Correct underlying disorder which is often hyperglycemia
Often volume depleted due to osmotic diuresis and normal saline provides adequate volume repletion
No treatment needed 
Give normal saline, but be cautious of raising the serum sodium more than 10 mmol/L/day and causing osmotic demyelination syndrome (central pontine myelinolysis)
Water restrict
Treat underlying cause
Water restriction
Diuresis
Treat underlying cause
Hyponatremia
Thiazide diuretic use
Hypernatremia
Na-wasting nephropathy 
Hypercapnia
Osmotic diuresis 
Hypermagnesemia
Aldosterone deficiency