Normal testicular function without detectable follicle-stimulating hormone. A novel mutation in the follicle-stimulating hormone receptor gene leading to apparent constitutive activity and impaired agonist-induced desensitization and internalization

► We report a novel mutation in the follicle-stimulating hormone receptor. ► The N431I mutation in the FSHR leads to constitutive activation. ► The N431I mutation in the FSHR leads to impaired desensitization. ► The N431I mutation in the FSHR leads to impaired internalization. ► The mutation (N431I) was detected in an asyntomatic man with normal spermatogenesis.