Low Sodium (Hyponatremia) Explained
What a low sodium reading actually means on your blood test, the most common causes (usually medications, not salt intake), and when it becomes a medical emergency.
The Short Answer
The Australian reference range for serum sodium is 135 to 145 mmol/L. Anything below 135 is “hyponatremia”. The number one cause is medications — particularly thiazide diuretics (Microzide, Natrilix) and SSRI antidepressants (Zoloft, Lexapro, Cipramil). Other common causes are heart failure, severe vomiting/diarrhoea, lung infection, and drinking large amounts of water during endurance exercise. Hyponatremia is rarely caused by eating too little salt.
What Does Low Sodium Mean?
Sodium is the dominant electrolyte in your bloodstream — it controls how much water sits inside your blood vessels and how much shifts into your cells. Your body keeps it in an extremely tight range (135-145 mmol/L) because cells, especially brain cells, are exquisitely sensitive to changes.
The crucial concept: a low sodium reading almost never means you are short of salt. It means there is too much water in your blood for the salt that is there. Hyponatremia is fundamentally a water problem. The body has either retained too much water (the most common scenario) or lost salt without replacing it.
Hypovolaemic
Total body water is LOW but salt loss exceeds water loss. Causes: vomiting, diarrhoea, diuretics, burns. Patient looks dehydrated.
Euvolaemic
Total body water is normal but ADH is too high. Causes: SIADH (drugs, lung disease), hypothyroidism, glucocorticoid deficiency. Patient looks normal.
Hypervolaemic
Total body water is HIGH but blood vessel volume is sensed as low. Causes: heart failure, cirrhosis, nephrotic syndrome. Patient has oedema.
What Causes Low Sodium?
More than 60% of community-acquired hyponatremia is caused by medications. The next biggest buckets are heart failure, gut losses, and SIADH (often from a lung problem).
Thiazide diuretics (hydrochlorothiazide, indapamide)
The number one drug cause of low sodium, especially in women over 65. Often appears within 2-4 weeks of starting. Stop or switch to a different antihypertensive.
SSRI antidepressants (sertraline, escitalopram, citalopram)
SSRIs trigger SIADH (excess ADH). Develops within the first 1-2 weeks. Can be life-threatening in elderly women on co-prescribed thiazides.
SIADH from lung disease
Pneumonia, COPD exacerbation, and small-cell lung cancer all release ectopic ADH causing dilutional hyponatremia. Always order a chest X-ray.
Heart failure
Low cardiac output activates ADH and renin-angiotensin, retaining water beyond what the failing heart can pump. Hyponatremia is a marker of severe disease.
Cirrhosis (advanced liver disease)
Splanchnic vasodilation reduces effective blood volume, driving water retention. A sodium below 130 mmol/L in cirrhosis is a poor prognostic sign.
Adrenal insufficiency (Addison disease)
Cortisol and aldosterone deficiency cause salt wasting. Often combined with low blood pressure, hyperkalaemia, and skin pigmentation.
Hypothyroidism (severe)
Severe untreated hypothyroidism reduces water clearance. Less common than other causes but worth checking TSH.
Excess water intake (psychogenic polydipsia, marathons, MDMA)
Drinking faster than kidneys can excrete (more than 1 L/hour) dilutes blood sodium. Classic in endurance athletes and party drug users.
Gastrointestinal losses (vomiting, diarrhoea)
When losses are replaced with water rather than electrolyte solutions. Common in elderly with gastroenteritis or post-bariatric surgery patients.
Carbamazepine, oxcarbazepine, desmopressin
Anticonvulsants and ADH analogues directly increase water reabsorption. Important to monitor sodium when starting these drugs.
Symptoms by Severity
Symptoms depend on both the depth of hyponatremia and how quickly it developed. A sodium of 125 mmol/L that developed over weeks may cause minimal symptoms; the same level over hours can cause seizures.
Mild
Often no symptoms. Subtle gait instability and falls in elderly. Mildly impaired attention.
Moderate
Headache, nausea, lethargy, irritability, muscle cramps. Reduced concentration. Increased fall risk.
Severe
Vomiting, confusion, drowsiness, slurred speech, ataxia. Seizures may begin.
Critical
Generalised seizures, decreased consciousness, coma, brainstem herniation. Acute drop is medical emergency.
When to See a Doctor — Red Flags
Same-day GP or ED
- New nausea, headache, or confusion with sodium under 130 mmol/L
- Sodium below 125 mmol/L (any symptoms)
- Recent fall, especially in elderly with low sodium
- New diuretic or SSRI started in the last 4 weeks with low sodium
- Persistent vomiting or diarrhoea with low sodium
Within a week
- Mild low sodium (130-134 mmol/L) without symptoms — for cause workup
- Low sodium plus heart failure, liver disease, or known kidney disease
- Endurance athlete with weakness or unusual fatigue post-event
- Long-term diuretic user with new lethargy or confusion
Diagnostic Next Steps
A low sodium result triggers a structured workup. Your GP follows a clinical algorithm to classify the type and find the cause:
Step 1: Confirm the result and assess volume status
Repeat sodium with a complete electrolyte panel including glucose (to rule out pseudo-hyponatremia from hyperglycaemia). Examination for dry mucous membranes (volume low) versus oedema (volume high) versus normal (euvolaemic).
Step 2: Serum osmolality, urine sodium, urine osmolality
The diagnostic gold standard. Low serum osmolality (under 275 mosm/kg) confirms true hyponatremia. Urine sodium above 20 mmol/L with high urine osmolality = SIADH or diuretic. Urine sodium under 20 mmol/L = volume depletion or oedematous state. MBS items 66536 and 66554.
Step 3: Hormone tests
TSH (rule out hypothyroidism). 9am cortisol (rule out adrenal insufficiency). If both are abnormal, an endocrinology referral follows.
Step 4: Imaging if needed
Chest X-ray (rule out lung cancer, pneumonia causing SIADH). CT brain if confused or head injury suspected. Sometimes a CT chest if the chest X-ray is normal but SIADH persists.
How Low Sodium Is Treated
Treatment depends entirely on cause and severity. Critical rule: sodium must never rise faster than 8-10 mmol/L in 24 hours, or a devastating brain injury called osmotic demyelination syndrome (ODS, formerly central pontine myelinolysis) can occur.
Drug-induced (most common)
Stop or switch the offending medication. Thiazide-induced hyponatremia usually corrects within 1-2 weeks of stopping. SSRI-induced hyponatremia may need a different antidepressant class (mirtazapine, agomelatine).
SIADH / dilutional
Fluid restriction to 800-1200 mL/day. Treat the underlying cause (lung infection, drug). Severe persistent cases may need tolvaptan (vaptan, blocks ADH receptor) under specialist care.
Volume depletion
Slow IV normal saline (0.9% sodium chloride) under hospital supervision. Oral rehydration solution (Hydralyte, Gastrolyte) for milder cases. Treat the underlying gut illness.
Severe symptomatic
Hospital admission. Hypertonic (3%) saline by slow infusion targeting a 4-6 mmol/L rise in the first 6 hours, then stop. Frequent sodium monitoring. Always under specialist or ICU supervision.
Common Sodium Myths
Myth: “Low sodium means I should eat more salt”
False in 99% of cases. Hyponatremia is almost always a water problem. Eating more salt rarely fixes it and can worsen heart failure or hypertension. The real fix is correcting fluid balance, stopping a guilty medication, or treating an underlying disease.
Myth: “Drinking eight glasses of water a day is always healthy”
Not for everyone. People on SSRIs, thiazides, with heart failure, cirrhosis, or SIADH must restrict their fluid intake. There is no universal hydration target. Healthy adults can rely on thirst — chronic over-drinking is a leading cause of mild hyponatremia in retirees who religiously drink water.
Myth: “Sports drinks prevent hyponatremia in marathons”
Partially. Sports drinks contain about 20 mmol/L of sodium — far less than blood (135-145 mmol/L). They reduce risk compared to water but cannot fully prevent hyponatremia if you over-drink. Drinking to thirst is the safest strategy in endurance events.
Myth: “My sodium is 134, I must be at risk of seizures”
Mild chronic hyponatremia (130-134 mmol/L) is rarely associated with seizures. The risk increases sharply below 125 mmol/L and especially with rapid drops. Mild stable hyponatremia mostly increases fall risk in elderly — serious but not seizure-level dangerous.
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This guide is based on the European Renal Best Practice (ERBP) and US Hyponatremia Expert Panel guidelines, Royal College of Pathologists of Australasia reference ranges, and Therapeutic Guidelines (eTG complete). SmarterBlood provides educational information only and does not replace personalised advice from a qualified medical practitioner.