Does Methimazole Cause Hyperkalemia?
No, methimazole does not cause hyperkalemia. Methimazole is not listed among the medications known to cause hyperkalemia in any major guidelines or comprehensive reviews of drug-induced hyperkalemia.
Evidence from Hyperkalemia Guidelines
The most authoritative sources on drug-induced hyperkalemia—including the European Society of Cardiology expert consensus document 1, Mayo Clinic Proceedings guidelines 1, and comprehensive reviews of medications causing hyperkalemia 2, 3, 4—provide extensive lists of culprit medications. None of these sources identify methimazole as a cause of hyperkalemia 1, 2, 3, 4.
Medications That Actually Cause Hyperkalemia
The established drug classes that cause hyperkalemia include 1, 2:
- RAAS inhibitors: ACE inhibitors, ARBs, mineralocorticoid receptor antagonists, direct renin inhibitors
- Potassium-sparing diuretics: Spironolactone, amiloride, triamterene
- Immunosuppressants: Calcineurin inhibitors (cyclosporine, tacrolimus)
- Antimicrobials: Trimethoprim-sulfamethoxazole, pentamidine, high-dose penicillin G 1, 2
- Other agents: NSAIDs, heparin, digitalis, beta-blockers 1, 3
Methimazole and Potassium: The Actual Relationship
The relationship between methimazole and potassium is the opposite of what the question suggests:
- Hyperthyroidism causes hypokalemia, not hyperkalemia, through increased Na-K-ATPase pump activity that drives potassium into cells 5
- Methimazole treats hyperthyroidism and therefore helps normalize potassium levels that were previously low 5, 6
- Studies combining methimazole with potassium-containing compounds (like potassium iodide or potassium bromide) use these combinations to treat hyperthyroidism more rapidly—the potassium component serves as an iodine or bromide carrier, not because methimazole depletes potassium 7, 6
Clinical Caveat
When treating hyperthyroid patients with methimazole and potassium supplementation (for concurrent hypokalemic periodic paralysis), clinicians must monitor for rebound hyperkalemia as the hyperthyroid state resolves and potassium shifts back out of cells 5. This is a consequence of correcting the underlying thyroid disorder, not a direct effect of methimazole itself.