Which electrolyte disturbance is NOT characteristic of digoxin toxicity?

• A. Hypomagnesemia
• B. Hypokalemia
• C. Hypocalcemia
• D. Hypermagnesemia

Answer: (C)

Digoxin toxicity stems from inhibition of the Na+/K+-ATPase pump, which raises intracellular sodium and, via the Na+/Ca2+ exchanger, increases intracellular calcium. That rise in calcium drives the strong inotropic effect but also raises the risk of arrhythmias.

Because of this mechanism, certain electrolyte patterns commonly accompany toxicity. Low potassium makes digoxin bind more readily to the pump, so hypokalemia increases toxicity risk. Low magnesium also predisposes to toxicity, since magnesium helps modulate the pump’s sensitivity. In more severe cases, the pump blockade lets potassium shift out of cells, leading to hyperkalemia, which signals a poorer prognosis.

Calcium dynamics, however, differ: the drug’s effect relies on increased intracellular calcium, so serum calcium is usually normal or elevated rather than reduced. Therefore, low calcium is not a typical feature of digoxin toxicity, making it the best answer.