• Hyponatremia is a plasma sodium (Na+) concentration of ≤135 mEq/L.
  • Hyponatremia itself does not provide information about the total body water (TBW) state of the patient. Patients with hyponatremia may be hypervolemic, hypovolemic, or euvolemic.
  • System(s) affected: endocrine/metabolic, renal, cardiovascular, central nervous system (CNS)



  • Most common electrolyte disorder seen in the general hospital population
  • Predominant age: all ages
  • Predominant sex: male = female


  • 2.5% of hospitalized patients
  • 20–30% in hospitalized patients (1)
  • 7.7% outpatients (1)

Geriatric Considerations
Elderly patients have a decreased renal mass placing them at risk decreased urinary concentration and decreased response to antidiuretic hormone. Additionally, they have a lower TBW state, a decreased thirst mechanism, and decreased renal blood flow, which will also affect the glomerular filtration rate. Presenting symptoms may be frequent falls and gait disturbances.

Pediatric Considerations
Children <16 years of age have less intracranial space and are at increased risk of brain herniation from cerebral edema.

Etiology and Pathophysiology

  • Volume status and serum osmolality must be ascertained to determine etiology in order to direct management.
  • Hypertonic hyponatremia: serum osmolarity (Osm) > 295 mOsmol/kg
    • Water shifts from intracellular fluid (ICF) to extracellular fluid (ECF), resulting in dilution.
      • Unchanged TBW and Na+
      • Causes: hyperglycemia, mannitol, sorbitol, radiologic contrast
  • Isotonic hyponatremia (“pseudohyponatremia”): serum Osm 275 to 295 mOsmol/kg
    • Excessive osmoles leading to dilution
    • Unchanged TBW and Na+
    • Causes: hyperlipidemia, hyperproteinemia (e.g., multiple myeloma), laboratory artifact, irrigant solutions
  • Hypotonic hyponatremia: serum Osm <275 mOsmol/kg
    • Subdivided by volume status into hypovolemic, euvolemic, or hypervolemic
    • Hypovolemic hyponatremia: low TBW and low Na+
  • Signs include orthostatic hypotension, decreased skin turgor, dry mucous membranes.
  • If urine Na+ <30 mmol/L, it indicates extrarenal loss such as GI loss (vomiting, diarrhea), third-spacing (pancreatitis, burns), skin loss (burns, cystic fibrosis, sweating), and heat-related illnesses.
  • If urine Na+ >30 mmol/L, it indicates renal loss such as cerebral salt wasting, adrenal insufficiency, diuretics, and osmotic diuresis.
  • Euvolemic hyponatremia: mild to moderate increase in TBW, normal Na+ (most common subtype)
  • Signs include a nonedematous state.
  • If urine Osm >100 mOsm/kg, causes include syndrome of inappropriate antidiuretic hormone (SIADH), hypothyroidism, adrenal insufficiency, medications (e.g., thiazide diuretics, loop diuretics, carbamazepine, clofibrate, cyclosporine, levetiracetam, oxcarbazepine, SSRIs, TCAs, vincristine).
  • If urine Osm <100 mOsm/kg, causes include primary polydipsia, beer potomania, and exercise induced hyponatremia.
  • Hypervolemic hyponatremia: increased TBW and Na+
  • Signs include edematous state.
  • Urine Na+ <30 mmol/L
  • Causes include congestive heart failure (CHF), cirrhosis, nephrotic syndrome, hypoalbuminemia, and psychogenic polydipsia.


  • Polymorphisms have been demonstrated.
  • Mutations have been associated with nephrogenic syndrome of inappropriate antidiuresis (NSIAD; SIADH).

General Prevention

Depends on underlying etiology

Commonly Associated Conditions

  • Hypothyroidism
  • Hypopituitarism
  • Cirrhosis
  • CHF
  • Nephrotic syndrome
  • Adrenocortical hormone deficiency
  • HIV patients
  • SIADH is associated with cancers, pneumonia, tuberculosis, encephalitis, meningitis, head trauma, cerebrovascular accident, and HIV infection.
  • Traumatic brain injury
  • Marathon runners in hot environments
  • Beer potomania
  • Tea-and-toast diet
  • Ecstasy use

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