OBJECTIVE: The incidence of type 1 diabetes mellitus (T1DM) and hypothyroidism as immune-related adverse events (irAEs) after programmed cell death-1 inhibitor (PD-1i) administration has not yet been sufficiently evaluated in a real clinical setting. To assess the incidence of T1DM and hypothyroidism among PD-1is and to identify the risk factors associated with hypothyroidism using a large claims database. METHODS: This cohort study used the Shizuoka Kokuho database in Japan from 2012 to 2018, including approximately 2.2 million people. We enrolled 695 PD-1i-treated patients. T1DM and hypothyroidism as irAEs were identified using International Classification of Diseases 10th Revision and Anatomical Therapeutic Chemical classification codes. Risk factors for hypothyroidism were explored using the multivariable Fine and Gray regression model after adjusting for age group and sex, treating death as a competing risk. RESULTS: The cumulative incidences of T1DM and hypothyroidism were 0.3% and 8.3%, respectively. We described the detailed onset timing of irAEs in patients with T1DM and hypothyroidism; hypothyroidism was observed evenly within 1 year of the PD-1i prescription. Sex and certain cancer types, such as lung and urothelial cancers, were significantly associated with subdistribution hazard ratio (sHR) (female: sHR, 2.04 [95% CI, 1.20-3.47]; lung cancer: sHR, 0.55 [95% CI, 0.32-0.95]; and urothelial carcinoma: sHR, 2.40 [95% CI, 1.05-5.49]). CONCLUSION: The incidence of T1DM and hypothyroidism as irAEs and associated risk factors identified in this analysis were comparable to those found in previous studies. The use of a large claims database to detect irAEs, such as T1DM and hypothyroidism, may lead to safer use of PD-1is.