Hyponatraemia

Key facts

• Hyponatraemia means the sodium level in the blood has fallen below the normal range, typically under 135 mmol/L, letting cells (including brain cells) swell with water.
• Common causes include drinking excessive water quickly, heart failure, liver cirrhosis, kidney disease, hormonal fluid-balance disorders, and medicines such as diuretics and antidepressants.
• Mild cases often cause no symptoms; more severe drops bring nausea, headache, fatigue and confusion, and can progress to seizures or loss of consciousness.
• Treatment depends on the cause: fluid restriction is first-line, while a hormonal condition called SIADH may need tolvaptan to help the kidneys shed excess water.

What is actually going on

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.

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. Athletes and manual workers who sweat heavily and rehydrate with plain water alone can also dilute their 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, since 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 stops the kidneys excreting excess fluid, a condition called syndrome of inappropriate antidiuretic hormone (SIADH), tolvaptan, a vasopressin receptor antagonist within hormone therapy, 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.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.