Several forms of hypogonadism exist, depending on the cause. A GnRH deficiency (1) results in gonadotropin deficiency (hypogonadotropic hypogonadism). This disorder has mostly a congenital origin. A disorder on the level of the pituitary (2) usually results in low levels of circulating FSH and LH (hypogonadotropic hypogonadism). In this case, the gonads do not receive enough input in order to produce mature germ cells and adequate amounts of steroid hormones.
On the other hand, FSH and LH levels can be very high, due to dysfunctioning gonads (3) (hypergonadotropic hypogonadism).
Causes of hypergonadotropic hypogonadism in men: Klinefelter syndrome, orchitis (due to mumps or tuberculosis), radiation, trauma
Causes of hypogonadotropic hypogonadism in men: panhypopituitarism, Kallmann syndrome, anorexia.