Effect of testosterone treatment during puberty in boys with Klinefelter syndrome (The TIPY Study): protocol for a nationwide randomised, double-blinded, placebo-controlled study.

Klinefelter syndrome (KS) is associated with multiple adult phenotypic characteristics and health risks.

• KS is characterised by the presence of an extra X chromosome in males (47,XXY).
• Associated adult phenotypic characteristics include infertility, hypogonadism, and increased risk of type II diabetes, cardiovascular disease, and osteoporosis.
• Individuals with KS often experience mental health challenges and functional impairments that significantly impact their quality of life.

No evidence-based recommendations currently exist for testosterone replacement therapy (TRT) in adolescents with KS.

• TRT in adolescence is considered first-line treatment by some physicians for patients with KS and biochemical signs of hypogonadism.
• Comprehensive evidence on TRT's effectiveness in preventing typical phenotypic traits associated with KS remains limited.
• The absence of evidence-based recommendations motivated the design of the TIPY study.

The TIPY study is designed as a multicentre, national, randomised, double-blind, placebo-controlled intervention study recruiting from four tertiary paediatric endocrine units in Denmark.

• Participants will be recruited from four tertiary paediatric endocrine units in Denmark that manage boys with KS.
• Participants will be randomised to transdermal placebo or transdermal testosterone (Androgel; Besins Healthcare, Paris, France).
• The study registration number is NCT06294990 and the Clinical trials information system number is 2023-505854-16-00.
• The study has been approved by the Danish National Medical Research Ethics Committee and the Danish Medicines Agency.

The primary endpoint of the TIPY study is change in body fat percentage over two years of TRT during puberty.

• The treatment duration is two years.
• Thorough clinical and biochemical evaluation will be performed at baseline, after 12 months, and after 24 months.
• Additional visits for minor evaluations will occur every 3 months.

Secondary endpoints of the TIPY study encompass a wide range of physiological, metabolic, and neuropsychological outcomes.

• Secondary endpoints include changes in pubertal development and virilisation, growth and body proportions, bone mineralisation, and muscle strength.
• Metabolic secondary endpoints include lipid and glucose metabolism, systemic inflammation, and methylation.
• Additional secondary endpoints include fertility and effects on cognitive and psychopathological features of KS.
• Neuropsychological assessment will be conducted at baseline and after 24 months of treatment.

Testosterone dose titration is conducted every 3 months by a single physician to maintain free testosterone between +1 and +3 SD for age.

• Dose titration is based on individual measurements of serum concentrations of testosterone.
• A single physician conducts all dose titrations to ensure consistency.
• The target free testosterone range is between +1 and +3 SD for age.