Radioactive iodine (131I) has previously been reported to prolong survival in dogs with thyroid carcinoma. This study aimed to describe tumour response and progression-free interval (PFI) in dogs with thyroid carcinomas treated with 131I. Secondary aims were to describe overall survival time (OST) and prognostic factors. A bi-institutional retrospective review of records identified 66 dogs with thyroid carcinoma treated with 131I from January 2010 to April 2020. Response was described using RECIST or a subjective response assessment where specific tumour measurements were not available. Forty-eight dogs (72.7%) were treatment naïve and 18 dogs (27.3%) had received prior therapy at the time of 131I treatment. Objective responses were available for 34 dogs and subjective responses for 58 dogs. The overall response rate was 35.3% (four complete and eight partial responses). Improvement of clinical signs was seen in 76.2% of dogs (32/42). Kaplan–Meier-estimated median PFI (95% confidence interval [CI]) was 301 (217–578) days and OST (95% CI) was 564 (421–865) days. Prior therapy was associated with a lower hazard for progression (hazard ratio [HR] 0.260 95% CI 0.123–0.548, p =.0004). Treatment of thyroid carcinoma using 131I can effectively alleviate clinical signs and reduce disease burden in a proportion of dogs.