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Hyponatremia is a topic covered in the 5-Minute Clinical Consult.

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  • 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


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

2.5% of hospitalized patients

Geriatric Considerations
The elderly have lower TBW, a decreased thirst mechanism, and decreased urinary concentrating ability; their kidneys are less responsive to antidiuretic hormone (ADH), and they show decreased renal mass, renal blood flow, and glomerular filtration rate, putting them at higher risk for hyponatremia.

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

Etiology and Pathophysiology

  • Assess serum osmolality and volume status to determine etiology. The etiology directs the management.
  • Hypertonic hyponatremia: serum osmolarity (Osm) >285 mOsm
  • Shift of water 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 280 to 285 mOsm
    • Excessive osmoles leading to dilution
    • Unchanged TBW and Na
    • Causes: hyperlipidemia, hyperproteinemia (e.g., multiple myeloma)
  • Hypotonic hyponatremia: serum Osm <280 mOsm
    • Subdivided by volume status into hypovolemic, euvolemic, or hypervolemic
  • Hypovolemic hyponatremia: subtype of hypotonic hyponatremia with decreased TBW and Na
    • Signs include orthostatic hypotension, decreased skin turgor, dry mucous membranes.
    • Urine Na <20 mmol/L; indicates extrarenal loss
      • Causes include GI loss (vomiting, diarrhea), third spacing (pancreatitis, peritonitis, burns, rhabdomyolysis), skin loss (burns, cystic fibrosis, sweating), and heat-related illnesses.
    • Urine Na >20 mmol/L; indicates renal loss
      • Causes include cerebral salt-wasting syndrome, adrenal insufficiency, diuretics, osmotic diuresis.
  • Euvolemic hyponatremia: subtype of hypotonic hyponatremia with increased TBW and normal Na
    • Signs include a nonedematous state.
    • Urine Osm >100 mOsm/kg
      • Causes include syndrome of inappropriate antidiuretic hormone (SIADH), hypothyroidism, adrenal insufficiency, medications (e.g., carbamazepine, clofibrate, cyclosporine, levetiracetam, opiates, oxcarbazepine, phenothiazines, SSRIs, TCAs, vincristine).
    • Urine Osm <100 mOsm/kg
      • Causes include primary polydipsia, beer potomania.
  • Hypervolemic hyponatremia: subtype of hypotonic hyponatremia with increased TBW and Na
    • Signs include edematous state.
    • Urine Na <20 mmol/L
      • Causes include congestive heart failure (CHF), cirrhosis, nephrotic syndrome, hypoalbuminemia.
    • Urine Na >20 mmol/L
      • Causes include renal failure.

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

General Prevention

Depends on underlying etiology

Commonly Associated Conditions

  • Hypothyroidism
  • Hypopituitarism
  • Adrenocortical hormone deficiency
  • HIV patients
  • SIADH is associated with cancers, pneumonia, tuberculosis, encephalitis, meningitis, head trauma, cerebrovascular accident, HIV infection.
  • Acute neurologic patients, brain injury
  • Marathon runners in hot environments

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Stephens, Mark B., et al., editors. "Hyponatremia." 5-Minute Clinical Consult, 27th ed., Wolters Kluwer, 2019. Medicine Central, im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116309/all/Hyponatremia.
Hyponatremia. In: Stephens MB, Golding J, Baldor RA, et al, eds. 5-Minute Clinical Consult. 27th ed. Wolters Kluwer; 2019. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116309/all/Hyponatremia. Accessed April 26, 2019.
Hyponatremia. (2019). In Stephens, M. B., Golding, J., Baldor, R. A., & Domino, F. J. (Eds.), 5-Minute Clinical Consult. Available from https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116309/all/Hyponatremia
Hyponatremia [Internet]. In: Stephens MB, Golding J, Baldor RA, Domino FJ, editors. 5-Minute Clinical Consult. Wolters Kluwer; 2019. [cited 2019 April 26]. Available from: https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116309/all/Hyponatremia.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - Hyponatremia ID - 116309 ED - Stephens,Mark B, ED - Golding,Jeremy, ED - Baldor,Robert A, ED - Domino,Frank J, BT - 5-Minute Clinical Consult, Updating UR - https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116309/all/Hyponatremia PB - Wolters Kluwer ET - 27 DB - Medicine Central DP - Unbound Medicine ER -