Decreased total body water resulting in relative hypertonicity and hypernatremia
Usually resulting from inadequate fluid intake or increased free water loss
Often confused with hypovolemia in which total body water is lost with a relative hypotonic or isotonic state
Dehydration
Emotional or psychiatric causes
Diabetic ketoacidosis 
Pain, anger, or anxiety
Diabetic ketoacidosis
Response to fever 
Syndrome of inappropriate antidiuretic hormone secretion
SIRS 