Most people understand electrolytes through sports drink marketing. Gatorade teaches you they matter for performance. What it doesn't teach you is that electrolyte imbalances can affect your heart rhythm, nerve function, muscle contraction, and in severe cases, consciousness.
Electrolytes are minerals dissolved in your blood and body fluids that carry an electrical charge. They're involved in virtually every physiological process that uses electrical signalling, which is most of them. This article covers what electrolytes are, when testing is useful, and how to interpret results.
A note before we get into it
General information only. I'm not an emergency physician or a nephrologist. Severe electrolyte imbalances can be medical emergencies. If you have symptoms of significant electrolyte disturbance (confusion, cardiac irregularities, severe weakness, seizures), seek medical attention immediately.
This article covers the routine monitoring context: what electrolytes are, when testing is indicated, and how to interpret results as part of a broader health picture.
What electrolytes actually do
Nerve conduction. Every nerve impulse in your body, from brain signals to muscle commands to sensory feedback, depends on the movement of sodium and potassium across cell membranes. Without the right balance, nerve signals misfire, slow down, or stop.
Muscle contraction. Your muscles (including your heart) contract in response to electrical signals carried by electrolytes. Potassium and calcium are particularly critical. Abnormal levels can cause anything from cramps to cardiac arrhythmias.
Heart rhythm. Your heart is an electrical organ. Its rhythm depends on precise electrolyte balance, particularly potassium and sodium. Severe imbalances can cause life-threatening arrhythmias.
Fluid balance. Sodium is the primary regulator of fluid distribution between your bloodstream and your tissues. Too little sodium and fluid shifts into tissues (oedema). Too much and blood pressure rises.
Acid-base balance. Bicarbonate is the body's primary buffer against acidity. The kidneys regulate bicarbonate levels to maintain blood pH within an extremely narrow range. Disruption affects every enzyme and protein in the body.
The individual players
Sodium
Normal range: 135-145 mmol/L
Sodium regulates fluid balance, blood pressure, and nerve/muscle function. It's the most abundant electrolyte in extracellular fluid.
Low sodium (hyponatraemia)
More common than most people realise, particularly in older adults, athletes, and people on certain medications. Symptoms range from mild (headache, nausea, confusion) to severe (seizures, coma). Causes include excessive water intake (dilutional), diuretic medications, kidney disease, heart failure, liver disease, and SIADH (syndrome of inappropriate antidiuretic hormone).
High sodium (hypernatraemia)
Usually caused by dehydration or inadequate water intake. Symptoms include thirst, confusion, and in severe cases, neurological dysfunction.
Potassium
Normal range: 3.5-5.0 mmol/L
Critical for heart rhythm, muscle contraction, and nerve function. The most important intracellular electrolyte.
Low potassium (hypokalaemia)
Can cause muscle weakness, cramps, constipation, and cardiac arrhythmias. Most commonly caused by diuretic medications, vomiting, diarrhoea, and inadequate dietary intake. ECG changes appear at lower levels, including T-wave flattening and U-waves.
High potassium (hyperkalaemia)
A medical concern because it directly affects cardiac conduction. Can cause tingling, muscle weakness, and life-threatening arrhythmias. Common causes include kidney disease (reduced excretion), ACE inhibitors/ARBs, potassium-sparing diuretics, and excessive supplementation. ECG changes (peaked T-waves) are an early warning sign.
Chloride
Normal range: 98-106 mmol/L
Works alongside sodium to maintain fluid balance and acid-base equilibrium. Chloride shifts usually mirror sodium shifts.
Clinical relevance
Chloride abnormalities are rarely the primary finding. They typically accompany sodium or acid-base disturbances and help characterise the underlying cause.
Bicarbonate
Normal range: 22-32 mmol/L
Your body's primary acid-base buffer. The kidneys regulate bicarbonate to keep blood pH between 7.35 and 7.45.
Low bicarbonate (metabolic acidosis)
Can indicate kidney disease, diabetic ketoacidosis, severe diarrhoea, or lactic acidosis. Symptoms include rapid breathing, confusion, and fatigue.
High bicarbonate (metabolic alkalosis)
Can result from prolonged vomiting, excessive antacid use, or diuretic therapy.
Urea and Creatinine
Included as kidney function markers
These are included in the EUC panel as kidney function markers rather than electrolytes proper.
Urea rises when kidney filtration declines or with dehydration, high protein intake, or GI bleeding.
Creatinine is the basis for eGFR calculation, the primary kidney function measure. Covered in detail in the kidney function article.
Why electrolyte imbalances happen
Most healthy people with normal kidney function and a varied diet maintain electrolyte balance without thinking about it. Imbalances typically arise when something disrupts the input-output equilibrium.
Fluid loss. Sweating, vomiting, diarrhoea, and burns all deplete electrolytes. Replacing fluid with water alone (without electrolytes) dilutes remaining electrolytes, the classic athlete hyponatraemia scenario.
Kidney dysfunction. The kidneys are the primary regulators of electrolyte balance. When kidney function declines, the ability to excrete potassium, retain sodium, and maintain acid-base balance is impaired. See the kidney function article.
Medications. A major and frequently underappreciated cause. See the dedicated section below.
Chronic illness. Heart failure, liver cirrhosis, adrenal insufficiency, and diabetes can all disrupt electrolyte balance through various mechanisms.
Dietary extremes. Very low sodium diets, excessive water intake, prolonged fasting, or eating disorders can produce electrolyte disturbances.
Alcohol. Chronic heavy drinking affects electrolyte balance through poor dietary intake, vomiting, liver dysfunction, and direct effects on kidney handling of magnesium and potassium.
The athlete angle: sweat, hydration, and performance
Exercise-associated hyponatraemia is the most common electrolyte scenario in otherwise healthy people.
What happens: During prolonged exercise (particularly in heat), you lose both water and sodium through sweat. If you replace the water but not the sodium by drinking large volumes of plain water, blood sodium drops. This is dilutional hyponatraemia, and it can range from mild (headache, nausea, confusion) to severe (seizures, coma, documented in marathon runners and military recruits).
Who's at risk: Endurance athletes, people training in heat, people who drink excessive water during exercise, and people taking NSAIDs (which can impair kidney water excretion).
Prevention: Replace electrolytes during prolonged exercise through electrolyte drinks, salt capsules, or sodium-containing foods. Don't over-hydrate with plain water. Drink to thirst rather than forcing fluid intake on a rigid schedule.
Potassium and magnesium are also lost in sweat, though in smaller quantities than sodium. Chronic depletion through sustained heavy training can contribute to cramping, fatigue, and impaired recovery. Magnesium is discussed in the athletes article.
Medications that shift electrolytes
Medication-related electrolyte disturbances are extremely common and frequently under-monitored.
Thiazide diuretics (hydrochlorothiazide, indapamide): Commonly prescribed for hypertension. Can cause low sodium, low potassium, and low magnesium. Regular electrolyte monitoring is standard practice when prescribed.
Loop diuretics (furosemide, bumetanide): Used for heart failure and fluid overload. Potent sodium and potassium wasters. Require close electrolyte monitoring.
ACE inhibitors and ARBs (ramipril, perindopril, irbesartan, candesartan): Can raise potassium by reducing its kidney excretion. Usually mild, but clinically important in combination with kidney disease or potassium-sparing diuretics.
Potassium-sparing diuretics (spironolactone, amiloride): Raise potassium by design. Require monitoring, particularly in combination with ACE inhibitors or kidney disease.
Laxatives (chronic use): Can deplete potassium and sodium through GI losses.
Corticosteroids (prednisone, dexamethasone): Can raise sodium and lower potassium through mineralocorticoid effect.
Lithium: Affects kidney sodium handling. Requires regular electrolyte and kidney monitoring.
If you're on any of these medications, regular EUC testing isn't optional. It's part of safe prescribing.
Who should be testing
Anyone on diuretics, ACE inhibitors, ARBs, or lithium. Medication-related monitoring, typically every 3-12 months depending on the drug and clinical context.
Anyone with kidney disease. Electrolyte disturbances become increasingly common as kidney function declines.
Athletes training intensively in heat. Particularly endurance athletes and those with a history of cramping or exercise-associated symptoms.
Older adults. Hyponatraemia is common in older adults, often related to medications, reduced kidney function, or reduced thirst sensation.
Anyone with chronic vomiting, diarrhoea, or GI illness. Fluid and electrolyte losses from the GI tract can be significant.
Anyone with heart failure or liver disease. Both conditions disrupt fluid and electrolyte balance.
Anyone over 40 who's never had electrolytes checked. A baseline EUC alongside kidney function is reasonable.
How to prepare
No specific fasting required for EUC, though if combined with other tests (lipids, glucose), fasting may be needed for those.
Stay normally hydrated. Don't over-hydrate or restrict fluids before the test.
Continue medications unless your clinician advises otherwise. The purpose of testing is often to see how medications are affecting your electrolytes.
Mention all supplements, particularly potassium supplements, salt tablets, or magnesium.
Understanding your results
The EUC panel is interpreted as a pattern, with individual values in context with each other and with kidney function.
All within range: Normal electrolyte balance. Kidneys are regulating effectively.
Low sodium (below 135): Evaluate cause: medications, fluid intake, kidney function, hormonal (SIADH, cortisol). Mild hyponatraemia is common; severe hyponatraemia is urgent.
Low potassium (below 3.5): Check medications (diuretics), GI losses, dietary intake. Even mild hypokalaemia warrants attention because of cardiac implications.
High potassium (above 5.0): Check kidney function, medications (ACE inhibitors, ARBs, spironolactone). Confirm with repeat testing. Haemolysed samples (where red blood cells rupture during collection) can falsely elevate potassium.
Elevated urea and creatinine with electrolyte disturbances: Suggests kidney involvement. Further investigation warranted. See the kidney function article.
Tests to consider through Bloody Good
The electrolyte panel
EUC Blood Test (Electrolytes, Urea & Creatinine) — the complete panel: sodium, potassium, chloride, bicarbonate, urea, creatinine, eGFR.
Combined panels
Electrolytes and Liver Function Test (ELFT) — EUC plus full liver panel in one test.
Additional context
Magnesium Blood Test — not included in standard EUC; important for cramping, fatigue, and cardiac function.
Calcium Blood Test — closely linked to electrolyte and kidney function.
Full Blood Count (FBC) — baseline health context.
Comprehensive coverage
The Bloody Good Test includes electrolytes alongside 100 biomarkers covering kidney, liver, metabolic, hormonal, and nutritional markers. For a complete baseline that includes EUC in context, this is the most efficient option.
What to do after testing
If electrolytes are normal: Reassuring. Your kidneys are regulating effectively and your current fluid, dietary, and medication status is maintaining balance. Retest per your clinician's recommendation, typically every 6-12 months if on relevant medications.
If sodium is mildly low: Review fluid intake (are you over-hydrating?), medications (diuretics?), and dietary sodium. Your GP may adjust medications or recommend dietary changes. Retest in 4-6 weeks.
If potassium is low: Your GP will assess the cause, most commonly medications. Dietary potassium increase (bananas, avocados, potatoes, spinach) or potassium supplementation may be recommended. If on a diuretic, your GP may add a potassium-sparing agent or adjust the dose.
If potassium is high: Confirm with repeat testing (exclude haemolysis). If confirmed, medication review is essential. ACE inhibitors, ARBs, and potassium-sparing diuretics are the usual suspects. Kidney function must be checked. Dietary potassium restriction may be advised.
If multiple electrolytes are abnormal: Suggests a systemic issue: kidney disease, hormonal disorder (adrenal insufficiency), or severe GI losses. Your GP will investigate further.
For athletes: If your episode was exercise-related, the fix is usually straightforward. Electrolyte replacement during prolonged exercise, drinking to thirst rather than forcing fluids, and ensuring adequate dietary sodium. Retest after adjusting your hydration strategy.
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General information only. This article is not medical advice and is not a substitute for care from a qualified health professional. If you have concerning symptoms or urgent health issues, seek medical attention promptly.