Low Sodium Symptoms: Causes, Signs & What to Do
Low sodium is the most common electrolyte abnormality and causes headache, confusion, and in severe cases brain herniation. This page covers the specific symptoms, likely causes, normal ranges, and when to act.
Low serum sodium (hyponatremia) is the most common electrolyte disorder in both outpatient and hospital settings. Low sodium draws water into brain cells osmotically, causing them to swell. The brain is confined within the skull, so cerebral edema produces the neurological symptoms that characterize severe cases. See the Sodium biomarker overview for how it is measured alongside other electrolytes.
What Low Sodium Means
Sodium concentration below 135 mmol/L indicates that water is disproportionately high relative to sodium — either from water retention (most common) or sodium loss. The hypothalamus-pituitary-kidney axis normally defends sodium concentration tightly, so persistent hyponatremia almost always reflects a pathological state. The clinical picture ranges from mild (nausea, headache) to life-threatening (seizures, brain herniation) depending on severity and rate of onset.
Symptoms of Low Sodium
Mild to moderate (125 to 135 mmol/L):
- Nausea and loss of appetite
- Headache
- Fatigue and low energy
- Muscle cramps and weakness
- Irritability and mood changes
Severe (below 125 mmol/L, or rapid fall):
- Confusion and disorientation
- Vomiting
- Seizures
- Coma
- Brain herniation in extreme cases (from unchecked cerebral edema)
What Causes Low Sodium
- SIADH (syndrome of inappropriate antidiuretic hormone) — the most common cause; ADH is secreted inappropriately from lung cancer, CNS disease, nausea, pain, or many medications
- Heart failure (reduced cardiac output triggers ADH and fluid retention)
- Liver cirrhosis (portal hypertension and reduced oncotic pressure)
- Nephrotic syndrome (protein loss reduces oncotic pressure)
- Thiazide diuretics (impair diluting ability of the kidney)
- Hypothyroidism (reduces free water clearance)
- Adrenal insufficiency (low aldosterone, low cortisol both contribute)
- Psychogenic polydipsia (extreme water intake overwhelming excretion)
- Postoperative hyponatremia (from pain/stress-driven ADH)
Normal Sodium Levels
| Measure | Reference Range | |---|---| | Sodium | 135-145 mmol/L | | Mild concern | 130-134 mmol/L | | Urgent threshold | Below 130 mmol/L warrants prompt evaluation | | Emergency threshold | Below 120 mmol/L |
When to See Your Care Team
Book a 1:1 consultation with a licensed care team lead if sodium is below 130 mmol/L, or if you have confusion, headache, or weakness with a low reading. Sodium below 120 mmol/L with acute symptoms is a medical emergency. As with hypernatremia, rapid correction of chronic hyponatremia is also dangerous — osmotic demyelination syndrome from correcting too fast can cause permanent neurological damage.
Frequently Asked Questions
What is SIADH?
Syndrome of Inappropriate ADH secretion — the most common cause of hyponatremia. ADH is secreted in excess and causes the kidneys to retain free water, diluting serum sodium. Causes include small cell lung cancer (ectopic ADH production), CNS disorders, pain, nausea, and many medications including SSRIs, opioids, and carbamazepine.
Can drinking too much water cause low sodium?
Yes, in extreme cases. Psychogenic polydipsia — compulsive water drinking, usually in people with psychiatric conditions — can overwhelm the kidneys’ diluting capacity, causing hyponatremia. Exercise-associated hyponatremia in endurance athletes occurs similarly from excessive water intake during events.
Why is rapid correction of low sodium dangerous?
When sodium is chronically low, brain cells adapt by expelling osmolytes to prevent swelling. If serum sodium is corrected too rapidly, water rushes out of brain cells faster than they can re-adapt, causing osmotic demyelination syndrome — irreversible damage to myelin sheaths in the pons and elsewhere. The maximum safe correction rate is 10 to 12 mmol/L per day.
Does low sodium affect the heart?
Yes. Severe hyponatremia can cause cardiac arrhythmias from impaired membrane potential. Heart failure itself is also a common cause of hyponatremia, creating a cycle where the treatment (diuretics) can worsen the electrolyte imbalance.
