Unknown (40 to 50 percent)
Primary hypogonadism (30 to 40 percent)
	Androgen insensitivity
	Congenital or developmental testicular disorder (e.g., Klinefelter syndrome)
	Cryptorchidism
	Medication (e.g., alkylating agents, antiandrogens, cimetidine [Tagamet], ketoconazole [Nizoral], spironolactone [Aldactone])
	Orchitis, including mumps orchitis
	Radiation
	Systemic disorder
	Testicular trauma
	Varicocele
	Y chromosome defect
Altered sperm transport (10 to 20 percent)
	Absent vas deferens or obstruction
	Epididymal absence or obstruction
	Erectile dysfunction
	Retrograde ejaculation
Secondary hypogonadism (1 to 2 percent)
	Androgen excess state (e.g., tumor, exogenous administration)
	Congenital idiopathic hypogonadotropic hypogonadism
	Estrogen excess state (e.g., tumor)
	Infiltrative disorder (e.g., sarcoidosis, tuberculosis)
	Medication effect
	Multiorgan genetic disorder (e.g., Prader-Willi syndrome)
	Pituitary adenoma
	Trauma