Abstract
An 80-year-old woman with a lung nodule in the right lower lobe and pleural thickening was diagnosed with lung adenocarcinoma by a surgical lung biopsy. The oncogene panel test showed a positive RET-fusion gene mutation, and selpercatinib was administered as a first-line treatment. She developed severe hyponatremia and was subsequently diagnosed with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Discontinuing selpercatinib and initiating demethyltetracycline gradually improved SIADH. After confirming that sodium levels had returned to normal, we resumed and continued selpercatinib at a reduced dose without a recurrence of SIADH. This case suggests that selpercatinib could cause SIADH as an adverse event.