The following factors contribute to hyponatremia: The antidiuretic effect of vasopressin on the kidneys Direct impairment of renal water excretion by angiotensin II Decreased glomerular filtration rate (GFR) Stimulation of thirst by angiotensin II Urine sodium excretion is usually < 10 mEq / L (< 10 mmol / L), and urine osmolality is high relative to serum osmolality.