By Sodium is one of the most important minerals in your blood. It helps control how water moves in and out of cells, supports nerve signals, and keeps muscles working. When blood sodium drops too low, a condition called hyponatraemia (also spelled hyponatremia), the effects can range from barely noticeable to life-threatening.
One well-known cause is SIADH, where the body releases too much of the water-balancing hormone vasopressin. This guide explains hyponatraemia, SIADH, other common causes, symptoms, and treatment.
What Is Hyponatraemia?
Hyponatraemia means the concentration of sodium in the blood is lower than normal. Most laboratories define it as a blood sodium level below 135 mmol/L. The normal range is usually around 135 to 145 mmol/L, though exact ranges vary slightly between labs.
| Sodium Level (mmol/L) | Commonly Described As |
|---|---|
| 135–145 | Normal range |
| 130–134 | Mild hyponatraemia |
| 125–129 | Moderate hyponatraemia |
| Below 125 | Severe (profound) hyponatraemia |
How quickly sodium falls matters as much as the number. A rapid fall, over less than 48 hours, is more likely to cause serious symptoms than a slow fall to the same level.
Is It a Lack of Salt?
Usually, no. In most cases, hyponatraemia happens because there is too much water relative to sodium, which dilutes the sodium in the blood. This is why it is closely linked to vasopressin, the hormone that controls how much water the kidneys keep. Learn the basics in what is vasopressin?
What Is SIADH?
SIADH stands for Syndrome of Inappropriate Antidiuretic Hormone secretion. “Inappropriate” means vasopressin (ADH) is being released even though the body does not need to save water. The kidneys keep holding on to water, which dilutes the blood sodium.
You may also see the term SIAD (syndrome of inappropriate antidiuresis), which includes cases where the kidneys behave as if vasopressin is present.
Common Causes of SIADH
- Lung conditions: pneumonia, tuberculosis, and some lung cancers, especially small cell lung cancer
- Brain conditions: head injury, stroke, meningitis, brain bleeding or tumours
- Medicines: certain antidepressants (such as SSRIs), some anti-seizure medicines (such as carbamazepine), some antipsychotics, and some cancer treatments
- After surgery, often related to pain, nausea, and stress
- Severe nausea or pain
- Some cancers that produce vasopressin-like substances
Other Common Causes of Hyponatraemia
- Diuretics (water tablets), particularly thiazides, used for blood pressure
- Heart failure, liver cirrhosis, and kidney disease, where the body retains fluid
- Losing fluids through vomiting or diarrhoea and replacing them with plain water only
- Drinking extremely large amounts of water in a short time
- Exercise-associated hyponatraemia in endurance athletes who drink more fluid than they lose
- Hormone problems, such as an underactive thyroid or adrenal insufficiency. See thyroid tests explained and cortisol explained.
- Too much desmopressin, a medicine used for diabetes insipidus and bedwetting. See diabetes insipidus explained.
Older adults are at higher risk, especially if they take several medicines.
Symptoms
Mild hyponatraemia often causes no symptoms and may only be found on a blood test. As sodium falls further or faster, water moves into brain cells and they swell, causing symptoms such as:
Mild to Moderate
- Nausea and loss of appetite
- Headache
- Tiredness and low energy
- Muscle cramps or weakness
- Unsteadiness and falls, especially in older people
- Difficulty concentrating
Severe (Medical Emergency)
- Vomiting
- Confusion or agitation
- Extreme drowsiness
- Seizures
- Loss of consciousness
If someone has severe confusion, seizures, or becomes unresponsive, call emergency services immediately.
How Is It Diagnosed?
Doctors use a combination of:
- Blood sodium and blood osmolality
- Urine osmolality and urine sodium
- An assessment of fluid status: is the person dehydrated, normally hydrated, or fluid-overloaded?
- Tests for thyroid and adrenal function, kidney function, and glucose
- A review of medicines
- Further tests, such as a chest X-ray or brain scan, if a cause like SIADH is suspected
SIADH is diagnosed when sodium and blood osmolality are low, the urine is inappropriately concentrated, the person is not dehydrated or fluid-overloaded, and other causes have been ruled out.
Treatment
Treatment depends on the cause, how low the sodium is, how fast it fell, and the symptoms.
- Treat the cause: stop or change a medicine, treat an infection, manage heart or liver disease.
- Fluid restriction: a common first step for SIADH, limiting daily fluid intake under medical guidance.
- Salt or urea tablets in some cases.
- Medicines called vaptans (such as tolvaptan), which block vasopressin’s effect on the kidneys, used in selected patients under specialist care.
- Hypertonic (strong) saline given in hospital for severe symptoms.
- Fluids if the cause is dehydration and sodium loss.
Why Slow Correction Matters
If long-standing low sodium is corrected too quickly, it can cause a rare but serious brain injury called osmotic demyelination syndrome. That is why sodium is raised carefully, with frequent blood tests, in hospital.
Prevention Tips
- Ask your doctor or pharmacist whether any of your medicines can lower sodium, and whether you need regular blood tests.
- During endurance events, drink to thirst rather than forcing large amounts of water.
- If you are unwell with vomiting or diarrhoea, use oral rehydration solutions rather than only plain water, and seek advice if symptoms persist.
- If you take desmopressin, follow your doctor’s fluid advice carefully.
Frequently Asked Questions
Can drinking too much water cause low sodium?
Yes. Drinking very large amounts in a short time, more than the kidneys can remove, can dilute blood sodium. This is rare in healthy people drinking normally.
Should I eat more salt if my sodium is low?
Not without medical advice. Most hyponatraemia is due to excess water, not too little salt, and treatment depends on the cause.
Is SIADH permanent?
Often not. It frequently improves once the underlying cause, such as an infection or a medicine, is treated.
The Bottom Line
Hyponatraemia is common, especially in older adults and people taking certain medicines. It usually happens because the body holds on to too much water, often due to vasopressin, as in SIADH. Mild cases may cause no symptoms, but severe cases are a medical emergency. If you are at risk, regular blood tests and a medicine review can make a big difference. Explore more in our hormones section.
Sources
- NHS
- MedlinePlus, US National Library of Medicine
- You and Your Hormones, Society for Endocrinology
This article is for general education and is not medical advice. Always speak to a qualified healthcare professional about your health. Read our editorial policy.
