What is Testosterone?
Testosterone is one of the most talked-about hormones in men's health, yet it is also one of the most misunderstood. Whether you have come across it in the context of gym performance, low energy, or a conversation with your GP, understanding what testosterone actually is and what it does in your body is the first step to making informed decisions about your health.
This guide covers everything you need to know, written specifically with UK men in mind.
What Is Testosterone?
Testosterone is a steroid hormone belonging to the androgen group. It is produced primarily by the testicles in men, and in much smaller amounts by the adrenal glands in both men and women. Its production is controlled by luteinising hormone (LH), which is released by the pituitary gland in the brain.
It is the principal male sex hormone and plays a central role in a wide range of physical and psychological functions throughout a man's life.
What Does Testosterone Do in the Body?
Testosterone is not a single-purpose hormone. Its effects span multiple systems in the body.
In the brain, it stimulates libido and supports cognition, memory and mood. In the kidneys, it promotes the production of red blood cells.
More broadly, testosterone is responsible for:
- Development of the testes and prostate during foetal development
- The physical changes that occur during puberty, including voice deepening, facial and body hair growth, and genital development
- Muscle mass and strength
- Bone mineral density
- Sperm production and fertility
- Sex drive and erectile function
- Energy levels, motivation and general sense of wellbeing
When testosterone levels fall, men may experience physical, psychological and metabolic effects that can compromise their cardiovascular health, metabolic function, general wellbeing, sexuality and fertility.
When Does Testosterone Peak and What Happens Over Time?
Testosterone production begins before birth and surges dramatically during puberty. Levels typically reach their peak in the late teens and early twenties.
After the age of 30, total testosterone levels decline by roughly 1% per year. For most men this gradual decline does not cause noticeable problems. However, for some men the decline is more pronounced, and symptoms begin to emerge that affect quality of life.
It is worth noting that this decline is not inevitable for everyone. Research shows that 75% of men maintained normal testosterone levels into old age, suggesting that low testosterone is not simply an unavoidable consequence of ageing. Lifestyle factors including obesity, poor sleep, high alcohol consumption and certain medications can all accelerate the decline.
What Are Normal Testosterone Levels in the UK?
This is where things get slightly complicated, because reference ranges vary between UK laboratories.
A normal testosterone level for adult men is generally considered to be around 8.7 to 29 nmol/L. Most UK laboratories use reference intervals similar to this range.
In practical terms, the British Society for Sexual Medicine (BSSM) provides the most widely used clinical guidance in the UK:
- Levels greater than 12 nmol/L typically do not require treatment. Levels between 8 and 12 nmol/L may require a trial of testosterone replacement therapy if symptoms of deficiency are present. Levels below 8 nmol/L usually require treatment.
One important nuance is that total testosterone is not the full picture. Most testosterone in the blood is bound to proteins and not available for use by the body. Around 40 to 50% is weakly bound to albumin and 50 to 60% is tightly bound to sex hormone binding globulin (SHBG). This leaves only around 1 to 3% circulating as free testosterone. Free testosterone is what the body can actually use, which is why many specialists now test both markers rather than total testosterone alone.
It is also worth knowing that many men begin experiencing symptoms at levels that are still technically within the normal range. Many men report a noticeable drop in performance, energy and mental sharpness when levels dip below 15 nmol/L, a figure still considered well within the normal zone
What Are the Symptoms of Low Testosterone?
Low testosterone, clinically referred to as testosterone deficiency (TD) or hypogonadism, presents with a range of symptoms that can be easy to dismiss or attribute to stress, ageing or lifestyle.
Common symptoms include:
- Reduced sex drive
- Difficulty achieving or maintaining erections
- Loss of morning erections
- Fatigue and low energy
- Reduced muscle mass and increased body fat
- Low mood or depression
- Poor concentration and brain fog
- Reduced bone density
- Decreased body and facial hair
Testosterone deficiency is a well-established and significant medical condition. Its diagnosis and management can be challenging, requiring both clinical symptoms and confirmed low blood levels before treatment is considered.
It is important to note that these symptoms overlap with many other conditions, which is why blood testing is essential before drawing any conclusions.
Who Is Most at Risk of Low Testosterone?
Testosterone deficiency is more common in older men, particularly those with obesity, type 2 diabetes, chronic kidney disease and other long-term health conditions. Certain medications including opiates, anabolic steroids, antipsychotics and anticonvulsants can also suppress testosterone levels.
Factors such as obesity, chronic illness, medications and lifestyle can significantly influence testosterone levels at any age. Addressing these through weight loss, improved sleep hygiene and reduced alcohol consumption may improve testosterone levels naturally without requiring medical intervention
How Is Low Testosterone Diagnosed in the UK?
Diagnosis requires both clinical symptoms and confirmed low blood levels. A diagnosis should never be made on symptoms alone.
The standard diagnostic process in the UK typically involves:
- A morning blood test, ideally taken between 7am and 10am when levels are at their highest
- Measurement of total testosterone, and in many cases free testosterone, SHBG, LH and FSH
- A repeat test to confirm consistently low levels before any treatment is considered
Within the NHS, testosterone replacement therapy is generally reserved for men with confirmed biochemical hypogonadism. Men with borderline testosterone levels but significant symptoms may not meet NHS treatment thresholds, depending on overall clinical assessment.
Many men who do not qualify under NHS criteria, or who face long waiting times, choose to pursue assessment and treatment through private TRT clinics, which apply the BSSM guidelines and often offer faster access to blood testing and consultation.
Does Testosterone Affect Women Too?
Yes, though this is widely underappreciated.
Testosterone is an important female hormone. Women actually produce more testosterone than oestrogen physiologically. Approximately half of a woman's endogenous testosterone comes from the ovaries and half from the adrenal glands.
Testosterone contributes to libido, sexual arousal and orgasm by increasing dopamine levels in the central nervous system. It also maintains normal metabolic function, muscle and bone strength, urogenital health, mood and cognitive function.
In women, testosterone levels naturally decline with age and can fall sharply during and after menopause. In clinical trials of women with hypoactive sexual desire disorder, approximately two thirds responded positively to testosterone therapy, compared to one third using a placebo.
Summary
Testosterone is a fundamental hormone that affects far more than just muscle and libido. It plays a role in bone health, cardiovascular function, mental clarity, mood and metabolic health. When levels drop below what your body needs, the effects can be wide-ranging and genuinely impact quality of life.
If you suspect your testosterone levels may be low, the right first step is a morning blood test and a conversation with a qualified healthcare professional. Understanding your numbers in context, alongside your symptoms, gives a far more accurate picture than either alone.
This article is intended for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making any decisions about hormone therapy.
