Testosterone replacement therapy (TRT) offers clinically meaningful benefits for men with confirmed testosterone deficiency, also known as hypogonadism. When prescribed appropriately following diagnostic blood tests and specialist assessment, TRT can improve physical health, sexual function, and psychological wellbeing. However, treatment is only suitable for men with biochemically proven low testosterone levels accompanied by relevant symptoms. In the UK, TRT is regulated as a prescription-only medicine and requires ongoing medical supervision to monitor efficacy and safety. Understanding what benefits testosterone treatment can realistically provide—and for whom—is essential for informed decision-making about this long-term therapy.

What Is Testosterone Replacement Therapy?

Testosterone replacement therapy (TRT) is a medical treatment prescribed to men with clinically confirmed testosterone deficiency, also known as hypogonadism. This condition occurs when the testes produce insufficient amounts of testosterone, the primary male sex hormone responsible for numerous physiological functions throughout the body. In the UK, TRT is available through the NHS and private healthcare providers, but only following appropriate diagnostic testing and clinical assessment.

The therapy works by supplementing the body's natural testosterone levels through various delivery methods. In the UK, these commonly include:

• Intramuscular injections (such as testosterone undecanoate, given every 10-14 weeks after initial loading)

• Transdermal gels (applied daily to the skin)

• Short-acting testosterone injections (typically every 2-4 weeks)

• Patches and implants (less commonly available in the UK)

Diagnosis and initiation of TRT requires:

• Two separate early-morning blood tests (between 7-11am) showing low testosterone levels

• Total testosterone <8 nmol/L strongly suggests deficiency

• Levels between 8-12 nmol/L require additional testing including SHBG and calculated free testosterone

• Presence of symptoms consistent with testosterone deficiency

• Additional hormone tests including LH, FSH and prolactin to determine the cause

• Exclusion of reversible causes such as obesity, medications, or underlying health conditions

• Assessment of contraindications, particularly prostate disease and cardiovascular risk factors

According to NICE Clinical Knowledge Summaries, testosterone replacement should only be initiated when there is biochemical evidence of hypogonadism accompanied by relevant clinical symptoms. The Medicines and Healthcare products Regulatory Agency (MHRA) emphasises that TRT is not indicated for age-related decline in testosterone alone, nor should it be used for performance enhancement or bodybuilding purposes. Proper medical supervision throughout treatment is essential to monitor efficacy and safety.

Physical Benefits of Testosterone Treatment

Men with confirmed testosterone deficiency who receive appropriate replacement therapy often experience improvements in body composition and physical function. Testosterone plays a crucial role in maintaining muscle mass and bone density, and its deficiency can lead to sarcopenia (muscle wasting) and reduced bone mineral density. Clinical studies have demonstrated that TRT can increase lean body mass and reduce fat mass, particularly visceral adipose tissue, though individual responses vary.

Key physical benefits may include:

• Increased muscle strength and mass – Testosterone stimulates protein synthesis in skeletal muscle, potentially enhancing muscle fibre size and function

• Improved bone mineral density – May benefit men with osteoporosis or osteopenia, though evidence for fracture risk reduction is limited

• Reduction in body fat – Especially central adiposity, which is associated with metabolic syndrome

• Enhanced physical performance – Some men report improvements in exercise capacity and reduced fatigue

The metabolic effects of testosterone are still being researched. Some studies suggest TRT may improve insulin sensitivity in men with both type 2 diabetes and hypogonadism, though results are inconsistent. It is important to emphasise that TRT should not be considered a primary treatment for diabetes or metabolic disorders.

Physical benefits typically emerge gradually over three to six months of treatment and are most pronounced when TRT is combined with regular exercise and a balanced diet. The magnitude of benefit varies between individuals and depends on baseline testosterone levels, age, overall health status, and adherence to treatment. Benefits are generally only seen when testosterone levels are restored to the mid-normal physiological range.

Patients should maintain realistic expectations and understand that testosterone replacement addresses deficiency rather than providing enhancement beyond normal levels.

Mental and Emotional Benefits

Testosterone deficiency can affect mood, cognition, and overall psychological wellbeing. Men with hypogonadism frequently report symptoms such as low mood, irritability, poor concentration, and reduced motivation. The relationship between testosterone and mental health is complex, and evidence for psychological benefits from TRT is mixed.

Some clinical trials have shown that TRT may lead to improvements in mood and reduction in depressive symptoms in men with confirmed testosterone deficiency, particularly those with mild to moderate depressive symptoms. However, it is essential to recognise that testosterone replacement is not a treatment for primary psychiatric disorders.

Potential mental and emotional benefits may include:

• Improved mood and sense of wellbeing in some men

• Reduced irritability

• Enhanced motivation and drive

• Better concentration

• Decreased symptoms of depression in some individuals with confirmed deficiency

Research into testosterone's effects on cognitive function continues, with some studies suggesting modest improvements in certain cognitive domains. However, the evidence remains inconsistent, and TRT should not be viewed as a cognitive enhancer or treatment for dementia.

Men experiencing significant mood disturbances should undergo comprehensive psychiatric assessment using validated tools such as the PHQ-9, as depression and anxiety disorders require specific psychological or pharmacological interventions in line with NICE guidance.

Patients should be aware that psychological improvements, if they occur, typically take several weeks to months to manifest and may be subtle. If depressive symptoms persist or worsen despite adequate testosterone replacement, referral to mental health services is appropriate. The psychological benefits of TRT are most evident in men with clear biochemical deficiency and should be monitored as part of ongoing clinical review.

Sexual Health Improvements with Testosterone Therapy

Sexual dysfunction is one of the most common presenting complaints in men with testosterone deficiency, and improvements in sexual function represent a primary treatment goal for many patients. Testosterone plays a vital role in male sexual physiology, influencing libido, erectile function, and overall sexual satisfaction. However, it is crucial to understand that not all sexual problems are caused by low testosterone, and TRT is only effective when hypogonadism is the underlying cause.

In men with confirmed testosterone deficiency, replacement therapy typically leads to improvements in sexual desire (libido) within three to six weeks of treatment initiation. This is often one of the earliest benefits patients notice. The effect on erectile function is more variable, generally less pronounced than the impact on libido, and may take up to six months to develop fully. Testosterone contributes to erectile function through multiple mechanisms, including effects on nitric oxide synthesis in penile tissue.

Sexual health benefits may include:

• Increased libido and sexual desire – Often the most consistent improvement

• Improved erectile function – Particularly in men where low testosterone is a contributing factor

• Enhanced sexual satisfaction – For both the patient and their partner

• Increased frequency of spontaneous erections – Including nocturnal erections

It is important to note that erectile dysfunction has multiple potential causes, including vascular disease, diabetes, neurological conditions, psychological factors, and medications. According to NICE Clinical Knowledge Summaries, men with erectile dysfunction should undergo comprehensive assessment, and if testosterone deficiency is identified, it should be treated appropriately. However, if erectile problems persist despite testosterone normalisation, additional treatments such as phosphodiesterase-5 inhibitors (e.g., sildenafil) are often required.

Patients should be counselled that sexual function improvements are gradual and that unrealistic expectations can lead to disappointment. If sexual dysfunction does not improve after six months of adequate testosterone replacement, further investigation and alternative treatments should be considered. Open communication with healthcare providers about sexual health concerns is essential for optimal management.

Who Is Suitable for Testosterone Treatment in the UK?

Testosterone replacement therapy is a prescription-only treatment in the UK, and specific clinical criteria must be met before initiation. The decision to commence TRT should be made by a specialist with expertise in endocrinology or andrology, following comprehensive assessment and investigation. Not all men with low testosterone levels require treatment, and the presence of symptoms is essential for justifying therapy.

Suitable candidates for TRT typically include men with:

• Biochemically confirmed hypogonadism – Two early-morning testosterone measurements showing deficiency (<8 nmol/L) or borderline levels (8-12 nmol/L) with low calculated free testosterone

• Symptomatic testosterone deficiency – Including reduced libido, erectile dysfunction, fatigue, mood changes, or loss of muscle mass

• Primary hypogonadism – Testicular failure due to genetic conditions (e.g., Klinefelter syndrome), chemotherapy, radiation, or trauma

• Secondary hypogonadism – Pituitary or hypothalamic disorders affecting testosterone production

Before initiating TRT, clinicians must exclude reversible causes of low testosterone, such as obesity, excessive alcohol consumption, certain medications (particularly opioids and glucocorticoids), poorly controlled diabetes, and sleep apnoea. Weight loss and lifestyle modifications alone can significantly improve testosterone levels in overweight men and should be attempted first.

Contraindications to TRT include:

• Prostate cancer or high suspicion of prostate malignancy

• Male breast cancer

• Desire for fertility in the near future (TRT suppresses sperm production)

• Polycythaemia (haematocrit >0.54)

Cautions (requiring careful assessment before TRT):

• Severe lower urinary tract symptoms

• Uncontrolled heart failure

• Unstable cardiovascular disease

• Untreated obstructive sleep apnoea

Men considering TRT should undergo baseline investigations including full blood count, prostate-specific antigen (PSA), liver function tests, lipid profile, LH, FSH, prolactin, and SHBG with calculated free testosterone. According to British Society for Sexual Medicine guidelines, prostate examination and PSA testing are essential in men over 40 before commencing therapy. Shared decision-making is crucial, with patients fully informed about potential benefits, risks, and the need for long-term treatment and monitoring. TRT should never be initiated for performance enhancement, anti-ageing purposes without confirmed deficiency, or based on symptoms alone without biochemical confirmation.

Potential Risks and Side Effects to Consider

Whilst testosterone replacement therapy can provide significant benefits for men with confirmed deficiency, it is not without potential risks and adverse effects. All patients considering TRT must be fully informed about these risks to make an educated decision about treatment. Regular monitoring throughout therapy is essential to detect and manage complications early.

Common side effects include:

• Skin reactions – Particularly with transdermal preparations, including irritation, rash, or allergic contact dermatitis

• Acne and oily skin – Due to increased sebaceous gland activity

• Fluid retention – Mild ankle swelling in some patients

• Breast tenderness or gynaecomastia – Caused by peripheral conversion of testosterone to oestradiol

• Testicular atrophy – Shrinkage of the testes due to suppression of natural testosterone production

• Reduced fertility – TRT suppresses sperm production and should not be used in men wishing to conceive

More serious potential risks include:

Polycythaemia – Testosterone stimulates red blood cell production, which can lead to elevated haematocrit levels. This increases blood viscosity and potentially raises the risk of thrombotic events. Regular full blood count monitoring is mandatory, and dose reduction, increased dosing interval, or temporary cessation may be required if haematocrit exceeds 0.54. Treatment can be restarted once haematocrit returns to normal range.

Cardiovascular considerations – The relationship between TRT and cardiovascular risk remains under investigation. The MHRA advises caution in men with pre-existing cardiovascular disease, and patients should be monitored for signs of heart failure, hypertension, or ischaemic heart disease.

Prostate health – Whilst there is no definitive evidence that TRT causes prostate cancer, testosterone can stimulate growth of existing prostate malignancy. PSA levels and prostate symptoms must be monitored regularly. A rapid rise in PSA or abnormal digital rectal examination warrants urological referral.

Sleep apnoea – TRT may worsen obstructive sleep apnoea in susceptible individuals, and patients with this condition require careful assessment before treatment.

Gel transfer risk – Men using testosterone gels must take precautions to prevent transfer to women and children through skin contact. This includes washing hands after application, allowing the gel to dry completely, covering the application site with clothing, and avoiding skin-to-skin contact at the application site.

When to contact your GP:

• Chest pain, shortness of breath, or leg swelling

• Severe headaches or visual disturbances

• Difficulty urinating or blood in urine

• Prolonged or painful erections (priapism) – though this is rare with TRT

• Mood changes, aggression, or depression

• Yellowing of skin or eyes (jaundice)

Patients on TRT require regular follow-up appointments, typically at three months, six months, and then annually. Monitoring should include symptom review, physical examination, testosterone levels (aiming for mid-normal range), full blood count, PSA, and liver function tests. Testosterone levels should be checked at appropriate times based on the formulation used (e.g., before daily gel application, mid-interval for short-acting injections, before the next dose for long-acting injections). The decision to continue treatment should be reviewed regularly, weighing ongoing benefits against potential risks.

Suspected adverse reactions to testosterone therapy should be reported through the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions