A serum sodium concentration of < 135 mEq/L (some sources may use a cutoff of < 136 mEq/L). Etiologies include thiazide diuretic use, adrenal insufficiency, and syndrome of inappropriate antidiuretic hormone secretion. Manifestations include altered mental status, nausea, vomiting, muscle weakness, and hyporeflexia. If severe, can cause seizures, coma, and death.