🧭 When to suspect

Hypokalaemia is a serum potassium < 3.5 mmol/L and is the most common electrolyte abnormality in clinical practice. It is frequently asymptomatic and picked up incidentally on a U&E, but can cause muscle weakness, cramps, fatigue, constipation and – most importantly – cardiac arrhythmias. In primary care the two dominant causes are non-potassium-sparing diuretics (loop diuretics such as furosemide; thiazide/thiazide-like such as bendroflumethiazide or indapamide) and gastrointestinal losses (vomiting, diarrhoea, laxative misuse).

The key skills are to grade severity, decide who can be replaced safely in the community versus who needs admission, identify and treat the cause, and never overlook a patient at high arrhythmia risk (on digoxin, or with heart failure, ischaemic heart disease or left ventricular hypertrophy (LVH)). Other causes include transcellular shift (insulin, beta-2 agonists such as salbutamol, alkalosis, refeeding), mineralocorticoid excess – primary hyperaldosteronism (Conn's syndrome) and Cushing's syndrome – renal tubular acidosis (RTA), the inherited tubulopathies (Bartter and Gitelman syndromes), and coexisting hypomagnesaemia, which both lowers potassium and makes it refractory to replacement.

Serum potassium	Severity	Usual action
3.0–3.4 mmol/L	Mild	Oral replacement in primary care; treat cause; recheck U&E
2.5–2.9 mmol/L	Moderate	Oral replacement plus ECG; recheck early; admit if high arrhythmia risk
< 2.5 mmol/L	Severe	Admit for intravenous replacement and cardiac monitoring

Crucially, severity is clinical as well as biochemical: significant symptoms or ECG changes make any hypokalaemia an emergency, regardless of the number. Consider an endocrine cause – particularly Conn's syndrome – in anyone with hypokalaemia and hypertension, or recurrent unexplained hypokalaemia.

Source: NHS Specialist Pharmacy Service

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