MedSchool: Ace Your OSCEsThe Medical Company
GET - On the App Store




  • Hyponatraemia is an imbalance between sodium and water balance in the body, producing a reduced serum sodium concentration of <135 mmol/L.


  • Hyponatraemia may occur due to loss of salt through the kidneys or elsewhere, through dilution or through incorrect sampling.
    • Causes of Hyponatraemia

    • Salt Depletion

    • Extrarenal - vomiting, diarrhoea, third spacing
    • Renal - loop or thiazide diuretic abuse, Addison’s, renal tubular acidosis
    • Dilution

    • Osmotic shift - hyperglycaemia, mannitol
    • Excess water intake - primary polydipsia, beer potomania
    • Antidiuresis - SIADH, hypothyroidism, ACTH deficiency
    • Fluid overload - heart failure, cirrhosis, nephrotic syndrome, renal failure
    • Factitious

    • Pseudohyponatraemia - lab error, hyperproteinaemia, hyperlipidaemia
    • Causes of SIADH

    • Malignancy - lung cancers, pancreatic cancer, colorectal cancer, bladder cancer, lymphoma, sarcoma
    • CNS - tumours, demyelination, ischaemic stroke, haemorrhage, infection, vasculitis, traumatic brain injury
    • Pulmonary - pneumonia, tuberculosis, abscess, vasculitis
    • Drugs - desmopressin, SSRIs, TCAs, carbemazepine, levetiracetam, haloperidol, cyclophosphamide, vincristine
    • Postoperative state

Clinical Features

  • Mild or gradual onset hyponatraemia generally presents with few symptoms, while severe or rapid onset hyponatraemia can be life-threatening.
  • Acute hyponatraemia may present with nausea, vomiting, headaches or confusion; as the sodium level falls patients may present with seizures (including status epilepticus) or coma.
  • Chronic hyponatraemia may also present with nausea, vomiting or headaches. Patients may also report polydipsia, lethargy or muscle cramps.

Approach to Diagnosis

  • In order to further investigate the cause of hyponatraemia, first check the serum and urine sodium and osmolality at the same time.
    • Approach to Diagnosis
  • Normal serum osmolality (280 - 285 mOsm/kg) indicates either a spurious result (pseudohyponatraemia) or excessive administration of isotonic solutes like isotonic mannitol.  Pseudohyponatraemia is the false laboratory measurement of sodium due to an increased solid phase of plasma, such as with hyperproteinaemia or hyperlipidaemia.
  • Elevated serum osmolality (>285mOsm/kg) indicates movement of water from cells to the interstitium and intravascularly, driven by extracellular solutes such as sugars. This can occur with hyperglycaemia and hypertonic mannitol. A sodium correction should be performed to account for serum glucose - see the Calculators section for this tool.
  • Low serum osmolality (<280 mOsm/kg) suggests excess total body water in relation to total body sodium, due to sodium loss or water retention. Examine the patient's fluid status to further delineate a cause.
  • Low serum osmolality with dehydration suggests salt and water loss from the kidneys or elsewhere. Low urinary sodium (<20 mEq/L) in these patients is suggestive of GI or sequestrational loss, such as due to vomiting, diarrhoea or third spacing. Normal urinary sodium (>20 mEq/L) is suggestive of renal loss, such as due to diuretics, mineralocorticoid deficiency, renal tubular acidosis, cerebral salt wasting or salt wasting nephropathy.
  • Low serum osmolality with euvolaemia is suggestive of redistribution. Urine osmolality <100 mOsm/kg may be due to primary polydipsia or beer potomania syndrome; urine osmolality >100mOsm/kg suggest SIADH, hypothyroidism or glucocorticoid deficiency.
  • Low serum osmolality with hypervolaemia is suggestive of water retention. Low urinary sodium (<20 mEq/L) in this case is suggestive of renal failure as a cause, while normal urinary sodium (>20 mEq/L) suggests other causes of fluid overload such as heart failure, liver cirrhosis, nephrotic syndrome or hypoalbuminaemia.
Want more info like this?
  • Your electronic clinical medicine handbook
  • Guides to help pass your exams
  • Tools every medical student needs
  • Quick diagrams to have the answers, fast
  • Quizzes to test your knowledge
Sign Up Now


 Acker CG, Johnson JP, Palevsky PM, Greenberg A. Hyperkalemia in hospitalized patients: causes, adequacy of treatment, and results of an attempt to improve physician compliance with published therapy guidelines. Archives of Internal Medicine. 1998 Apr 27;158(8):917-24.
Adrogué HJ, Madias NE. Hypernatremia. New England Journal of Medicine. 2000 May 18;342(20):1493-9.
 Adrogue HJ, Madias NE. Hyponatremia. New England Journal of Medicine. 2000 May 25;342(21):1581-9. Assadi F. Hypophosphatemia: an evidence-based problem-solving approach to clinical cases. Iranian journal of kidney diseases. 2010 Jul 1;4(3):195. Ayuk J, Gittoes NJ. How should hypomagnesaemia be investigated and treated?. Clinical endocrinology. 2011 Dec 1;75(6):743-6. Berend K, van Hulsteijn LH, Gans RO. Chloride: the queen of electrolytes?. European journal of internal medicine. 2012 Apr 30;23(3):203-11. Bourke E, Yanagawa N. Assessment of hyperphosphatemia and hypophosphatemia. Clinics in laboratory medicine. 1993 Mar;13(1):183-207. Braun MM, Barstow CH, Pyzocha NJ. Diagnosis and management of sodium disorders: hyponatremia and hypernatremia. American family physician. 2015 Mar 1;91(5). Carroll MF, Schade DS. A practical approach to hypercalcemia. American family physician. 2003 May 1;67(9):1959-68. Chung HM, Kluge R, Schrier RW, Anderson RJ. Postoperative hyponatremia: a prospective study. Archives of Internal Medicine. 1986 Feb 1;146(2):333-6. Cooper MS, Gittoes NJ. Diagnosis and management of hypocalcaemia. BMJ: British Medical Journal. 2008 Jun 7;336(7656):1298. Cundy T, Dissanayake A. Severe hypomagnesaemia in long?term users of proton?pump inhibitors. Clinical endocrinology. 2008 Aug 1;69(2):338-41. Einhorn LM, Zhan M, Walker LD, Moen MF, Seliger SL, Weir MR, Fink JC. The frequency of hyperkalemia and its significance in chronic kidney disease. Archives of internal medicine. 2009 Jun 22;169(12):1156-62. Eisenhut M. Causes and effects of hyperchloremic acidosis. Critical Care. 2006 Jun 29;10(3):413. Fichman MP, Vorherr H, Kleeman CR, Telfer NA. Diuretic-induced hyponatremia. Ann Intern Med. 1971 Dec 1;75(6):853-63. Fong J, Khan A. Hypocalcemia. Canadian Family Physician. 2012 Feb 1;58(2):158-62.
Fukumoto S, Namba N, Ozono K, Yamauchi M, Sugimoto T, Michigami T, Tanaka H, Inoue D, Minagawa M, Endo I, Matsumoto T. Causes and Differential Diagnosis of Hypocalcemia—Recommendation Proposed by Expert Panel Supported by Ministry of Health, Labour and Welfare, Japan—. Endocrine journal. 2008;55(5):787-94.
 Gennari FJ. Hypokalemia. New England Journal of Medicine. 1998 Aug 13;339(7):451-8. Halperin ML, Bohn D. Clinical approach to disorders of salt and water balance: emphasis on integrative physiology. Crit Care Clin. 2002; 18: 249–272. Hannon MJ, Thompson CJ. The syndrome of inappropriate antidiuretic hormone: prevalence, causes and consequences. European journal of endocrinology. 2010 Jun 1;162(Suppl1):S5-12. Hruska KA, Mathew S, Lund R, Qiu P, Pratt R. Hyperphosphatemia of chronic kidney disease. Kidney international. 2008 Jul 2;74(2):148-57. Huang CL, Kuo E. Mechanism of hypokalemia in magnesium deficiency. Journal of the American Society of Nephrology. 2007 Oct 1;18(10):2649-52. Imel EA, Econs MJ. Approach to the hypophosphatemic patient. The Journal of Clinical Endocrinology & Metabolism. 2012 Mar 1;97(3):696-706. Katz MA. Hyperglycemia-induced hyponatremia—calculation of expected serum sodium depression. New England Journal of Medicine. 1973 Oct 18;289(16):843-4. Mandal AK. Hypokalemia and hyperkalemia. Medical Clinics of North America. 1997 May 1;81(3):611-39. Muggia FM. Overview of cancer-related hypercalcemia: epidemiology and etiology. InSeminars in oncology 1990 Apr 1 (Vol. 17, No. 2, pp. 3-9). Elsevier. Noakes TD, Norman RJ, Buck RH, Godlonton J, Stevenson K, Pittaway D. The incidence of hyponatremia during prolonged ultraendurance exercise. Medicine and Science in Sports and Exercise. 1990 Apr;22(2):165-70. Palevsky PM, Bhagrath R, Greenberg A. Hypernatremia in hospitalized patients. Annals of internal medicine. 1996 Jan 15;124(2):197-203. Perazella MA. Drug-induced hyperkalemia: old culprits and new offenders. The American journal of medicine. 2000 Sep 30;109(4):307-14. Soifer JT, Kim HT. Approach to metabolic alkalosis. Emergency Medicine Clinics. 2014 May 1;32(2):453-63. Topf JM, Murray PT. Hypomagnesemia and hypermagnesemia. Reviews in endocrine & metabolic disorders. 2003 May 1;4(2):195-206.