Are Klinefelter boys hypogonadal?

Male hypogonadism implies decreased function of one or more testicular cell population, i.e. germ, Leydig and/or Sertoli cells. In the normal prepubertal boy, Sertoli cells are very active, as indicated by high anti‐Müllerian hormone (AMH) and inhibin B secretion, whereas the functional activity of Leydig cells is minimal, as evidenced by low testosterone production, and germ cells do not undergo the full spermatogenic process. Klinefelter syndrome is the most frequent cause of hypogonadism in the adult male. In this review, we discuss whether the gonadal failure is already established during infancy and childhood. In Klinefelter syndrome, there is increased germ cells degeneration from mid‐foetal life – resulting in a decreased number at birth – which persists during infancy and childhood and becomes dramatic during puberty. Controversial results exist in the literature regarding Leydig cell function in Klinefelter boys: while some authors have found normal to low testosterone levels in infancy and childhood, others have reported normal to high values. Sertoli cell products AMH and inhibin B are normal in prepubertal boys and only decline during mid‐ to late puberty. Conclusion:  Klinefelter syndrome is a primary hypogonadism affecting all testicular cell populations. Germ cells are affected from foetal life, and a severe depletion occurs at puberty. Leydig cell function may be normal or mildly affected in foetal and early postnatal life. Sertoli cell function is not impaired until mid‐ to late puberty, as reflected by normal AMH and inhibin B in Klinefelter boys.