Checkpoint inhibitor (ICI) immunotherapy leverages the body’s own immune system to attack cancer cells but leads to unwanted autoimmune side effects in up to 60% of patients. Such immune related adverse events (IrAE) lead to treatment interruption, permanent organ dysfunction, hospitalization and premature death. Thyroiditis is one of the most common IrAE, but the cause of thyroid IrAE remains unknown. Here we present a novel mouse model in which checkpoint inhibitor therapy leads to multi-organ autoimmune infiltrates and show that activation and infiltration of Type 3 immune cells including IL17A+ RORγt+ CD4+ (T helper 17 or Th17) and gamma delta 17 (γδT17) T cells promote thyroid IrAE development. In parallel, Th17 and γδT17 cells were similarly expanded in cancer patients treated with ICI. Furthermore, antibody-based inhibition of IL-17A, a clinically available therapy, significantly reduced thyroid IrAE development in ICI-treated mice. Finally, combination of IL-17A neutralization with ICI treatment in a mouse tumor model did not reduce ICI anti-tumor efficacy and indeed showed a trend toward enhancement. These studies suggest that targeting Th17 and γδ17 function may reduce IrAE without impairing ICI anti-tumor efficacy and may be a generalizable strategy to address IL17-mediated IrAE.