Hyponatraemia

Overview 
Theory 
Diagnosis 
Management 
Follow up 
Resources 

Last reviewed: 17 May 2025

Last updated: 28 Feb 2024

Hyponatraemia is the most common electrolyte disorder encountered in clinical practice.

Can occur in settings of volume depletion, volume overload, or euvolaemia.

Serum osmolality, urine osmolality, and urine sodium concentration help to determine the underlying cause.

Cerebral oedema is a medical emergency and occurs more frequently when hyponatraemia develops over <48 hours. Patients should be treated promptly with hypertonic 3% saline.

Other treatment depends on the underlying cause and may include fluid restriction and stopping causative medications.

Overly rapid correction of serum sodium concentration can lead to the life-threatening complication of myelinolysis and should be avoided.

Definition

Hyponatraemia is defined as a serum sodium concentration of <135 mmol/L. Normal serum sodium concentration is in the range of 135-145 mmol/L.[1] It is a disorder of water balance reflected by an excess of total body water relative to electrolytes (total body sodium and potassium) leading to low plasma osmolality (i.e., <275 mmol/kg).[2] Hyponatraemia is generally caused by an increase in renal water reabsorption due to release of vasopressin (arginine vasopressin also known as antidiuretic hormone) along with water intake, and can occur in situations of volume depletion, volume overload, or normal volume. It should be noted that loss of sodium is minor compared with gains in water in most types of hyponatraemia.

This topic principally focuses on hypotonic (hypo-osmolar) hyponatraemia, the most common type of hyponatraemia.

History and exam

Key diagnostic factors

presence of risk factors
high fluid intake
fluid losses
history of diabetes mellitus
history of cirrhosis, nephrosis, congestive heart failure
nausea/vomiting
mild cognitive symptoms
altered mental status, seizures, coma
low urine output
weight changes
orthostatic hypotension
abnormal jugular venous pressure
poor skin turgor
dry mucous membranes
absence of axillary sweat
oedema
rales or crackles on lung auscultation
polyuria

Full details

Other diagnostic factors

history of hyperlipidaemia or paraproteinaemia

Full details

Risk factors

older age
hospitalisation
selective serotonin-reuptake inhibitor (SSRI) use
thiazide diuretic use
underlying medical conditions
severe hypothyroidism
adrenal insufficiency
malignancy
use of other medications
MDMA (ecstasy) use

Full details

Diagnostic investigations

1st investigations to order

serum sodium concentration
serum electrolytes, urea, creatinine, and glucose
serum osmolality
urine sodium concentration
urine osmolality
urine electrolytes
urine flow rate
electrolyte-free water excretion
fractional excretion of sodium
thyroid-stimulating hormone
serum cortisol level and/or adrenocorticotrophic hormone test
serum lipids and serum protein electrophoresis

Full details

Investigations to consider

CT brain, chest, abdomen/pelvis
other tests targeted at evaluating the underlying cause

Full details

Treatment algorithm

ACUTE

acute onset (<48 hours) and/or symptomatic

ONGOING

chronic onset (≥48 hours) or asymptomatic

overcorrection of serum sodium concentration

Contributors

Authors

Judith H. Veis, MD

Section Director

Nephrology

MedStar Washington Hospital Center

Washington

Disclosures

JHV declares that she has no competing interests.

Peer reviewers

Gregor Lindner, MD

Director

Department of Internal & Emergency Medicine

Hirslanden Klinik Im Park

Zurich

Switzerland

Disclosures

GL is an author of a number of references cited in this topic.

Michael E. Ullian, MD

Professor of Medicine

Division of Nephrology

Department of Medicine

Medical University of South Carolina

Charleston

Disclosures

MEU declares that he has no competing interests.

References

Our in-house evidence and editorial teams collaborate with international expert contributors and peer reviewers to ensure that we provide access to the most clinically relevant information possible.

Key articles

Spasovski G, Vanholder R, Allolio B, et al; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014 Feb 25;170(3):G1-47.Full text Abstract

Adrogué HJ, Tucker BM, Madias NE. Diagnosis and management of hyponatremia: a review. JAMA. 2022 Jul 19;328(3):280-91. Abstract

Verbalis JG, Grossman A, Höybye C, et al. Review and analysis of differing regulatory indications and expert panel guidelines for the treatment of hyponatremia. Curr Med Res Opin. 2014 Jul;30(7):1201-7. Abstract

Hoorn EJ, Zietse R. Diagnosis and treatment of hyponatremia: compilation of the guidelines. J Am Soc Nephrol. 2017 May;28(5):1340-9.Full text Abstract

Reference articles

A full list of sources referenced in this topic is available to users with access to all of BMJ Best Practice.

Differentials
- Hypertonic hyponatraemia
- Pseudohyponatraemia
- Assessment of hyponatraemia
More Differentials
Guidelines
- Clinical practice guidelines for the management of exercise-associated hyponatremia: 2019 update
- Clinical practice guideline: maintenance intravenous fluids in children
More Guidelines
Patient information
Hyponatraemia (low blood sodium)
More Patient information
Calculators
Sodium Correction in Hyperglycemia (Hillier 1999)
Osmolality Estimator (serum)
More Calculators
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Hyponatraemia

Summary

Definition

History and exam

Key diagnostic factors

Other diagnostic factors

Risk factors

Diagnostic investigations

1st investigations to order

Investigations to consider

Treatment algorithm

acute onset (<48 hours) and/or symptomatic

chronic onset (≥48 hours) or asymptomatic

overcorrection of serum sodium concentration

Contributors

Authors

Judith H. Veis, MD

Disclosures

Peer reviewers

Gregor Lindner, MD

Disclosures

Michael E. Ullian, MD

Disclosures

References

Key articles

Reference articles

A full list of sources referenced in this topic is available to users with access to all of BMJ Best Practice.

Differentials

Guidelines

Patient information

Calculators

Log in or subscribe to access all of BMJ Best Practice

Log in or subscribe to access all of BMJ Best Practice

Log in to access all of BMJ Best Practice