A failure of dihydrotestosterone formation in a male embryo is responsible for the phenotype (Figs 11.27 & 11.28). Since testosterone itself regulates LH secretion, plasma LH levels are usually normal or minimally elevated. As a result, testosterone and oestrogen production rates are those of normal men, and gynaecomastia does not develop. Affected individuals have a defect in the 5a-reductase 2 gene.
