Hyponatremia, hypo-osmolality, and inappropriately concentrated urine are characteristic of SIADH. The syndrome results from excessive secretion of vasopres­sin. This could be due to ectopic tumor secretion of vasopressin, various central nervous system complica­tions, or it could be drug induced. The increased vasopressin causes excessive water retention, resulting in dilutional hyponatremia and hypo-osmolality. Vol­ume expansion inhibits the renin-angiotensin system. Plasma aldosterone therefore tends to be low, which partly explains the inappropriately concentrated urine.