Hyponatraemia
Hyponatraemia means the sodium level in the blood has fallen below the normal range (typically under 135 mmol/L). Sodium regulates how much water the body holds in and around cells, so when it drops, cells can swell, including those in the brain. The condition ranges from mild and symptom-free to a medical emergency requiring urgent hospital care.
Key points
- Heavy sweating combined with rehydrating on plain water alone, common among athletes and manual workers, can wash out sodium levels.
- As sodium drops further, fatigue, headache, nausea and difficulty concentrating can appear, with confusion or seizures signalling a more severe fall.
- Tolvaptan, used for the hormonal cause SIADH, helps the kidneys release excess water so sodium levels rise steadily rather than too fast.
Why sodium levels fall
The most common underlying causes include drinking excessive water in a short period, heart failure, liver cirrhosis, kidney disease, and disorders affecting the hormones that control fluid balance. Certain medications, particularly diuretics and antidepressants, can also trigger sodium loss. Hyponatraemia sometimes appears in athletes and manual workers who sweat heavily and rehydrate with plain water alone, diluting sodium without replacing it.
Recognising a low sodium level
Mild hyponatraemia often causes no symptoms at all and is found incidentally on a blood test. As sodium falls further, common signs include nausea, headache, fatigue, and difficulty concentrating. Severe or rapid drops can lead to confusion, seizures, loss of consciousness, and in rare cases, coma.
Seek urgent medical attention if any neurological symptoms appear, confusion, seizures, or extreme drowsiness, as the brain is highly sensitive to rapid changes in sodium.
Correcting hyponatraemia
Treatment depends entirely on the cause and on how quickly the sodium has fallen. Fluid restriction is first-line when the body is retaining too much water. In cases driven by a hormonal imbalance that prevents the kidneys from excreting excess fluid, a condition called syndrome of inappropriate antidiuretic hormone (SIADH), tolvaptan, a vasopressin receptor antagonist in the hormone therapy category, can help the kidneys shed water and raise sodium levels steadily. Sodium must never be corrected too quickly, as this carries its own neurological risks.