Potassium

What is Potassium?

Potassium (K⁺) is the predominant positively charged electrolyte inside cells. Around 98% of total body potassium sits intracellularly, which means serum potassium reflects only a narrow outer slice of the total pool. The kidneys regulate it tightly through the hormone aldosterone — excreting excess potassium in urine and retaining it when levels fall. The normal serum range in adults is 3.5–5.0 mmol/L, a narrow window compared with most other blood markers, and the reason even modest deviations deserve clinical attention. Potassium is critical for the cardiac action potential: the electrical conduction of heart cells is exquisitely sensitive to the ratio of potassium inside versus outside the cell. Too little potassium (hypokalaemia) increases the risk of ventricular arrhythmias and prolongs the QT interval; too much (hyperkalaemia) slows conduction and raises the risk of life-threatening bradycardia and ventricular fibrillation. Outside the heart, potassium disturbances manifest as muscle weakness, cramps, fatigue, and — with hypokalaemia — constipation. Hypokalaemia most commonly arises with loop or thiazide diuretics, prolonged diarrhoea or vomiting, inadequate dietary intake, and refeeding syndrome. Hyperkalaemia most often follows renal failure, use of ACE inhibitors, ARBs, potassium-sparing diuretics, or NSAIDs, and adrenal insufficiency.

Why is Potassium relevant?

Potassium matters because even small deviations outside the reference range can be clinically significant — in contrast to markers where only large outliers attract attention. This is why potassium is almost always part of a standard electrolyte or renal panel, especially for people with cardiovascular disease, kidney disease, or medications that affect potassium. The margin for 'normal' is narrower than for most other electrolytes, and the consequences of a deviation can be directly felt — or, at extreme values, directly dangerous. For people on diuretics, regular potassium monitoring is near-universal, because loop and thiazide diuretics actively drive potassium losses. ACE inhibitors and ARBs do the opposite — they raise potassium, particularly with declining kidney function. When these drug classes are combined, the opposing effects can partially cancel out, which makes monitoring even more important rather than less. In exercise and sweating, potassium is lost daily, but a normal diet rich in potassium — bananas, potatoes, tomatoes, legumes — typically replenishes this promptly. Magnesium deficiency is a frequently overlooked cause of refractory hypokalaemia: without sufficient magnesium the kidneys spill potassium in the tubules, making potassium supplementation largely ineffective until magnesium is restored. Refractory hypokalaemia — a potassium level that refuses to rise despite supplementation — is therefore a reason to check magnesium at the same time.

Potassium high or low — what it means

Read potassium alongside sodium, creatinine, and eGFR. Kidney function is the primary determinant of excretion capacity, and a falling eGFR is the classic driver of rising potassium. With an unexpectedly high result, the first step is always to rule out haemolysis: if red blood cells lyse in the tube — from delayed processing, excessive centrifugation, or a technically difficult draw — intracellular potassium leaks and can substantially inflate the measured value. A haemolysed sample usually shows a reddish tinge in the serum; when this is the case, always repeat the measurement. A difficult venepuncture where the patient had to pump their fist can cause the same artefact. A value outside the reference range in an otherwise asymptomatic person, measured under good conditions and confirmed on a second draw, always warrants identifying the cause. A mildly elevated potassium (5.0–5.5 mmol/L) in the context of declining kidney function or a newly started renally active medication calls for follow-up. Values above 6.0 mmol/L, especially with symptoms such as muscle weakness or palpitations, are an indication for prompt clinical assessment. At the lower end, a sustained potassium below 3.5 mmol/L in someone not on diuretics is a reason to investigate loss or intake issues, and to add a magnesium measurement at the same time.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.