SAT-481 Pembrolizumab-Induced Thyroiditis with Negative Thyroid Peroxidase Antibody

Introduction: Immune checkpoint inhibitors (ICI) have reformed oncology treatment through its immunomodulatory effect on T-lymphocytes to target metastatic and locally advanced cancers but have been known to produce immune-related adverse events (irAEs). Thyroiditis is a well-documented endocrinopathy occurring in patients receiving ICI; however, a Thyroid Peroxidase Antibody (TPO Ab) negative case of ICI induced thyroiditis suggests that its pathogenesis is independent of antibody mediated thyroid destruction and more associated with an alternative immunoregulatory mechanism. Case Description: A 60-year-old Caucasian, male with a 37-year smoking history and lung adenocarcinoma with metastasis to the brain was referred to Endocrinology clinic for evaluation of suppressed thyroid stimulating hormone (TSH) level. Patient was treated with a four-month course of IV Pembrolizumab every three weeks. TSH was