Sodium Blood Test: High and Low Sodium Levels Explained
Sodium is the primary electrolyte controlling fluid balance in your body. Both too little and too much carry serious risks — yet most people with sodium abnormalities have no obvious symptoms until the level is severely out of range.
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Sodium is the most abundant electrolyte in your extracellular fluid — the fluid outside your cells. It governs osmotic pressure, meaning it determines how much water stays in the bloodstream versus moves into tissues. Your body tightly regulates serum sodium within a narrow window through a system involving the kidneys, the hormone ADH (antidiuretic hormone), and the thirst mechanism.
Sodium also enables nerve impulse conduction and muscle contraction — including the heart. Unlike dietary sodium (table salt), which affects blood pressure over time, serum sodium on a blood test reflects your body's current fluid regulation, not simply how much salt you ate that day.
Reference Ranges at a Glance
| Category | Sodium (mEq/L) | Clinical Significance |
|---|---|---|
| Critically low | Below 125 | Severe hyponatraemia — seizures, coma risk. Urgent medical care. |
| Low (hyponatraemia) | 125 – 136 | Fatigue, nausea, confusion; investigate and treat underlying cause |
| Optimal | 136 – 142 | Well-regulated fluid balance |
| Normal (lab range) | 136 – 145 | Standard reference range |
| High (hypernatraemia) | 145 – 155 | Usually dehydration; investigate and rehydrate appropriately |
| Critically high | Above 155 | Severe dehydration or hormonal disorder — urgent evaluation |
Emergency
Investigate
Urgent
Low Sodium (Hyponatraemia): Causes
Hyponatraemia — serum sodium below 136 mEq/L — is the most common electrolyte abnormality seen in hospital blood tests and is frequently encountered in outpatient panels too. It rarely means you are not eating enough salt. It almost always reflects a problem with water regulation.
Diuretics (especially thiazides) — the most common medication cause. Thiazide diuretics (hydrochlorothiazide, indapamide) impair the kidney's ability to excrete free water, diluting sodium. Older adults on these drugs are at highest risk.
SIADH (Syndrome of Inappropriate ADH) — the body retains too much water due to inappropriate ADH release. Causes include lung disease (pneumonia, COPD), brain disorders, certain medications (SSRIs, carbamazepine), and many cancers.
Heart failure, liver cirrhosis, kidney disease — in these conditions, the body senses inadequate blood flow and releases ADH to retain water, diluting sodium even as total body sodium may be normal or high.
Overhydration (psychogenic polydipsia) — drinking excessive water can overwhelm the kidney's ability to excrete free water, particularly in psychiatric conditions or endurance events.
Hypothyroidism and adrenal insufficiency — both hormonal conditions can impair free water excretion and cause persistent mild hyponatraemia.
Symptoms of Low Sodium
Important: Correction Speed Matters
Chronic hyponatraemia that has developed slowly (over days to weeks) must be corrected slowly. Correcting sodium too rapidly can cause osmotic demyelination syndrome — severe neurological damage. Treatment of sodium abnormalities should always be managed by a healthcare provider; this is not a condition to self-treat.
High Sodium (Hypernatraemia): Causes
Hypernatraemia — sodium above 145 mEq/L — nearly always reflects insufficient water intake or excessive water loss. Unlike hyponatraemia, which can have complex hormonal causes, hypernatraemia is most often a simpler problem: not enough water in relative to how much is being lost.
Dehydration — the most common cause. Inadequate fluid intake, particularly in elderly or unwell people who may have a reduced thirst sensation. Also common during fever, burns, or heavy sweating.
Diabetes insipidus — a condition where the kidneys cannot concentrate urine, causing massive water loss and rising sodium. Central (brain ADH deficit) or nephrogenic (kidney unresponsive to ADH) — both raise sodium.
Diarrhoea or vomiting — GI losses contain more water than sodium, concentrating the blood. Particularly dangerous in infants and older adults.
Osmotic diuresis — very high blood glucose in uncontrolled diabetes pulls water into urine, concentrating sodium. A high sodium alongside very high glucose can indicate a hyperosmolar emergency.
When Sodium Abnormalities Are Emergencies
Sodium below 120 mEq/L or above 160 mEq/L can be life-threatening and requires urgent hospital evaluation. Similarly, sodium that has changed rapidly (even within the normal-to-abnormal range) is more dangerous than a chronic mild abnormality at the same level.
Sodium and the Bigger Picture
Serum sodium is most informative when read alongside other electrolytes — particularly potassium, chloride, and bicarbonate — and together with kidney function markers (creatinine, eGFR). Your doctor will also consider your medication list, symptoms, fluid balance, and urine sodium before drawing any conclusions. A single mildly abnormal sodium value on an otherwise normal panel rarely represents serious disease, but it always warrants a follow-up retest and clinical review.
Supporting Healthy Fluid Balance
Emergency note: If you are experiencing symptoms of a medical emergency, call emergency services (911 in the US / 999 in the UK / 112 in the EU) immediately. Do not rely on this website in an emergency situation.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Reference ranges, supplement dosages, and nutritional information mentioned are general educational guidance from published research—not personalised recommendations. Do not use this content to self-diagnose or self-treat any condition. Always consult a qualified healthcare provider before making any changes to your health regimen, medications, or supplements.
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