Defined as sodium concentration <135meq/L
Patients often not symptomatic until <120meq/L, although this level varies by patients and may be higher if the change occurred abruptly
Too fast of sodium correction (>10 mmol/L/day), especially if chronic, can cause osmotic demyelination syndrome (central pontine myelinolysis)
Hyponatremia
Thiazide diuretic use
Hypernatremia
Na-wasting nephropathy 
Hypercapnia
Osmotic diuresis 
Hypermagnesemia
Aldosterone deficiency