In short
A sodium below 135 mmol/L is usually a water problem rather than a salt problem. Here's what pushes it down, why unsteadiness and confusion matter more than the number itself, and why correcting it too fast is dangerous.
Emergency if: A seizure, marked drowsiness or confusion, or persistent vomiting with headache — particularly after drinking a large amount of water in a short time — needs emergency assessment.
On your report
Sodium, Serum Osmolality, Potassium, Urea / BUN +3 more
On this page
01
What the Number Means
Sodium in blood is normally 135–145 mmol/L (the same as mEq/L on some reports). Below 135 is hyponatremia: 130–134 is mild, 125–129 moderate, and below 125 is severe. It is one of the most common abnormalities on a routine panel, especially in older adults.
The crucial idea is that sodium is a concentration, not a total. In most cases the body has roughly the right amount of salt but is holding on to too much water, which dilutes it. That is why the treatment is often about water rather than about eating more salt.
02
Why It Happens
Medicines are a leading cause — thiazide diuretics above all, along with some antidepressants, carbamazepine and certain painkillers. Heart failure, liver cirrhosis and kidney disease all make the body retain water. Hypothyroidism and adrenal insufficiency are less common but treatable causes worth excluding.
Another group is SIADH, where the hormone that tells the kidneys to hold water is released when it should not be — triggered by chest infections, some cancers, brain injury or medicines. Drinking a very large volume of water in a short time can also do it, which is why it appears after endurance events.
03
What It Feels Like
A sodium that has drifted down slowly often causes nothing obvious, or only vague tiredness and poor concentration. Even so, mild long-standing hyponatremia is linked to unsteadiness, falls and fractures in older people, so it is not something to shrug off just because there are no dramatic symptoms.
A sodium that drops quickly is far more dangerous, because the brain has had no time to adapt to the change. Headache, nausea and vomiting, muscle cramps, drowsiness, confusion and, at the extreme, seizures reflect fluid shifting into brain cells and need urgent assessment.
04
How It Is Sorted Out
Working out the cause usually needs more than the sodium itself: how hydrated you look, a blood and urine osmolality, urine sodium, kidney function, and thyroid and cortisol levels. The answers separate dilution from salt loss, which matters because the two are managed in opposite ways.
Treatment follows the cause. Fluid restriction suits SIADH, stopping or changing a diuretic suits drug-induced cases, and replacing salt and water suits someone who is genuinely depleted after vomiting or diarrhoea. Severe or symptomatic cases are treated in hospital with careful monitoring.
05
Why Correcting It Slowly Matters
Raising sodium too quickly can injure the brain's white matter — a rare but devastating complication called osmotic demyelination. Because of it, doctors deliberately limit how far sodium is allowed to rise in any 24-hour period, typically to single figures, even when the starting value is alarming.
This is the reason not to self-treat a low reading with salt tablets, extra salty food or sports drinks. It is also why a repeat blood test is scheduled soon after any change in treatment: the trajectory is watched as closely as the number.
When to act
Emergency — act now
A seizure, marked drowsiness or confusion, or persistent vomiting with headache — particularly after drinking a large amount of water in a short time — needs emergency assessment.
Same-day — call promptly
New confusion, unsteadiness or repeated falls in someone taking a diuretic or antidepressant, or a reported sodium below 130 with any symptoms.
Routine — see a doctor
A sodium of 130–134 with no symptoms — book a review to check medicines, thyroid and kidney function, and repeat the test rather than ignoring it.
Values mentioned in this story
Each one opens a visual guide with its normal range and what moves it — upload a report and we place your own numbers on the same scale.
Sources
- Low blood sodium — MedlinePlus Medical Encyclopediamedlineplus.gov
- Sodium blood test — MedlinePlus Medical Encyclopediamedlineplus.gov
- Osmotic demyelination syndrome — MedlinePlus Medical Encyclopediamedlineplus.gov
- Fluid and Electrolyte Balance — MedlinePlusmedlineplus.gov
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