• Severe: seizure, delirium, coma, herniation, neurogenic pulmonary edema
  • Non-severe: n/v, headache, confusion, dizziness, tremor, hyperreflexia, muscle cramps


  • Repeat electrolytes, including Ca/Mg/Phos
  • Serum osmolality
  • Cortisol level
  • Urine osmolality & urine sodium


  • Pseudohyponatremia: serum osmolality isn't actually low--> hyperglycemia, TGL>>1,500, multiple myeloma/IVIG, other osmoles (contrast, mannitol etc.)
  • Severe renal failure: typically only when GFR<15mL/min
  • Water intake> solute intake: psychogenic (schizophrenia), rapid intake (frat hazing), beer potomania, tea and toast
  • Hypovolemic
    • Non-renal: vomiting, diarrhea, NG suction, hemorrhage, burns
    • Renal: Diuretics (thiazides), post-obstructive diuresis, hypoaldosteronism/ adrenal insufficiency
  • Euvolemic: adrenal insufficiency or SIADH (medications: NSAIDs, psych meds, antiepileptics etc.)
  • Hypervolemic: CHF, cirrhosis, nephrotic syndrome

Urine osmolarity/ specific gravity

  • Concentrated urine: urine osmolality >300 mOsm, specific gravity >1.010
    • This is most hyponatremic patient's. Means they won't auto-correct their sodium and isotonic fluids may worsen their hyponatremia
  • Dilute urine: urine osmolality <<300 mOsm, specific gravity <1.010
    • Kidney is working correctly, they will autocorrect. Either due to too much intake of water or recovering from any other cause of hyponatremia

Urine Sodium

  • Low urine sodium (<20 mEq/L) suggests: Hypovolemic hyponatremia due to extra-renal volume loss OR hypervolemic hyponatemia (e.g. heart failure, cirrhosis)
  • Intermediate urine sodium (~20-40 mEq/L): Grey zone, provides no clear information
  • High urine sodium (>40 mEq/L) suggests: Euvolemic hyponatremia (SIADH or adrenal insufficiency OR hypovolemic hyponatremia due to renal salt wasting (e.g. diuretics, vomiting)

Isotonic fluid challenge

This is not generally recommended (because it may make matters worse). However, it's very common for patients to get 1-2 liters of isotonic crystalloid shortly after presentation

  • Sodium increase: This suggests that the patient has dilute urine (kidneys are working, they should autocorrect)
  • Sodium decreases: This proves the presence of concentrated urine (>>300 mOsm). So essentially the patient holds onto the water and excretes the sodium. Most commonly, this implies the presence of SIADH


  • If at risk for herniation: Symptomatic, acute onset, increased ICP (mass, premenopausal women)---> Hypertonic saline 2mL/kg (~150mL)
  • If at risk for overcorrection/ osmotic demyelination (give them fluids, they shut off ADH and pee out free water and become HYPERnatremic)---> DDAVP Clamp (prevents kidneys from secreting free water). Increase Na 6-8mM/ 24h
    • DDAVP 2micrograms IV q8h, fluid restrict, 3% saline using Sodium Correction Rate Calculator
    • Follow electrolyte q4-6h
      • If the sodium is too low, increase the rate of 3% saline
      • If the sodium is too high, calculate the amount of free water deficit calculator
  • If SIADH with persistent cause
    • Traditional method: loop diuretic + sodium--> 20mg IV furosemide q6h + 3% NaCl or oral salt tabs
    • Optimal method: oral urea (kidneys excrete urea with free water, controlled removal of water). Give 15-30g oral urea and restrict water intake to <1-1.5L/day
  • If heart failure: typically a mild hyponatremia, due to poor systemic perfusion
    • If hypoperfusing: give fluids
    • Volume overloaded: give furosemide (typically raises sodium)
    • If refractory congestion: 150mL 3% saline + 250mg IV furosemide q12h
  • If cirrhotic--> furosemide if volume overloaded, oral lactulose improves sodium if encephalopathic
  • If hypokalemic too---> administration of KCl increases sodium as much as NaCl would
    • So give them KCl, this will improve their sodium too. Use the estimated increase in serum sodium calculator

Estimated Increase in Serum Sodium w/ Potassium Calculator