Osimertinib-induced Syndrome of Inappropriate Secretion of Antidiuretic Hormone

Clinical Practice Points • We describe a case of syndrome of inappropriate secretion of antidiuretic hormone (SIADH) in a patient treated with osimertinib. • A 75-year-old woman was diagnosed with stage IV lung adenocarcinoma (T2aN0M1a) harboring an exon-19 deletion of epidermal growth factor receptor and was treated with osimertinib. After 14 days, she developed headache and nausea. The laboratory results fulfilled the diagnostic criteria of SIADH. • Osimertinib was discontinued, and the patient was treated with sodium intake and fluid restriction. The serum sodium level improved within 1 week. • After discontinuation of osimertinib treatment, the patient was started on gefitinib. She has been able to continue gefitinib treatment without development of hyponatremia.