Abstract: We report a case of thyrotoxicosis caused by a PD-1 (programmed death-1) inhibitor. The patient was a 45-year-old female diagnosed with metastatic squamous cell carcinoma. She experienced palpitation, hyperhidrosis and increased appetite 18 days after first PD-1 inhibitor pembrolizumab administration. Laboratory examinations showed elevated free triiodothyronine and free thyroxine, indicating autoimmune thyroiditis caused by PD-1 inhibitor. Her clinical symptoms were relieved afterβ-blocker was used. This case suggested us to strengthen the monitoring of thyroid function and antithyroid antibody levels in patients receiving PD-1 inhibitors, especially in the first 1-2 months of treatment and thus to timely identify and treat thyroid dysfunction.