- 01Low sodium is usually too much water, not too little salt
Serum sodium is a concentration. In most people with a low result, the body is holding extra water (often because ADH, the water-holding hormone, is switched on by a medicine, fluid intake or a condition), so a normal amount of sodium is diluted. Even when some sodium has been lost, the kidneys are still holding water, so eating more salt at home does not fix the cause.
A low sodium rarely means you need more salt. Serum sodium (serum is the clear, watery part of your blood) is the amount of sodium in each litre of it, and in most people a low reading means there is too much water diluting a normal amount of sodium 3. The usual culprits are medicines, drinking a lot of fluid, and conditions that make the body hold water. Less often the body has lost sodium, for example through vomiting, diarrhea or some medicines, and a clinician decides how to replace it. The normal range is about 135 to 145 mmol/L (millimoles per litre, the same number as mEq/L for sodium; see <a href="/learn/lab-units-explained">lab units explained</a>).
Where your sodium number sits
Example sodium result: 131 mmol/Lmmol/L
| Serum sodium (mmol/L) | Label | What it usually means |
|---|---|---|
| Above 145 | High (hypernatremia) | Too little water relative to sodium, most often in people who cannot feel thirst or reach a drink 4 |
| 135 to 145 | Normal reference range | Water and sodium are in balance |
| 130 to 134 | Mild low (mild hyponatremia) | Often a medicine or fluid effect; usually found by chance 1 |
| 125 to 129 | Moderate low | Needs prompt medical review, even without symptoms 1 |
| Below 125 | Profound low | Medical urgency; risk of brain swelling is highest when it fell quickly 1 |
The European guideline grades low sodium by the number, then says symptoms decide how serious it is. A sodium of 128 mmol/L with vomiting and confusion is treated as more serious than a sodium of 124 mmol/L found on a routine panel in someone who feels fine, because symptoms show the brain is affected 1. Both still need medical attention: the first in an emergency department, the second the same day. Unlike cholesterol, sodium has no separate "optimal" target inside the normal range; the reference range is the goal.
Why low sodium is usually a water problem
A sodium result is a concentration, the amount of sodium per litre of blood water, so it changes when water moves as well as when sodium does 3. Your kidneys control that concentration by deciding how much water to keep. The signal is antidiuretic hormone (ADH), the hormone that tells the kidneys to hold on to water. When ADH stays switched on when it should be off, the kidneys keep water you do not need, the water dilutes the sodium already in your blood, and the number falls. Adrogue and Madias describe most low sodium as water retention from impaired water excretion, not a body-wide shortage of sodium 3.
That is why eating more salt rarely helps and can mislead. In people with healthy kidneys, blood sodium stays in range across a wide span of everyday salt intakes, so an abnormal result seldom reflects diet alone 3. For how sodium fits with the other electrolytes on your panel, see <a href="/learn/electrolytes-explained">electrolytes explained</a>.
How extra water lowers a sodium result
Common causes of a low sodium result
In people who are otherwise well and seen outside hospital, a small set of causes accounts for most low sodium results 12.
| Cause | How it lowers sodium | Typical clue |
|---|---|---|
| Thiazide diuretics (water pills such as hydrochlorothiazide, chlorthalidone) | Make the kidneys lose sodium and impair water excretion 12 | Started or dose changed in the past weeks; older women at higher risk 2 |
| SSRIs and other antidepressants | Can switch on ADH inappropriately 12 | Recent start, older age |
| Carbamazepine and oxcarbazepine (seizure and nerve-pain medicines) | Increase ADH effect on the kidney 12 | Long-term use |
| Very high fluid intake | Water in exceeds what the kidneys can clear 3 | Large volumes daily, sometimes with a low-food diet |
| Low-solute diets ("beer potomania", "tea and toast") | Kidneys need salts and protein breakdown products from food (solutes) to flush water out; too little food plus lots of fluid traps water 3 | Heavy beer drinking or very little food |
| <a href="/topics/heart">Heart failure</a> and <a href="/topics/liver">liver cirrhosis</a> | The body senses too little blood reaching the organs and switches on ADH, so water is kept 1 | Swelling, fluid in the belly, known diagnosis |
| Kidney disease | Kidneys cannot dilute urine well 1 | Low eGFR (estimated kidney filtration) on the same panel |
| SIADH (syndrome of inappropriate ADH) | ADH released without a normal trigger, from lung disease, some cancers, pain, nausea, or after surgery 12 | Low sodium with no swelling and no dehydration 1 |
Some causes, such as thiazides, vomiting or diarrhea, do lose sodium, but the kidneys still hold water, and replacing salt at home is not the fix 1. Less common contributors include an underactive thyroid (see <a href="/conditions/hypothyroidism">hypothyroidism</a> and <a href="/learn/how-to-read-your-thyroid-panel">how to read your thyroid panel</a>) and low cortisol from adrenal insufficiency, which is why a clinician may add thyroid and cortisol tests when the cause is unclear 1. Kidney-related causes are covered in <a href="/learn/kidney-function-explained">kidney function explained</a>, <a href="/biomarkers/egfr">eGFR</a> and <a href="/conditions/chronic-kidney-disease">chronic kidney disease</a>.
Your clinician may review medicines that can lower sodium. Do not stop or change a prescribed medicine on your own; some, such as seizure medicines and antidepressants, are risky to stop suddenly.
Mild low sodium is not automatically harmless. Even a long-standing sodium in the low 130s has been linked to unsteadiness, attention problems and falls in older adults, so a mild low result that persists should have its cause found 2.
When the low sodium is not what it seems
Very high blood sugar
High blood glucose pulls water out of cells into the blood, which dilutes sodium. Measured sodium falls about 1.6 mmol/L (Katz, 1973) to 2.4 mmol/L (Hillier, 1999) for every 100 mg/dL that glucose rises above normal, so a low sodium with a very high glucose is read after a correction 1. For example, with a glucose of 500 mg/dL and a measured sodium of 131 mmol/L, the corrected sodium works out to roughly 137 to 141 mmol/L, in the normal range. A glucose that high is itself a medical urgency, so the priority is same-day care for the blood sugar; your clinician will interpret the sodium alongside it. See <a href="/biomarkers/glucose-fasting">fasting glucose</a> and <a href="/learn/blood-sugar-explained">blood sugar explained</a>.
Very high triglycerides or blood proteins
Pseudohyponatremia is a falsely low sodium caused by the test method. When blood contains a lot of fat (very high triglycerides) or protein (for example paraproteins, abnormal antibody proteins, in myeloma, a bone-marrow cancer), the common indirect measuring method under-reads sodium 1. A direct ion-selective electrode test (a lab method that measures sodium in undiluted blood) shows the true sodium. A normal serum osmolality (a measure of how concentrated the blood is) confirms the low reading is a measurement artefact 1. If your triglycerides are very high, see <a href="/learn/how-to-read-your-lipid-panel">how to read your lipid panel</a>.
Low sodium after marathons and long events
Exercise-associated hyponatremia happens mainly when athletes drink more fluid than they lose during long events such as marathons, triathlons and long hikes, while exercise itself keeps ADH switched on. Salt loss in sweat plays a smaller part 6. The 2015 international consensus statement advises drinking to thirst rather than to a set schedule, because letting thirst guide intake limits over-drinking while still providing enough fluid to prevent excessive dehydration 6.
What a high sodium means
A sodium above 145 mmol/L, called hypernatremia, almost always means too little water relative to sodium; diet is rarely the cause 4. A healthy adult who can feel thirst and reach water rarely develops it. It is most common in frail older adults, people with dementia or after a stroke, infants, and people with high fluid losses from fever, diarrhea or <a href="/learn/blood-sugar-explained">uncontrolled diabetes</a> 4. A rarer cause is diabetes insipidus, where the body does not make or respond to ADH and loses large volumes of dilute urine 4.
| Low sodium (below 135 mmol/L) | High sodium (above 145 mmol/L) | |
|---|---|---|
| Main problem | Too much water for the sodium present | Too little water for the sodium present |
| Who it often affects | People on thiazides or SSRIs, heavy fluid drinkers, heart or liver failure, endurance athletes | Frail older adults, infants, people who cannot drink on their own |
| Common symptoms | Often none if mild; nausea, headache, unsteadiness, confusion if lower or faster | Thirst, irritability, weakness, confusion |
| Is diet the usual cause? | No | No |
| Danger sign | Below 125 mmol/L or any neurological symptom | Confusion or drowsiness, especially in an older adult |
Why correction speed is a medical decision
When sodium drifts low over more than about 48 hours, brain cells adapt to the dilute blood. If sodium is then pushed back up too quickly, those cells can be injured, a rare but serious condition called osmotic demyelination. The US expert panel advises not raising a long-standing (chronic, more than 48 hours) low sodium by more than about 10 to 12 mmol/L in 24 hours, and 8 mmol/L in higher-risk people such as those with alcohol use disorder, malnutrition or liver disease 2. This is why treatment, whether fluid limits, stopping a medicine, or other therapy, is chosen and monitored by a clinician with repeat blood tests.
How sodium is tested and how often to recheck
- Sodium is part of the basic metabolic panel (BMP) and comprehensive metabolic panel (CMP), alongside <a href="/biomarkers/potassium-serum">potassium</a>, <a href="/biomarkers/chloride-serum">chloride</a>, <a href="/biomarkers/bicarbonate-co2">bicarbonate</a>, <a href="/biomarkers/glucose-fasting">glucose</a> and kidney markers. It is rarely ordered alone.
- Fasting is not needed for sodium itself; a fast is sometimes requested because the same panel measures glucose.
- Sodium is read with its neighbours: chloride usually moves in the same direction, and the <a href="/biomarkers/anion-gap">anion gap</a> uses sodium in its formula.
- When the cause is unclear, clinicians often add serum osmolality, urine sodium and urine osmolality to tell water excess from true sodium loss 1.
There is no general screening schedule for sodium. Retest timing depends on the level, symptoms, and cause: a profound or symptomatic low is rechecked within hours in hospital, while a mild low found by chance is usually rechecked once the likely cause has been reviewed, often within a few weeks; ask your clinician for a specific date. After starting a thiazide diuretic or an SSRI, especially in older adults, clinicians may recheck electrolytes in the first weeks. A single reading matters less than the direction over time; see <a href="/learn/trends-beat-single-readings">why trends beat single readings</a>.
For the potassium on the same panel, which behaves very differently, see <a href="/biomarkers/potassium-serum">serum potassium</a>; for all the electrolytes together, see the <a href="/topics/electrolytes">electrolytes topic</a>.
Citations
- Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1-G47.
- Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42.
- Adrogue HJ, Madias NE. Hyponatremia. N Engl J Med. 2000;342(21):1581-1589.
- Adrogue HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.
- Mohan S, Gu S, Parikh A, Radhakrishnan J. Prevalence of hyponatremia and association with mortality: results from NHANES. Am J Med. 2013;126(12):1127-1137.
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med. 2015;25(4):303-320.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.