AVP and inappropriate ADH syndrome: One of the causes of euvolemic hyponatremia is SIADH, as mentioned earlier, whereby the ADH secretion remains high despite the low osmolality, resulting in high urine osmolality and natriuresis; the serum osmolality is less than 275 mOsm/kg (normal range: 275-295 mOsm/kg); the urine electrolytes are significant for urine sodium >40 meq/L; and the urine osmolality is >100 mOsm/kg, usually reaching more than 300 mOsm/kg.