Urine osmolality >400 mosm/kg (renal water-conserving ability is functioning)

• Nonrenal losses

1. Excessive sweating, burns
2. Insensible respiratory tract losses
3. Diarrhea, vomiting, nasogastric suctioning, osmotic cathartics, eg, lactulose

• Renal losses

1. Diuretics
2. Osmotic diuresis, eg, hyperglycemia, mannitol, urea
3. Postobstructive diuresis
4. Diuretic phase of acute tubular necrosis (ATN)

• Hypertonic sodium gain

1. Salt intoxication (rare)
2. Hypertonic intravenous fluids, tube feeds, enemas
3. Primary hyperaldosteronism (hypernatremia usually mild and asymptomatic)
4. Central diabetes insipidus: idiopathic, head trauma, CNS mass
5. Nephrogenic diabetes insipidus: lithium, demeclocycline, prolonged urinary tract infection , interstitial nephritis, hypercalcemia, hypokalemia, congenital