Low sodium (hyponatremia): what your result means, causes and when it is urgent

A sodium under 135 mmol/L usually means the blood holds too much water for its sodium, not too little salt. Sodium is measured as a concentration, how much sodium sits in each liter of blood water, so a low result is mostly a story about water. Extra water from a hormone signal, a medicine, a weak heart or a long race dilutes the sodium a person already has, and the kidneys fail to clear it. There is no separate blood test for sodium deficiency: the sodium line on a basic or comprehensive metabolic panel is the test. About 1.7% of US adults have a low sodium at any one time, more often with age and with long-term conditions such as diabetes, lung disease and cancer ⁶. How fast the number fell matters as much as how low it is.

What to do with your result: if your sodium is 130 to 134 and you feel well, ask your clinician when to repeat it, whether your glucose was checked, and whether any medicine you started recently could lower sodium. If it is under 130, or you feel confused, sick or unsteady, call them today.

How low is too low? Mild, moderate and profound

The 2014 European guideline grades low sodium by the number: 130 to 134 mmol/L is mild, 125 to 129 is moderate, and under 125 is profound ¹. It also splits low sodium by speed. A drop documented within 48 hours is acute, and acute low sodium is more dangerous at any given number because brain cells have no time to adapt to the diluted blood ¹³. A sodium of 128 that has sat there for months can feel like very little; the same 128 reached overnight after a marathon can cause vomiting and confusion.

Low sodium symptoms

Low sodium symptoms come from the brain, because water moves into brain cells when the blood around them is diluted. Symptoms climb roughly in this order as sodium falls or falls faster:

1. None at all, which is the most common picture for mild, slow low sodium.
2. Tiredness, poor concentration and a foggy head (see fatigue and brain fog).
3. Unsteady walking, dizziness and falls, plus muscle cramps.
4. Nausea, headache and confusion.
5. Vomiting, drowsiness, seizures and coma, which belong to severe or fast drops.

Mild chronic low sodium still carries risk. In a Belgian study, people with so-called asymptomatic low sodium averaging about 128 mmol/L had measurable attention and gait problems and fell far more often than matched controls, about 21% against 5% ⁴. In older adults, falls can lead to fractures. A low sodium can also explain none of a person's symptoms: tiredness at 134 mmol/L usually has another cause, and the sodium is a clue to note and recheck.

First check the low sodium is real

Some low sodium results are artifacts of the sample. High blood sugar (as in poorly controlled diabetes) pulls water out of cells into the blood and lowers measured sodium by about 1.6 to 2.4 mmol/L for every 100 mg/dL of glucose above normal ². A sodium of 131 mmol/L with a glucose of 400 mg/dL corrects to roughly 136 to 138 mmol/L, a glucose problem with a normal sodium. Very high triglycerides or a large amount of abnormal protein (a paraprotein, as in myeloma) can make many lab machines read sodium falsely low while the serum osmolality, the total concentration of dissolved particles in the blood, stays normal ³. Blood drawn from an arm with a drip running into it can also read low. A normal serum osmolality points to one of these false lows; a value under 275 mOsm/kg confirms a true, diluted low sodium ¹.

What causes low sodium? Sorted by your fluid state

Doctors sort true low sodium by the body's fluid state, because the three groups need different treatment ¹². A person can be dry (low fluid volume), roughly normal, or overloaded with fluid and swollen. Urine sodium, how much salt the kidneys are letting into the urine, is one of the main clues.

Medicines are among the commonest causes, especially in older adults and in the first weeks of a new drug ¹. The usual suspects are thiazide diuretics (hydrochlorothiazide, chlorthalidone, indapamide), SSRI and SNRI antidepressants, carbamazepine and oxcarbazepine, desmopressin, some chemotherapy, and NSAIDs alongside other drugs. Any medicine change is a decision for the prescriber; stopping a heart or seizure medicine on your own can be dangerous.

Eating very little salt and protein works through a different route. The kidneys need salt and protein breakdown products to carry water out in urine, so a diet of mostly beer, or tea and toast, limits how much water a person can pass, and an ordinary amount of fluid then dilutes the blood ³. In the must-not-miss group sit adrenal insufficiency (Addison's disease, where low sodium is present in most people at diagnosis, often with high potassium and a low morning cortisol ⁷), severe hypothyroidism, and SIADH caused by a lung or brain tumor.

SIADH: when the water-keeping hormone stays on

ADH (antidiuretic hormone, also called vasopressin) tells the kidneys to hold on to water. Normally it switches off when blood gets dilute. In SIADH, the syndrome of inappropriate antidiuretic hormone, ADH keeps running, so the kidneys keep water they should pass. Common triggers are pain, nausea, recent surgery, pneumonia and other lung disease, head injury and brain conditions, some cancers (small cell lung cancer above all), and drugs such as SSRIs and carbamazepine ¹³.

Urine sodium is high in SIADH, usually over 30 mmol/L, because the body is not short of salt and the kidneys keep excreting what comes in. In plain terms, SIADH means the blood is truly diluted, the urine is not, the kidneys are still passing salt, and nothing else (dehydration, swelling, thyroid, adrenal, diuretics) explains it. The 2014 guideline turns that into a checklist your doctor works through ¹:

• Effective serum osmolality under 275 mOsm/kg (the blood is diluted)
• Urine osmolality over 100 mOsm/kg (the urine is not)
• Urine sodium over 30 mmol/L on a normal salt and water intake
• Normal fluid status: not dry and not swollen
• Normal thyroid and adrenal function
• No recent diuretic use

The tests that come after a low sodium result

1. 1Repeat the sodiumConfirms the result and shows whether it is stable, falling or recovering.
2. 2Glucose and serum osmolalitySeparate a true low sodium from a false one or a glucose effect.
3. 3Urine osmolality and urine sodiumShow whether the kidneys are holding water and whether they are saving or spilling salt.
4. 4Morning cortisol and TSHCheck the adrenal and thyroid causes before SIADH can be called.
5. 5Kidney panel and medicine reviewCreatinine, BUN and potassium, and a look at every new drug started in recent weeks.

How often it is rechecked depends on the number and the trend. A mild, stable low with no symptoms is usually repeated within days to a few weeks, often after a medicine change. A falling sodium, a moderate or profound low, or any treatment that raises sodium is checked far more often, in hospital every few hours during active correction ¹.

Low sodium and low chloride often appear together. Chloride travels with sodium, so the same water excess lowers both, and vomiting or diuretics drain both. Potassium adds a clue of its own: it tends to run high with adrenal insufficiency and low with diuretics or vomiting. That is why the whole electrolyte panel is read as a set (see electrolytes explained and the metabolic panel explained).

How to raise your sodium level, and why salt is rarely the answer

Because most low sodium is excess water, eating more salt does little for most people, and in heart failure or cirrhosis extra salt worsens the swelling. The treatment follows the cause ¹²:

• A drug cause: the prescriber may stop, swap or lower the drug.
• SIADH and overdrinking: fluid restriction, decided and monitored by a doctor.
• Genuine fluid and salt loss (vomiting, diarrhea): replacing both salt and fluid.
• Heart, liver, kidney, adrenal or thyroid causes: treating that condition, such as hormone replacement in Addison's disease.
• Salt tablets, urea powder or tolvaptan (a tablet that makes the kidneys pass more water): only under medical supervision, with frequent blood tests.

Speed is the main safety rule. Brain cells adapt to chronic low sodium over a few days, and raising sodium too fast can strip the insulating myelin from nerve fibers in the brainstem, a disabling injury called osmotic demyelination ³. Severe symptoms are treated in hospital with small, measured doses of concentrated saline and blood checks every few hours ¹.

A US expert panel set a lower limit of 8 mmol/L in 24 hours for people at higher risk of that brain injury: heavy alcohol use, malnutrition, low potassium or liver disease ².

Low sodium in older adults, athletes, children and pregnancy

Older adults carry the highest risk: they take more of the drugs that lower sodium, feel thirst less reliably and often eat less, and their low sodium shows up as unsteadiness and falls ⁴. Endurance athletes are the clearest acute example. In the 2002 Boston Marathon, 13% of studied finishers had a sodium of 135 mmol/L or less and 0.6% had 120 or less, and weight gain during the race, a sign of drinking more than was sweated out, was the strongest predictor ⁵. Sports-medicine consensus guidance is to drink to thirst during long events rather than to a fixed schedule ⁸. In a fast drop, children and premenopausal women are more prone to brain swelling at the same sodium level ².

In pregnancy sodium normally runs a few mmol/L lower, because the set point for thirst and ADH shifts downward. A reading of 133 or 134 mmol/L in pregnancy is often normal for pregnancy, though it is still one for the midwife or doctor to look at, especially with headache, vomiting or swelling.

CalculatorWater intake calculatorA general drink-to-need estimate for your day and your heat. It does not replace a fluid limit set by your doctor.Work it out

For the other end of the scale and the wider panel, start from the sodium marker page, the electrolytes topic and how kidney function is read.

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