SUN-335 - Thyrotoxic Periodic Paralysis as the Initial Presentation of Graves Disease Complicated by Methimazole-Induced Hepatocellular Injury: A Case Report

Background
Thyrotoxic periodic paralysis (TPP) is a rare complication of hyperthyroidism caused by an intracellular shift of potassium. Methimazole is the first-line treatment for Graves disease but can rarely cause liver injury. The combination of these two conditions is rare and poses potential management challenges.

Case Presentation
A previously healthy 25-year-old Hispanic male presented with acute lower extremity weakness. His work up revealed severe hypokalemia (2.2 mmol/L) and thyrotoxicosis (TSH < 0.097 μIU/mL, free T4 4.38 ng/dL, T3 589 ng/dL, positive TSI and TRAb). He was diagnosed with TPP associated with Graves’ disease. He improved quickly with potassium repletion and started on Methimazole 20 mg daily.

At one-month follow-up, he developed new hepatocellular injury (AST 671 U/L, ALT 367 U/L), with prior normal liver function tests (LFTs) on admission and on two week follow-up. Methimazole was promptly discontinued, and workup for other causes of abnormal LFTs was negative. The presentation was strongly suggestive of drug-induced liver injury (DILI). Radioactive iodine was expediated for definitive treatment of Graves’ disease.

Discussion
TPP is rare in the Hispanic population, with a prevalence of 0.5%. Hepatocellular injury caused by Methimazole is less common than cholestatic injury. There is very limited literature about the coexsitence of TPP and Methimazole-induced hepatocellular injury. We found one abstract discussing video-assisted total thyoidectomy in a patient with TPP, Methimazole-induced hepatotoxicity, and Hepatitis B.

Conclusion
Both TPP and methimazole-induced hepatocellular injury are uncommon but well-documented conditions; however, their coexistence is extremely rare. TPP requires adequate control of hyperthyroidism, while methimazole-induced liver injury requires discontinuation of the medication. Early recognition and transition to definitive therapy are key to safe and effective management.

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