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.