What is hyponatraemia?

Hyponatraemia is a condition where the concentration of sodium in the blood is lower than normal.

Sodium is a naturally-occurring chemical, present in table salt, known as an electrolyte, which is essential in the body. It helps to balance fluids and is needed for nerves and muscles to work properly.

This page provides additional information on hyponatraemia for patients who have been diagnosed with this condition by their doctor.

How are the kidneys and hyponatraemia connected?

Hyponatremia can be due to having too much fluid (water) in the body, causing the sodium in the blood to be diluted or because of losing too much sodium, for example in excessive sweating, diarrhoea or in the urine.

The kidneys help to keep the concentration of sodium normal in the blood and the body by controlling the amount of sodium or water lost in the urine. When the kidneys aren’t working well, sometimes they can’t control this delicate balance, which can cause or worsen hyponatraemia.

Hyponatraemia symptoms

Most people with hyponatraemia don’t get symptoms as the drop in sodium levels is usually small or has happened over a long time. Typically, it is noticed on a blood test by your doctor. However, symptoms of hyponatraemia can occur if the levels are very low or a sudden fall in sodium levels which needs urgent treatment at a hospital as this can lead to a coma and be life threatening.

Sometimes the symptoms of hyponatraemia can be similar to symptoms from the cause.

Symptoms of sudden or severe hyponatraemia can include:

Being sick (nausea)
Headaches
Feeling tired (fatigue)
Confusion
Muscle weakness
Seizures (a sudden change to brain activity which may result in loss of control of movements or becoming unconscious or unresponsive)

It is important you seek medical advice, and urgent medical attention where appropriate, if you are concerned about your health.

A seizure can be a medical emergency, and you should call 999 if:

It is the first time the person has had a seizure
The seizure lasts over 5 minutes
The seizure leads straight into another one
The person doesn’t recover consciousness
The person doesn’t recover properly from the seizure, for example having difficulty breathing afterwards

What causes hyponatraemia?

Several factors can lead to hyponatraemia by either causing too much fluid (water) in the body or too much sodium to be lost. Causes include:

Kidney disease
Heart failure: extra fluid can build up in the body if the heart is pumping less efficiently
Vomiting or diarrhoea: can cause loss of sodium
Diuretics: such as thiazide diuretics which decrease the amount of sodium the kidney reabsorbs from urine meaning more is lost
Diseases affecting the adrenal gland
Liver disease
Excessive water intake
Very high blood sugar: this can draw excess fluid into the blood
Other medications: such as some antidepressants, antipsychotics, and drugs to treat seizures

Treatment of hyponatraemia

Treatment for hyponatraemia aims to restore a healthy level of sodium in the blood.

If sodium levels only drop slightly over a long period and the individual doesn’t have any symptoms, depending on the cause, immediate treatment may not be needed. But prompt medical support is important for people who experience a sudden drop in sodium (within 48 hours), have symptoms, have very low sodium levels or where certain causes are suspected.

One or more of these treatment options may be suggested by their healthcare provider:

Treatment of underlying conditions: such as heart failure, liver disease, diabetes, or kidney disease
Limiting fluid intake
Change of medications: hyponatraemia causing medications may be stopped or switched to alternatives
Sodium infusion: where a sodium containing solution is given directly into a vein (IV)

References

Tidy C. Hyponatraemia [online]. Patient.info, Navigate Health Ltd: London, UK; Feb 2022 [Accessed 28 August 2025]. Available from: Hyponatraemia: Symptoms and Treatment | Doctor

Lewis, J. Overview of Sodium's Role in the Body [online]. MSD Manual, Merck & Co, Inc., Rahway, NJ, USA; Jun 2025 [Accessed 28 August 2025]. Available from: Overview of Sodium's Role in the Body - Hormonal and Metabolic Disorders - MSD Manual Consumer Version

NICE [online]. Nice.org, England; Jul 2025 [Accessed 28 August 2025]. Available from: Hyponatraemia | Health topics A to Z | CKS | NICE

Campellone J. Seizures [online]. Medlineplus.gov, USA; April 2023 [Accessed 2 September 2025]. Available from: Seizures: MedlinePlus Medical Encyclopedia

NHS [online]. Nhs.uk, England; Dec 2023 [Accessed 2 September 2025]. Available from: What to do if someone has a seizure (fit) - NHS

Rout, P. and Badireddy, M. Hyponatremia [online]. StatPearls Publishing: Treasure Island (FL); Jan 2026 [Accessed 25 March 2026]. Available from: <PMID: 29262111>

Reviewed March 2026 by Professor Simon Davies.