What Does a Low Sodium Mean?

A low blood sodium, known medically as hyponatraemia, nearly always means there is too much water in your blood relative to salt. It is a water balance problem far more often than a dietary salt problem. Eating more salt is rarely the answer, and is occasionally the wrong one.

Normal is roughly 135 to 145 mmol/L, and millimoles per liter and milliequivalents per liter are numerically the same for sodium, so both units read identically.

How low, and how fast#

Speed matters as much as the number. The brain adapts to a slowly falling sodium over weeks, so someone can sit at 128 mmol/L and feel almost normal. The same level reached in forty eight hours can cause headache, nausea, unsteadiness and confusion, because the brain has had no time to adjust.

• 130 to 134 mmol/L: mild, very common, often no symptoms
• 125 to 129 mmol/L: moderate, may cause nausea, headache, poor concentration
• Below 125 mmol/L: severe, risk of confusion, falls and seizures

The usual causes#

Medicines lead the list, and they are the most reversible. Thiazide diuretics are the classic culprit. SSRI antidepressants, carbamazepine, some antipsychotics and proton pump inhibitors all contribute in some people.

Beyond medicines, the common causes are:

• Loss of fluid through vomiting, diarrhea or heavy sweating, replaced with plain water
• Heart failure, liver cirrhosis and kidney disease, where the body retains water
• SIADH, where the body holds on to water inappropriately, often triggered by chest infections, brain conditions, pain, nausea or cancers
• Underactive thyroid and adrenal insufficiency
• Drinking very large volumes of water quickly

A falsely low reading can also occur when blood glucose or triglycerides are very high, so the result is interpreted alongside those.

What a doctor does with it#

The first move is usually to repeat the test, because errors happen and because a trend is more useful than a snapshot. After that the question is how much fluid is in your body: whether you look dehydrated, normally hydrated or fluid overloaded. That single judgement splits the causes into three manageable groups.

Typical tests alongside it are kidney function, glucose, thyroid function, and paired urine sodium and urine osmolality, which reveal whether the kidneys are holding water inappropriately. A careful medicine review is part of every assessment.

Treatment follows the cause. Sometimes it is stopping or changing a medicine. Sometimes it is limiting fluid intake. Sometimes, when sodium is low and symptoms are present, it is intravenous treatment in hospital with careful monitoring.

At work#

In hot environments, sodium can fall when people sweat heavily for hours and rehydrate with plain water alone. Symptoms look like heat exhaustion: headache, nausea, cramps, muscle weakness. Where sweating is prolonged and heavy, replacing electrolytes rather than water alone is the standard occupational health approach.

Where this fits#

Sodium sits within the electrolyte and kidney panel, and the numbers beside it, particularly potassium, urea and creatinine, change what it means. The wider guide to understanding your blood test results explains how to read that group together.