What Does a Low Sodium Mean?
A doctor explains what a low blood sodium means, why it is usually a water problem rather than a salt problem, and which levels need urgent attention.
See a doctor promptly if
- Confusion, drowsiness, unsteadiness or a seizure in someone with low sodium, which needs emergency care
- A sodium below about 125 mmol/L on a confirmed sample
- Low sodium developing quickly over hours or days, especially after starting a new medicine
What your sodium result means
| Result | In short | What it means |
|---|---|---|
| Under 120 mmol/L | Severe | Yes. At this level confusion, drowsiness and seizures become real risks, and it needs emergency care. |
| 120 to 129 mmol/L | Moderate | Low enough to cause symptoms and low enough to need a same-day answer about why. |
| 130 to 134 mmol/L | Mild | Slightly below range, common, and usually a water problem rather than a salt problem. Medicines are the first thing reviewed. |
| 135 to 145 mmol/L | Normal | Yes, and the range is narrow on purpose: the body defends this number closely. |
| 146 to 149 mmol/L | Mildly high | Usually not enough water rather than too much salt. It is read alongside how much you have been drinking and any recent illness. |
| 150 mmol/L or above | Clearly high | High enough to need an explanation the same day, particularly in an older person or anyone who has been unwell. |
One result is a snapshot. Ranges differ a little between labs, and what a number means depends on the rest of you. Check it against the guidelines.
The short version
- A low blood sodium is called hyponatremia. It almost always means there is too much water in the blood compared with salt. It is not usually too little salt in the diet.
- Normal is about 135 to 145 mmol/L (the same as mEq/L). 130 to 134 is mild and often causes no symptoms. Below 125 can cause confusion and needs prompt medical care.
- Medicines are one of the most common causes, and one of the easiest to fix. The main ones are thiazide diuretics, some antidepressants and some anti-epileptics.
Try the result lookupone number, free, nothing to installType the number you are holding and it lands in a band, with what usually happens next.
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 judgment 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, especially potassium, urea and creatinine, change what it means. The wider guide to understanding your blood test results explains how to read that group together.
Should this go to a doctor?
Three honest answers, a plain answer back.
On more than two daily medicines
A weekly organizer prevents the double-dose and missed-dose mistakes every doctor sees. The simplest ones work best, filled once a week at the same time.
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Common questions
Should I eat more salt to fix it?
Can drinking too much water cause it?
How low is dangerous?
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