Hyponatremia (low sodium)
A lower-than-normal blood sodium level, most often caused by too much water relative to salt rather than a lack of salt. Mild cases are common and symptomless; a severe or rapidly falling sodium can cause confusion and seizures, and correction must be done carefully.
What it is
Hyponatremia means the sodium concentration in the blood is low. Sodium is the main salt controlling the body's water balance, and, counterintuitively, low sodium is usually a problem of too much water diluting the blood rather than genuine salt loss. It is one of the commonest electrolyte abnormalities, especially in older people and hospital patients. The causes group into: retaining too much water — often from the hormone problem SIADH (where the body holds onto water inappropriately, triggered by many illnesses, medications, pain or surgery); conditions where fluid backs up such as heart failure, liver cirrhosis and kidney disease; true salt and water loss (from vomiting, diarrhoea, or diuretics); drinking excessive water; and hormonal causes like an underactive thyroid or adrenal glands. The effects depend heavily on how low the sodium is and how fast it fell: a mild, slow drop often causes no symptoms, while a low or rapidly falling level makes the brain swell, causing headache, nausea, confusion, and in severe cases seizures and reduced consciousness. Importantly, correcting a low sodium too quickly can itself harm the brain, so treatment is deliberate and monitored.
Key lab markers
- Sodium — low; the degree and how fast it fell guide urgency.
- Serum and urine osmolality — help work out the mechanism (dilution versus salt loss).
- Urine sodium — helps distinguish the causes.
- Kidney, liver and thyroid function, and cortisol — for the underlying cause.
- Glucose — a very high glucose can lower sodium (and is corrected for).
- A medication review — many drugs cause hyponatremia.
Symptoms
- Often none, if mild or slowly developing
- Nausea, headache
- Tiredness, difficulty concentrating
- Muscle cramps or weakness
- Confusion, irritability
- In severe or rapid cases: drowsiness, seizures, reduced consciousness
- Symptoms of the underlying cause (e.g. heart failure, an illness)
Related lab panels
When to discuss with a doctor
A low sodium found on a blood test should be assessed by a doctor to find the cause, which ranges from a medication effect or mild SIADH to heart, liver, kidney or hormonal problems — and the treatment depends entirely on the cause and the fluid state. Mild, slowly developing hyponatremia is often managed by treating the cause and adjusting fluids or medications. Seek urgent care for a low sodium with significant symptoms — marked confusion, drowsiness, seizures, or a rapid change — which needs careful, monitored treatment in hospital. A crucial principle is that sodium must be corrected gradually, because raising it too fast can cause serious brain injury, so this is very much a doctor-guided process. Mediora.AI flags a low sodium and shows it with osmolality and the related markers so the pattern is visible; the cause and treatment belong with your doctor.