Testosterone replacement therapy (TRT) is a medical treatment for clinically diagnosed testosterone deficiency, also known as hypogonadism. In the UK, there is no absolute minimum age for starting testosterone treatment; eligibility depends on confirmed biochemical deficiency and relevant clinical symptoms rather than age alone. Adolescents with delayed puberty may begin therapy under specialist supervision, whilst most adults commence treatment from late teens onwards once natural puberty is complete. For gender-affirming care, NHS England generally does not routinely commission cross-sex hormones for individuals under 18. This article explains the medical criteria, age-related considerations, and NHS pathways for accessing testosterone therapy safely and appropriately.

What Is Testosterone Replacement Therapy?

Testosterone replacement therapy (TRT) is a medical treatment used to restore testosterone levels in individuals with clinically diagnosed testosterone deficiency, also known as hypogonadism. Testosterone is a crucial hormone responsible for numerous physiological functions, including the development and maintenance of male sexual characteristics, bone density, muscle mass, red blood cell production, and overall wellbeing.

In the UK, TRT is prescribed when blood tests confirm persistently low testosterone levels accompanied by relevant clinical symptoms. These symptoms may include reduced libido, erectile dysfunction, fatigue, decreased muscle mass, mood changes, and reduced bone density. The treatment aims to alleviate these symptoms and improve sexual function, bone mineral density, and body composition. TRT does not prevent cardiovascular disease and should not be used for this purpose.

Testosterone can be administered through various formulations licensed for use in the UK, each with distinct pharmacological profiles. Common delivery methods include:

• Intramuscular injections (long-acting) – testosterone undecanoate (Nebido) 1000 mg, typically administered with a second dose at 6 weeks, then every 10–14 weeks

• Intramuscular injections (short-acting) – testosterone mixed esters (Sustanon 250) 250 mg, typically administered every 2–4 weeks

• Transdermal gels – applied daily to the skin (e.g., Testogel, Tostran), providing steady hormone levels

Other formulations such as transdermal patches, oral capsules, and subcutaneous implants are not routinely available in the UK or have limited availability. The choice of formulation depends on individual patient factors, preferences, contraindications, and clinical response. TRT requires ongoing monitoring by healthcare professionals to ensure efficacy, safety, and appropriate dosing. It is important to note that TRT is distinct from testosterone use for gender-affirming care, which follows different clinical pathways and guidelines.

At What Age Can You Start Testosterone Treatment in the UK?

In the UK, there is no absolute minimum age for starting testosterone treatment, as eligibility is determined primarily by clinical need rather than age alone. The presence of confirmed testosterone deficiency with relevant symptoms—not age or natural age-related decline alone—is required for treatment. The context and indication for treatment significantly influence when therapy may be initiated.

For adolescents with delayed or absent puberty due to hypogonadism, testosterone therapy may be initiated during the teenage years. The timing is individualised by paediatric endocrinologists based on the underlying diagnosis, bone age, and pubertal assessment. Treatment in this age group aims to induce normal pubertal changes and prevent psychological distress associated with delayed development.

For adults, testosterone therapy is most commonly prescribed from the late teens onwards once natural puberty would be expected to have completed. The majority of men seeking TRT for late-onset hypogonadism are typically over 40 years of age. It is important to note that age-related testosterone decline alone is not an indication for treatment; diagnosis requires both biochemical deficiency and clinical symptoms.

For transgender individuals, access to testosterone therapy as part of gender-affirming treatment in the UK has changed significantly. Under current NHS England policy, cross-sex hormones are generally not routinely commissioned for individuals under 18 years of age. Adults seeking gender-affirming testosterone therapy should request referral to specialist adult gender dysphoria services through their GP. Each case is assessed individually according to established protocols and current NHS England service specifications.

Ultimately, the decision to start testosterone treatment at any age requires thorough medical assessment, confirmed biochemical deficiency (where applicable), and consideration of the individual's overall health status and treatment goals.

Medical Criteria for Starting Testosterone Therapy

The decision to initiate testosterone therapy in the UK follows strict clinical criteria consistent with NICE Clinical Knowledge Summaries (CKS) and guidance from the Society for Endocrinology and the British Society for Sexual Medicine (BSSM). Diagnosis requires both biochemical evidence of testosterone deficiency and the presence of relevant clinical symptoms.

Biochemical diagnosis requires at least two separate fasting morning blood samples (taken between 8–11 am) showing total testosterone levels below the reference range, typically less than 12 nmol/L, though some guidelines use 8 nmol/L as the threshold. Single measurements are insufficient due to natural fluctuations in testosterone levels. When total testosterone is borderline (8–12 nmol/L) or sex hormone-binding globulin (SHBG) is abnormal, calculated free testosterone should be assessed. Additional hormonal assessments may include luteinising hormone (LH), follicle-stimulating hormone (FSH), and prolactin to determine the underlying cause and type of hypogonadism. If secondary hypogonadism is suspected (low or normal LH with low testosterone, raised prolactin, or visual symptoms), pituitary evaluation including other pituitary hormones and possibly MRI should be arranged, and specialist endocrinology referral is required.

Clinical symptoms that warrant investigation include:

• Reduced libido and sexual dysfunction

• Erectile dysfunction

• Decreased energy and persistent fatigue

• Reduced muscle mass and strength

• Increased body fat, particularly central adiposity

• Mood disturbances, including depression and irritability

• Decreased bone mineral density or osteoporosis

• Reduced facial and body hair growth

Contraindications and cautions must be carefully evaluated before initiating treatment. Absolute contraindications include prostate cancer, male breast cancer, and haematocrit ≥54%. TRT should not be started if haematocrit is ≥54%; if haematocrit rises to ≥54% during treatment, therapy should be withheld or the dose reduced, the formulation changed, and the cause investigated. Relative contraindications include severe lower urinary tract symptoms, untreated obstructive sleep apnoea, uncontrolled heart failure, and a desire to preserve fertility in the near term.

Before starting TRT, baseline investigations typically include full blood count (including haematocrit), liver function tests, lipid profile, and assessment of cardiovascular risk factors. Prostate assessment should be risk-based; in men over 40 or those with risk factors, prostate-specific antigen (PSA) and digital rectal examination may be performed. Any raised PSA or abnormal digital rectal examination warrants urgent referral to urology in line with NICE guidance on suspected cancer (NG12).

Age-Related Considerations and Safety Factors

Age plays a crucial role in determining the appropriateness, safety, and monitoring requirements for testosterone therapy. Different age groups present distinct physiological considerations and potential risks that must be carefully evaluated.

In younger men (under 40), testosterone deficiency is less common and often indicates an underlying pathological cause such as Klinefelter syndrome, pituitary disorders, testicular injury, or previous chemotherapy. In this age group, preserving fertility is often a priority, and patients should be counselled that TRT typically suppresses sperm production. Alternative treatments such as human chorionic gonadotropin (hCG) may be appropriate in hypogonadotrophic hypogonadism when preserving fertility, and should be managed by specialists. Selective oestrogen receptor modulators (e.g., clomifene) are used off-label in men and should only be prescribed under specialist supervision.

In middle-aged men (40–65 years), testosterone therapy requires careful cardiovascular risk assessment. The MHRA advises that testosterone therapy should only be used for confirmed hypogonadism and that cardiovascular risk should be assessed before and during treatment. Evidence on cardiovascular safety is mixed; the TRAVERSE trial (2023) showed non-inferiority for major adverse cardiovascular events but higher rates of atrial fibrillation, pulmonary embolism, and acute kidney injury in the testosterone group. Shared decision-making and ongoing monitoring are essential, particularly in men with multiple cardiovascular risk factors.

In older men (over 65), additional safety considerations include:

• Prostate health monitoring – risk-based PSA testing and clinical assessment, as testosterone may stimulate pre-existing prostate conditions; raised PSA or abnormal examination requires urgent urology referral per NICE NG12

• Polycythaemia risk – older men are more susceptible to elevated haematocrit, requiring regular full blood count monitoring

• Sleep apnoea – TRT may worsen obstructive sleep apnoea, which is more prevalent in older age groups

• Bone health benefits – older men may benefit from TRT's positive effects on bone mineral density, though evidence for fracture reduction is limited

Monitoring protocols vary by age but typically include 3-monthly reviews initially, then 6–12 monthly once stable. Assessments should include symptom review, blood pressure, full blood count (with haematocrit measured at baseline, 3–6 months, then annually), testosterone levels, PSA (in men over 40 or at risk), and liver function tests. Haematocrit should remain below 54%; if it reaches or exceeds this threshold, TRT should be withheld or adjusted and the cause investigated. Patients using transdermal gels should be advised on precautions to avoid skin-to-skin transfer to others, particularly women and children. Monitoring should also include assessment for worsening sleep apnoea, fluid retention, acne, and mood changes.

How to Access Testosterone Treatment Through the NHS

Accessing testosterone treatment through the NHS requires following established referral pathways and meeting clinical criteria. The process typically begins with your general practitioner (GP) and may involve specialist endocrinology services.

Initial steps include:

1. GP consultation – Discuss your symptoms with your GP, who will take a detailed medical history and perform a physical examination. Be prepared to describe symptoms such as fatigue, reduced libido, erectile dysfunction, or mood changes, and their impact on your quality of life.

2. Blood tests – Your GP will arrange at least two fasting morning blood samples (between 8–11 am) to measure testosterone levels, typically taken on separate occasions at least one week apart. Additional hormonal tests may be requested to investigate the underlying cause.

3. Specialist referral – If blood tests confirm testosterone deficiency and symptoms are present, your GP may refer you to an endocrinologist or specialist in sexual medicine. Referral criteria and waiting times vary by region, with some areas experiencing significant delays due to service capacity. Once initiated by a specialist, ongoing prescribing and monitoring may follow local shared care agreements with your GP.

For gender-affirming testosterone therapy, the pathway differs significantly. Individuals should request referral to a specialist adult gender dysphoria clinic through their GP. Under current NHS England policy, cross-sex hormones are generally not routinely commissioned for individuals under 18 years of age. Current NHS waiting times for adult gender dysphoria services can be substantial, often exceeding two years in some regions. Some individuals may choose to access private gender services while awaiting NHS appointments.

Patient safety advice:

• Never purchase testosterone online or from unregulated sources – counterfeit products pose serious health risks

• Attend all monitoring appointments to detect potential adverse effects early

• Call 999 or attend A&E immediately if you experience: severe chest pain, sudden breathlessness, or symptoms of deep vein thrombosis (severe leg swelling, pain, redness)

• Contact your GP promptly if you experience: urinary symptoms, persistent headaches, visual changes, or other concerning symptoms

• Inform all healthcare providers that you are receiving testosterone therapy

• Report any suspected side effects via the MHRA Yellow Card Scheme at https://yellowcard.mhra.gov.uk/

NICE Clinical Knowledge Summaries emphasise that testosterone therapy should only be prescribed by healthcare professionals experienced in hormone management, with ongoing monitoring to ensure treatment remains appropriate and safe.

Frequently Asked Questions