MON-499 Nivolumab-Induced Hypothyroidism Is Irreversible in Most Patients

Background Thyroid dysfunction caused by the immune checkpoint inhibitor (ICPI) is common, however mild dysthyroidism could occur easily in cancer patients due to other causes. The aim of this study was to investigate the incidence and clinical course of ICPI-induced hypothyroidism requiring thyroid hormone replacement. Patients and methods We analyzed baseline and follow up thyroid function tests of cancer patients treated with nivolumab between March 2016 and March 2019 at Chonnam University Hwasun Hospital retrospectively. Results Among 265 cancer patients treated with nivolumab therapy, six patients were excluded from the study because they were on thyroid hormone replacement therapy before starting nivolumab therapy. Twenty-one patients (8.1%) newly developed thyroid dysfunction during nivolumab therapy and sixteen patients (6.2%) required thyroid hormone replacement therapy due to drug-induced hypothyroidism. Cancer diagnoses included lung cancer (n=7), renal cell carcinoma (n=4), malignant melanoma (n=2), hepatocellular carcinoma (n=2), and esophageal cancer (n=1). Six patients (37.5%) showed thyrotoxic phase prior to overt hypothyroidism and the others (n=10, 62.5%) revealed hypothyroidism without thyrotoxic phase. Most ICPI-induced hypothyroidism was irreversible, only one patient was able to discontinue thyroid hormone replacement after quitting nivolumab therapy. Conclusion A significant number of patients treated with nivolumab developed ICPI-induced hypothyroidism requiring thyroid hormone replacement and its clinical course was irreversible in most patients.