Normal physiology of puberty and normal GnRH, LH, FSH, and hCG secretion have been reviewed. Systemic disorders can affect the neuroendocrine axis and cause varying degrees of hypogonadism by acting at different levels in the axis. As both hypothalamic abnormalities and intrinsic pituitary abnormalities can cause an abnormal FSH/LH response to GnRH, this test does not distinguish hypothalamic from pituitary mechanisms of hypogonadism. Therefore, only in disorders that have been demonstrated to have a structural pituitary abnormality (e.g., iron or granulomatous infiltration of the gonadotrophs) can we be certain that the disorder has its effect at the level of the pituitary. Abnormalities leading to hypersecretion (both ectopic and eutopic) of gonadotropins have also been described. To date, ectopic production of FSH and LH has not been unequivocally demonstrated. Systemic disorders cause mainly hypogonadism, many of the symptoms of which are reversible with control or cure of the disease. The effect of hypersecretion of gonadotropins on the reproductive system depends on the age at which the tumor (ectopic/eutopic) occurs.