The diagnosis of hypogonadotropic hypogonadism in the male adult is made by an initial clinical evaluation for symptoms and signs of hypogonadism and determination of serum levels of total testosterone, LH, and FSH on at least two different days, in the absence of acute/subacute illness and preferably before 10 a.m., after an overnight fast combined with serum PRL. This evidence concerns the gene PRL and hypogonadotropic hypogonadism.