Copeptin measurement alone has proven useful in hyponatremic disorders to differentiate patients with primary polydipsia (PP), who have low copeptin plasma concentration and low urine osmolality, from other causes of hyponatremia such as SIADH, but do not allow to differentiate SIADH from other causes hyponatremic disorders such as diuretic-induced hyponatremia, hypovolemic hyponatremia, and hypervolemic hyponatremia (97). Here, AVP is linked to inappropriate ADH syndrome.