How do you workup hyponatremia (Na < 135),"First measure serum osmolarity-If >295 = Hypertonic hyponatremia (hyperglycemia or osmotic diuresis)-If 280-295 = Pseudohyponatremia (high protein or TG)-If <280, assess ECF-If low = hypovolemia (↓TBW, ↓↓Na). Measure urine Na. If <10 it is due to extrarenal loses (diarrhea, vomit, sweat, 3rd spacing). If > 20, it is due to renal loses (diuretic, low aldosterone, ATN)-If normal = euvolemia hyponatremia (↑TBW, -Na). Due to SIADH or psychogenic polydypsia-If high = Hypervolemic hyponatremia. Due to CHF, RF
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