Conditions · Metabolism (biochemistry)
Hypokalaemia !
Blood potassium is low, usually from diuretics, vomiting or diarrhoea; it causes weakness and arrhythmias.
Updated 2026-11-01
Structures involved
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In embryology
In physiology
- How arrhythmias arise
- The ECG in potassium and calcium disorders
- The resting membrane potential
- The Nernst and Goldman equations
- Primary and secondary active transport
- Aldosterone: actions and control
- Islets and insulin: secretion and actions
- Water and electrolyte absorption and how diarrhea happens
- Vomiting and its control
- Prolonged fasting and starvation
- Refractory periods and excitability
- Transport in the loop of Henle
- Distal tubule and collecting duct
- Potassium balance
- Reasoning through fluid and electrolyte imbalances
- Where and how diuretics act
- Renal handling of H⁺ and bicarbonate
- Metabolic alkalosis and its compensation
- Renal handling of calcium, phosphate and magnesium
Why it happens
What it is
A low serum potassium from renal or gut losses, a shift of potassium into cells or, rarely, poor intake.
Who gets it
It is common in hospital patients and people taking diuretics; acute diarrhoea often causes it in Latin American children.
Causes
- Renal loss: loop and thiazide diuretics, hyperaldosteronism, Cushing, Bartter and Gitelman syndromes, RTA types 1 and 2, hypomagnesaemia, amphotericin B
- Gut loss: diarrhoea, laxatives, fistulas, villous adenoma; vomiting acts mainly through renal loss secondary to alkalosis
- Shift into cells: insulin, β2 agonists (salbutamol), alkalosis, refeeding, hypokalaemic periodic paralysis (thyrotoxic or familial)
- Very low intake: anorexia, alcoholism
Risk factors
- Diuretic use
- Prolonged vomiting or diarrhoea
- Insulin treatment of ketoacidosis
- Laxative misuse or eating disorders
- Hypertension with aldosteronism
Pathophysiology
98 % of potassium is inside cells, held there by Na⁺/K⁺-ATPase; the gradient sets the resting potential. With low extracellular potassium the membrane hyperpolarises and repolarisation is prolonged: muscle weakness, ileus, cramps, rhabdomyolysis and, in the heart, U waves, flattened T waves, long QT and arrhythmias (more so with digoxin). In the kidney, hypokalaemia increases ammonium production and bicarbonate reabsorption (sustaining alkalosis) and causes ADH resistance (polyuria). Hypomagnesaemia increases renal potassium loss through ROMK channels, so it must be corrected.
Classification
- By mechanism: shift, renal loss (high urine potassium), extrarenal loss (low urine potassium)
- By severity: mild, moderate, severe by the laboratory and symptoms
Symptoms and signs
- Muscle weakness and fatigue
- Cramps and muscle pain
- Constipation or ileus
- Palpitations
- Polyuria
- Paralysis when severe
Diagnosis
- Clinical
- Review drugs (diuretics, insulin, salbutamol, laxatives), gut losses and blood pressure.
- Tests
- Serum potassium, magnesium, arterial gases (alkalosis or acidosis), urine potassium or urine potassium/creatinine ratio, renin and aldosterone if hypertensive, ECG.
- Imaging
- Adrenal CT if primary hyperaldosteronism is confirmed.
Treatment
- Medical
- Oral potassium replacement when mild or moderate without severe symptoms
- Medical
- Intravenous potassium with cardiac monitoring when severe or with arrhythmias, within each protocol's rate limits
- Medical
- Correct magnesium and treat the cause (adjust diuretics, potassium-sparing agents, treat aldosteronism)
- Follow-up
- Serial potassium and ECG checks
Complications
- Ventricular arrhythmias
- Paralysis and respiratory failure
- Rhabdomyolysis
- Paralytic ileus
- Chronic hypokalaemic nephropathy
Prognosis
Good once the cause is corrected; severe hypokalaemia in heart disease raises the risk of sudden death.
Prevention
- Monitoring potassium on diuretics
- Replacing potassium in ketoacidosis before insulin drives it down
- Replacing fluids and electrolytes in diarrhoea
Test yourself
Think of the answer, then tap the question to see it.
Which ECG findings does hypokalaemia produce?
Flattened T waves, U waves, ST depression and long QT, with a risk of arrhythmias.
Why does insulin lower potassium?
It activates Na⁺/K⁺-ATPase and drives potassium into cells.
Why must magnesium be corrected in hypokalaemia?
Hypomagnesaemia increases renal potassium secretion and makes hypokalaemia resistant to replacement.
How does vomiting cause hypokalaemia when gastric juice holds little potassium?
Through alkalosis and volume depletion, which activate aldosterone and increase renal potassium loss.
Related conditions
- Metabolic alkalosis
- Refeeding syndrome
- Hyperkalaemia
- Acute (viral) gastroenteritis
- Diabetic ketoacidosis
- Cushing syndrome
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Low blood potassium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Potassium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Magnesium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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