Evidence and service information checked, October 2026.
Low sodium is a blood-test finding that ranges from a mild abnormality to a medical emergency. The number matters, but so do symptoms, the speed of change and the reason it has fallen. It is not a result to correct yourself with extra salt or large changes in fluid intake.
What does low sodium on a blood test mean?
Low blood sodium, or hyponatraemia, usually means that there is too much water in relation to sodium in the blood. It does not simply mean that you have eaten too little salt. Medicines, illnesses and changes in the body's control of water can all contribute.
A result below 135 mmol/L is commonly described as hyponatraemia. Laboratory ranges may differ. A clinician needs to assess the result in context, including glucose, kidney function, medicines, fluid balance and any previous sodium measurements.
When not to wait for a routine appointment
Seek emergency care if low sodium is accompanied by confusion, a seizure, reduced consciousness, severe drowsiness, a severe headache or repeated vomiting. Call 999 for a seizure, collapse or impaired consciousness. Do not drive yourself if you are confused or faint.
A sodium level below 125 mmol/L is generally classed as severe and needs urgent clinical assessment, even if you feel relatively well. A rapid fall can be dangerous at a higher value too. If the laboratory or your clinician has asked you to attend hospital or seek urgent help, follow that advice immediately. Do not use a routine private booking or a repeat test on another day to delay care.
Why can low sodium be dangerous?
Sodium helps regulate the balance of water around cells and is involved in nerve and muscle function. A rapid fall can cause water to move into brain cells, leading to swelling and neurological symptoms. A slower fall may cause subtler symptoms, but still needs a safe, cause-specific plan.
Correcting sodium too quickly can also cause serious neurological harm. This is why treatment in hospital may involve carefully controlled fluids and repeated measurements. The goal is not to make the number normal as fast as possible.
What are the common causes?
Possible causes include some diuretics and other medicines, vomiting or diarrhoea, excessive water intake, and conditions affecting the kidneys, heart or liver. Hormonal conditions can be relevant. Sometimes the body retains water through inappropriate antidiuretic hormone activity, called SIADH.
These mechanisms need different treatments. Someone who has lost fluid and salt is not managed in the same way as someone retaining water. Advice to drink more, restrict water or take salt tablets can be harmful when applied to the wrong situation.
Medicines deserve a careful review
Tell the clinician about every prescription, over-the-counter medicine and supplement. The timing of a new medicine or dose change can be important. Do not stop a prescribed medicine abruptly on your own; urgent clinicians can advise on a safe change if one is needed.
Excessive drinking during endurance exercise can also contribute to low sodium. Hydration advice should fit the individual and activity. Forcing large amounts of water is not a reliable way to protect health.
What symptoms can it cause?
Symptoms can include nausea, headache, weakness, muscle cramps, poor concentration or unsteadiness. These symptoms are nonspecific and can have other causes. Severe confusion, seizures or reduced consciousness are emergency warning signs.
A person can sometimes have a low result without obvious symptoms, especially if it developed slowly. That does not mean the result can be ignored. The value, trend, other tests and clinical situation decide how urgently it needs assessment.
What do mild, moderate and severe mean?
The American Family Physician review describes sodium of 130 to 134 mmol/L as mild, 125 to 129 as moderate and below 125 as severe. These bands are useful descriptions, not a self-triage rule. Symptoms, speed of onset and other illness can make a result more urgent than the label suggests.
A new low result should be discussed with a clinician who can access the report and assess you. If you have not been given a plan, contact the requesting clinician promptly. A repeat measurement may be needed, but a repeat test should not postpone treatment when urgent features are present.
What investigations might be needed?
Depending on the situation, a clinician may repeat sodium and review glucose, kidney function and other electrolytes. Blood and urine measurements of concentration and urine sodium can help establish the mechanism. Assessment may also consider thyroid or adrenal function where appropriate.
These tests are interpreted alongside examination and fluid balance. A sodium result cannot be reliably treated from a screenshot of one line of a report. A normal-looking blood pressure or the absence of swelling does not settle the cause by itself.
Should you eat more salt or drink less water?
Not without individual advice. Hyponatraemia is often a problem of water balance, and increasing dietary salt may not address it. Fluid restriction is used in some circumstances but is not the correct response to every cause. Similarly, drinking extra water to feel healthier can make some forms worse.
Do not copy an online fluid allowance, buy salt tablets or use sports drinks as treatment for a diagnosed low sodium result. A clinician should explain what to do, what not to change and when the sodium will be checked again.
What happens after treatment?
Follow-up depends on the cause and severity. It may include a medicine change, management of an underlying condition and a planned repeat blood test. Ask what symptoms should trigger urgent reassessment and who is responsible for reviewing the next result.
If you were treated in hospital, keep the discharge plan and medicine list available. Feeling better does not replace the agreed follow-up. If symptoms recur, seek medical advice rather than adjusting fluid or medicines yourself.
Where a routine GP review fits
A non-emergency private GP review can help explain an already assessed, stable result or coordinate follow-up once urgent care is not required. It is not a substitute for hospital assessment of severe or symptomatic hyponatraemia. As checked in October 2026, The Online GP lists video consultations from £150 for up to 30 minutes; tests and other care may cost extra and are confirmed before booking.
Contact The Online GP for a non-urgent results review if you already have a safe follow-up plan or need help understanding a stable result. Tell the team the exact sodium value and any advice already given. If there are emergency symptoms or an urgent laboratory instruction, seek urgent care instead of waiting for a reply.
Sources and further reading
- Mayo Clinic: hyponatraemia, symptoms and causes.
- American Family Physician, 2023: diagnosis and management of sodium disorders.
This article does not provide a sodium-correction or fluid-prescribing protocol.
Written by The Wellness Team. This information supports, but does not replace, an individual medical assessment.