Absence, decreased frequency or decreased amplitude of pulsatile GnRH release results in the clinical syndrome of hypogonadotropic hypogonadism (HH). Deficient GnRH secretion may occur in isolation (idiopathic hypogonadotropic hypogonadism [IHH]), in association with anosmia (Kallmann syndrome [KS]) or as a result of a variety of structural and functional lesions of the hypothalamic-pituitary axis. The phenotypic expression of GnRH deficiency in the human demonstrates considerable heterogeneity, suggesting that patients with IHH and KS may represent part of a spectrum of isolated GnRH deficiency as opposed to representing discrete diagnostic subsets.