In patients who have defects in thirst regulation and vasopresin secretion, serum vassopresin levels are low or absent.[12] Measurements of urine electrolytes and osmolality are critical in determining the central, rather than renal, nature of the defect in water homeostasis. In adipsia, the fractional excretion of sodium is less than 1%, unless a coexisting defect in AVP secretion is present. In salt intoxication, the urine sodium concentrations are very high and fractional excretion of sodium is greater than 1%.