Klinefelter Syndrome and Testosterone Treatment in Puberty
The goal of this randomized clinical trial is to study the effect of testosterone replacement therapy during puberty in boys with Klinefelter syndrome (KS, 47,XXY).

The main questions to answer are how treatment with testosterone will affect body fat mass, lipid and glucose metabolism, growth and body proportions, bone mineralization as well as effects on neurocognitive development and emotional and social difficulties.

Participants will be randomized to two years treatment with testosterone or placebo.
- EligibilityCriteria: Inclusion Criteria:  47,XXY Klinefelter syndrome Age 10-14 years at inclusion Luteinizing Hormone > +2 standard deviations (SD) by ultrasensitive luteinizing hormone assay Free Testosterone<+2 standard deviations Signed consent from parents  Exclusion Criteria:  Previous or ongoing T treatment except for TRT because of micropenis Contraindications to testosterone treatment known hypersensitivity to testosterone or to any other constituent of the gel known or suspected prostatic cancer or breast carcinoma Participation in any other clinical trial - HealthyVolunteers: No - Gender: Male - MinimumAge: 10 Years - MaximumAge: 14 Years - StdAgeList: Child