Methimazole is the preferred first-line treatment for hyperthyroidism because of its efficacy and safety. Rarely, it can cause severe adverse effects such as hepatotoxicity, agranulocytosis, or vasculitis. Thyroid storm, a life-threatening complication of thyrotoxicosis, may be precipitated by infection or drug-induced injury. We describe a case of methimazole-induced hepatotoxicity complicated by impending thyroid storm that required urgent surgical intervention.

A 49-year-old male with newly diagnosed hyperthyroidism was started on methimazole 20–25 mg daily and propranolol. After one week, he developed pruritus, followed by fever, anorexia, and agitation. On admission, he was febrile and hypoxemic, with leukocytosis (17,680/mm³) and markedly elevated aminotransferases: alanine aminotransferase (ALT) 1862 U/L and aspartate aminotransferase (AST) 528 U/L. Thyroid function tests showed suppressed Thyroid stimulating hormone (TSH) with elevated free thyroxine (FT4) (53.62 pmol/L) and free triiodothyronine (FT3) (12.17 pmol/L), consistent with worsening thyrotoxicosis. He was admitted to the intensive care unit with a working diagnosis of methimazole-induced hepatotoxicity, impending thyroid storm, and sepsis. Management included saturated solution of potassium iodide, intravenous hydrocortisone, hepatoprotective agents, and antibiotics. Despite therapy, he developed hypotension, psychotic symptoms, and persistent biochemical thyrotoxicosis. On hospital day 3, he underwent urgent total thyroidectomy. Postoperatively, he developed atrial fibrillation, which was controlled with medical therapy. He stabilized and was discharged on levothyroxine. At follow-up, ALT and AST normalized to 44 U/L and 28 U/L, respectively.

Methimazole-induced hepatotoxicity is rare but potentially fatal. It often manifests with cholestatic or mixed patterns, but in this case presented as severe hepatocellular injury with markedly elevated aminotransferases. Concurrent infection likely contributed to impending thyroid storm. While medical therapy with corticosteroids, iodine, and beta-blockers is standard, urgent thyroidectomy is indicated when drug-induced hepatotoxicity precludes antithyroid therapy and medical stabilization fails.

This case highlights the importance of vigilance for hepatotoxicity in patients treated with methimazole. Marked elevations in aminotransferases, coupled with thyrotoxic crisis, necessitate prompt discontinuation of methimazole and consideration of early surgical intervention. Multidisciplinary and timely management can be lifesaving in severe hyperthyroidism.