
• “Testosterone therapy is a magic fix.” Not quite—lifestyle matters just as much.
• “Supplements work for everyone.” Not always, unless you’re deficient.
• “More testosterone = better performance.” Too much can be harmful (and illegal in sport).

FAQs about testosterone