HypoNa+

• Results from intake (PO or IV) and retention of water (failure to excrete)
  
• In normal individuals increased water causes fall in serum osmolality and thus suppression of ADH and water is excreted in dilute urine
• Patients with HypoNa+ usually have impaired renal water secretion usually from inability to suppress ADH

Laboratory Testing

• Chem7 (hyperglycemia or renal failure)
• Serum osmolality (usually reduced by excess water)
  
• Urine osmolality (usually indicated impaired water secretion)
• Urine sodium, potassium, and chloride concentrations (distinguish between hypoNa+ due to volume depletion and euvolemic hypoNa+)

Causes

• Hypovolemic (fluid loss w/ hypotonic replacement)
• U-Na+ < 20 mEq/L (Extra-renal loss)
  
• Fluid loss – vomiting, diarrhea
• U-Na+ > 20 mEq/L (Renal loss)
  
• Medications - diuretics
• Adrenal insufficiency
• Euvolemia
• U-Osm > 100 mOsm/kg
• SIADH
• U-Osm < 100 mOsm/kg
• Psychogenic polydipsia
• Hypervolemic (retention of water > Na+, edema states)
• U-Na+ < 20 mEq/L (Extra-renal loss)
• Edema / ascites - CHF, liver failure
  
• U-Na+ > 20 mEq/L (Renal loss)
• Renal failure
• Others
• HypoTH
• Pseudohyponatremia - lab error due to hyperlipidemia or hyperproteinemia
  

Treatment

• Treat underlying disease – thyroid, medications, fluid restriction
• Volume depleted patients – normal saline
• Adrenal insufficiency – glucocorticoids
• SIADH - fluid restriction, PO salt tablets, loop diuretic or vasopressin receptor antagonist
• Hypertonic saline – restricted to patients with seizures or impaired mental status

AVOID RAPID CORRECTION OF HYPONATREMIA (raise Na+ level by 4 to 6 meq/L per day)