• Sodium <135 mEq/L
  • Most common electrolyte disturbance (3–6% of hospitalized patients)


  • Low measured serum sodium but normal measured serum osmolarity
  • Occurs secondary to the displacement of sodium to aqueous phase of serum
  • Seen with elevated lipids or proteins
  • Lab or blood raw error
  • Disease examples include:
    • Multiple myeloma
    • Hyperlipidemia

Hyponatremia with Normal Osmolarity and Fluid Overload
  • Inappropriate retention of water
  • Disease examples include:
    • CHF
    • Cirrhosis
    • Renal failure
    • Nephrotic syndrome

Hyponatremia with Normal Osmolarity and Euvolemia
  • Patients tend to have increased total body water without marked edema
  • Purest form of dilutional hyponatremia
  • Disease examples include:
    • Endocrine abnormalities:
      • Hypothyroid
      • Stress
      • Syndrome of inappropriate antidiuretic hormone (SIADH)
    • Diseases that cause SIADH:
      • Pulmonary disease (tuberculosis, Legionella, aspergillosis, COPD)
      • CNS disorders (malignancy, sarcoid, infection)
      • Cancer (small cell lung, pancreas, duodenum)
      • HIV infection
    • Water intoxication (3–7% of institutionalized psychotic patients), can also occur in marathon runners
    • Mineralocorticoid abnormalities
    • Postoperative hyponatremia (particularly after transurethral prostatectomy)
    • Consumption of large amounts of beer (beer potomania)
    • MDMA (Ecstasy)

Hyponatremia with Normal Osmolarity and Hypovolemia
  • Deficits in total body water and total body sodium
  • Sodium deficits exceed water deficits
  • Possible etiologies include:
    • GI losses/colon prep
    • Sweating
    • Cerebral salt wasting (occurs after head injury or neurosurgical procedures)
    • Burns
    • Cystic fibrosis
    • Salt-wasting nephropathies
    • Diuretics

Drug Induced
  • Drugs may stimulate antidiuretic hormone (ADH) and cause hyponatremia:
    • Amiodarone
    • Barbiturates
    • Bromocriptine
    • Carbamazepine
    • Clofibrate
    • Cyclophosphamide
    • Opiates
    • Oxytocin
    • Vincristine, vinblastine
  • Drugs may increase sensitivity to ADH and cause hyponatremia:
    • Chlorpropamide
    • NSAIDs
  • Drugs may stimulate thirst and cause hyponatremia:
    • Amitriptyline
    • Ecstasy (MDMA)
    • Fluoxetine
    • Fluphenazine
    • Haloperidol
    • Sertraline
    • Thiothixene

Hyponatremia with Hyperosmolarity
  • Due to excessive osmotically active substances
  • Serum osmolarity >295
  • Possible etiologies include:
    • Elevated glucose (most common cause of hyponatremia)
    • Corrected Na+ = 0.016 × (measured glucose – to 100) + measured sodium
    • Mannitol infusion
    • Maltose and glycine

Pediatric Considerations
  • More prone to water intoxication
  • High incidence of iatrogenic hyponatremia due to dilute formula or rehydration with water only
  • If hyponatremia secondary to DKA, follow hydration per pediatric DKA recommendations
  • Age <16 and hypoxia greatest risk for iatrogenic hyponatremia

Pregnancy Considerations
Conivaptan and tolvaptan are class C drugs in pregnancy

Geriatric Considerations
  • Tend to develop more symptoms
  • Hyponatremia more common due to impaired water secretion and low sodium diets

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