Testosterone treatment plays a carefully calibrated role in managing delayed puberty in boys, particularly when puberty has not begun by age 14. Administered under specialist supervision, low-dose testosterone therapy initiates the physical changes of male development—deepening voice, muscle growth, and secondary sexual characteristics—whilst allowing the body's natural hormonal systems to activate. The approach differs significantly depending on whether the delay is constitutional (a normal variant requiring short-term priming) or due to permanent hypogonadism (requiring lifelong replacement). Understanding how testosterone influences pubertal development, the expected timeline of changes, and the importance of careful monitoring helps families navigate this sensitive period with confidence and realistic expectations.

What Is Delayed Puberty in Boys?

Delayed puberty in boys is defined as the absence of testicular enlargement by age 14 years, which represents the first physical sign of puberty in males. This condition affects a small percentage of adolescent boys and can cause significant psychological distress, affecting self-esteem, body image, and social development during a critical period of life.

There are several causes of delayed puberty, which clinicians typically categorise into three main groups. Constitutional delay of growth and puberty (CDGP) is the most common cause, accounting for approximately 60–65% of cases. This represents a normal variant where puberty occurs later than average but progresses normally once initiated, often with a family history of late development. Hypogonadotropic hypogonadism occurs when the hypothalamus or pituitary gland fails to produce adequate hormones (gonadotrophin-releasing hormone, luteinising hormone, or follicle-stimulating hormone) to stimulate testicular function. This may be congenital, such as Kallmann syndrome, or acquired through conditions affecting the brain. Primary hypogonadism involves testicular failure itself, which may result from chromosomal abnormalities like Klinefelter syndrome, previous chemotherapy, radiation, or testicular injury.

Other important causes include chronic systemic illnesses, inflammatory conditions, malnutrition, and excessive exercise, which can all delay pubertal development.

Diagnosing delayed puberty requires a thorough clinical assessment. Healthcare professionals will evaluate:

• Growth patterns – plotting height and growth velocity on appropriate charts

• Family history – particularly parental pubertal timing

• Physical examination – assessing testicular volume using an orchidometer (normal pre-pubertal volume <4ml)

• Bone age – X-ray assessment of skeletal maturation

• Hormonal investigations – early morning measurements of testosterone, luteinising hormone (LH), follicle-stimulating hormone (FSH), thyroid function, and prolactin

• Additional tests – may include inflammatory markers, coeliac screen, and kidney and liver function tests

Brain MRI may be considered when there are signs suggesting a central cause, such as headaches, visual disturbances, or multiple pituitary hormone deficiencies.

Accurate diagnosis is essential because the underlying cause determines whether testosterone treatment is appropriate and how it should be administered.

How Testosterone Treatment Works for Delayed Puberty

Testosterone therapy for delayed puberty aims to initiate and promote the physical changes of male puberty whilst allowing natural hormonal mechanisms to develop. The treatment works by supplementing the body's insufficient testosterone production, triggering the cascade of pubertal changes that would normally occur spontaneously.

Mechanism of action: Testosterone is an androgen hormone that binds to androgen receptors throughout the body, particularly in reproductive tissues, bone, muscle, and the central nervous system. Once administered, testosterone promotes protein synthesis, stimulates growth of the penis, deepens the voice through laryngeal growth, increases muscle mass and bone density, and stimulates the development of secondary sexual characteristics including facial and body hair. It's important to note that exogenous testosterone does not directly cause testicular enlargement; testicular growth primarily occurs when the body's own hypothalamic-pituitary-gonadal axis activates with LH and FSH production.

In the UK, testosterone treatment for delayed puberty typically involves intramuscular injections of testosterone esters, most commonly testosterone enantate (such as Sustanon). It should be noted that this use is often off-label and requires specialist paediatric endocrinology supervision. Initial doses are deliberately kept low—typically 50mg monthly for 3-6 months, potentially increasing to 100mg as needed—to mimic the natural, gradual rise in testosterone that occurs during normal puberty. This approach differs significantly from adult testosterone replacement, which uses higher doses.

The treatment strategy varies according to the underlying diagnosis:

• Constitutional delay: Short courses (3–6 months) of low-dose testosterone are used to 'prime' the system and initiate puberty, after which the body's own hormonal axis typically takes over

• Permanent hypogonadism: Long-term testosterone replacement is required, with doses gradually increased to reach adult levels

• Hypogonadotropic hypogonadism: In some cases, particularly when fertility preservation is a priority, gonadotrophin therapy (human chorionic gonadotrophin with or without FSH) may be considered as an alternative to testosterone, as this can stimulate testicular growth and development

It's important to understand that long-term exogenous testosterone can suppress the body's own gonadotrophin production and spermatogenesis, which has implications for fertility planning.

Alternative formulations such as testosterone gels exist but are less commonly used in adolescents due to concerns about dose consistency, transfer to others through skin contact, and the need for daily application. The choice of preparation is individualised based on clinical circumstances, patient preference, and specialist endocrinologist recommendation.

Expected Changes and Timeline with Testosterone Therapy

Understanding the timeline and sequence of changes during testosterone treatment helps manage expectations for both adolescents and their families. The progression of pubertal development follows a predictable pattern, though individual variation exists.

Early changes (1–3 months): The first noticeable effects typically include increased energy levels, improved mood, and subtle psychological changes. In boys with constitutional delay, their own hormone production may begin to activate, which can lead to testicular growth (not a direct effect of the testosterone treatment itself). Skin changes become apparent, with increased oiliness and the potential development of acne. Some adolescents notice early pubic hair growth or darkening of existing hair.

Intermediate changes (3–6 months): Penile growth becomes evident, progressing in both length and width. Voice changes begin, initially with voice 'cracking' before gradual deepening occurs over 12–24 months. Muscle mass increases, with improved strength and athletic performance. Body composition shifts, with decreased body fat percentage and increased lean muscle mass. Facial hair may begin appearing, typically starting with upper lip hair. Growth velocity increases significantly, with many boys experiencing a growth spurt during peak velocity.

Later changes (6–12 months and beyond): Facial and body hair continues to develop and coarsen, following the typical male pattern. The voice reaches its adult pitch. Adult body proportions develop, with shoulder broadening. Bone density continues to increase, which is particularly important for long-term skeletal health. In boys with constitutional delay who receive short-course treatment, spontaneous progression of puberty should continue after testosterone is stopped, with the body's own hormonal system maintaining development.

Important considerations: The rate of change varies considerably between individuals. Boys with constitutional delay typically show more rapid progression as their own testosterone production 'awakens', whilst those with permanent hypogonadism depend entirely on the administered testosterone. Careful dose titration is important to avoid accelerated bone age advancement, which could compromise final adult height. Psychological benefits—including improved self-confidence and reduced anxiety about development—often occur before physical changes become obvious to others, which can be particularly valuable during adolescence.

Monitoring and Safety During Treatment

Testosterone therapy in adolescents requires careful monitoring to ensure safety, optimise outcomes, and detect any adverse effects early. The monitoring protocol typically follows guidance from the British Society for Paediatric Endocrinology and Diabetes, with regular specialist endocrinology review.

Clinical monitoring includes:

• Growth assessment – Height and weight measured at each visit (typically every 3–6 months), plotted on appropriate growth charts. Growth velocity is calculated to ensure appropriate response

• Pubertal staging – Systematic assessment using Tanner staging for genital development, pubic hair, and testicular volume

• Blood pressure – Monitored at each visit as testosterone can occasionally affect cardiovascular parameters

• Physical examination – Checking for gynaecomastia (breast tissue development), which can paradoxically occur as testosterone converts to oestrogen

Laboratory investigations: Blood tests are performed periodically (typically every 6–12 months) to assess:

• Testosterone levels – Ensuring appropriate dosing

• Gonadotrophin levels (LH and FSH) – Particularly important in constitutional delay to confirm awakening of the natural hormonal axis; these should be interpreted when off exogenous testosterone, as treatment will suppress these hormones

• Bone age X-rays – Repeated annually to monitor skeletal maturation and predict final adult height

• Full blood count – Testosterone stimulates red blood cell production; excessive elevation (polycythaemia) requires dose adjustment

• Other tests – Additional monitoring may be performed according to local protocols and individual patient needs

Common adverse effects that require monitoring include:

• Acne – A common side effect that is usually manageable with topical treatments or, if severe, referral to dermatology

• Mood changes – Increased irritability or aggression occasionally reported; usually mild

• Accelerated bone maturation – Excessive doses can cause premature fusion of growth plates, potentially reducing final adult height

Safety considerations: Testosterone is a controlled drug under UK regulations. Prescriptions must be clear, and families should store medication securely. Injection technique training is provided if home administration is planned. If testosterone gel is used, careful precautions must be taken to prevent transfer to others (covering application sites, thorough handwashing, avoiding skin contact with others).

Patients and parents should report any suspected side effects to their healthcare professional and can also report directly to the MHRA Yellow Card scheme (website or app).

Any concerning symptoms—severe headaches, visual changes, chest pain, or significant mood disturbance—warrant immediate medical review.

When to Seek Medical Advice About Delayed Puberty

Recognising when to seek medical evaluation for delayed puberty is important for timely diagnosis and management. Early assessment allows for appropriate investigation, reassurance when development is simply late but normal, or treatment when indicated.

Initial consultation triggers: Parents or adolescents should arrange a GP appointment if:

• No signs of puberty by age 14 years – Specifically, no testicular enlargement (the first sign of male puberty)

• Puberty has started but not progressing – Development begins but then stalls for more than 12 months

• Significant height concerns – The boy is considerably shorter than peers or falling away from his previous growth trajectory

• Associated symptoms – Loss of sense of smell (suggesting Kallmann syndrome), headaches, visual problems, or other concerning features

• Psychological distress – Significant anxiety, low self-esteem, or social withdrawal related to delayed development

The GP will perform an initial assessment and typically refer to paediatric endocrinology if delayed puberty is confirmed or if there are concerning features requiring specialist investigation.

During testosterone treatment, seek urgent medical advice if:

• Severe or persistent headaches – Particularly with visual disturbance, which requires urgent review to exclude serious causes

• Chest pain or palpitations – Cardiovascular symptoms require prompt evaluation

• Severe mood changes – Significant depression, aggression, or behavioural concerns

• Allergic reactions – Rash, breathing difficulties, or swelling after injection

• Painful, persistent erections (priapism) – A rare but serious side effect requiring emergency treatment

• Signs of infection at injection sites – Increasing pain, redness, swelling, or discharge

For severe symptoms, contact NHS 111, arrange a same-day GP appointment, or attend A&E/call 999 if symptoms are severe or rapidly worsening.

Routine follow-up: Regular appointments with the specialist endocrinology team are essential. Between appointments, contact your specialist team if you have concerns about:

• Treatment response – Lack of expected changes after several months

• Side effects – Even if seemingly minor, as dose adjustments may be needed

• Practical issues – Difficulty with injections, missed doses, or medication supply problems

Most boys with delayed puberty, particularly constitutional delay, have excellent outcomes with appropriate management. Early engagement with healthcare services ensures optimal physical and psychological development during this critical life stage.

Frequently Asked Questions