Abstract Introduction/Objective Selpercatinib (Retevmo) is a highly selective rearranged during transfection (RET) kinase inhibitor indicated for treatment of solid tumors with RET gene mutation or fusion, such as medullary thyroid carcinoma or non-small cell lung carcinoma. Among selpercatinib’s side effects, severe hepatotoxicity was only seen in 3% of the patients. Here we report a case of selpercatinib-induced hepatotoxicity, which has not been well characterized before. Methods/Case Report The patient was a 56-year-old female who had medullary thyroid carcinoma with RET M918T mutation (pT3aN1b) status post total thyroidectomy. Recurrence occurred and selpercatinib treatment was initiated. Then the patient complained of mouth sores, tongue sensitivity, and lower extremity cramping. The dosage was reduced to half. The magnetic resonance imaging on day 67 of selpercatinib treatment showed heterogeneous enhancement in the right hepatic lobe, compatible with parenchymal liver disease/hepatitis, trace perihepatic ascites, and mild splenomegaly. On day 71, liver function abnormalities were detected (ALK 315 IU/L, ALT 745 IU/L, AST 899 IU/L, total bilirubin 7.7 mg/dL). Workups including autoimmune hepatitis serology, viral hepatitis, or other infections were all negative. She had no history of other drug use. On day 72, a liver biopsy was performed that demonstrated mild to moderate portal lymphocyte-predominant mixed inflammation with interface activity. The lobules showed marked inflammation with hepatocyte disorganization, acidophil bodies, hepatocyte dropout, and rare lipogranulomas. Bile duct injury and patchy canalicular cholestasis were also noted. Trichrome stain showed focal bridging fibrosis. Overall, the histologic findings were acute and chronic hepatitis pattern injury, favor selpercatinib-induced hepatotoxicity. Selpercatinib was discontinued. All liver enzymes gradually returned to normal levels on day 169. Results (if a Case Study enter NA) NA Conclusion Our study suggests that screening patients for abnormal liver function tests prior to and monitoring periodically during selpercatinib treatment is necessary. Based on the severity of hepatotoxicity, withholding, reducing the dose, or permanently discontinuing selpercatinib is recommended.