Which electrolyte abnormality increases the risk of digoxin toxicity?

• A. Hyperkalemia
• B. Hyponatremia
• C. Hypokalemia
• D. Hypercalcemia

Answer: (C)

Digoxin works by blocking the Na+/K+-ATPase pump in heart muscle cells. This increases intracellular calcium, boosting cardiac contractility. Potassium and digoxin compete for the same binding site on that pump. When potassium levels are low, there are more unoccupied pumps for digoxin to bind, so the drug’s effect on the pump is stronger. This makes toxicity more likely, with risks of arrhythmias and other symptoms.

Hyperkalemia is more a feature that appears once toxicity has started, as pump failure releases potassium. Hyponatremia or hypercalcemia don’t have as direct a role in increasing digoxin’s binding or toxicity risk as hypokalemia. So the electrolyte abnormality that raises the risk of digoxin toxicity is low potassium.