A 13 year old male neutered Domestic shorthair cat was referred to the Cornell University Hospital for Animals for treatment chronic hyperthyroidism. For several years, treatment with methimazole was marginally successful since thyroxine levels (T4) remained above normal limits and weight loss was still observed. Radioactive iodine (131I 6 millicuries) was given, but the subsequent total T4 did not normalize, and a second treatment (4 millicuries) was administered. The total T4 remained elevated after the second dose. Treatment failure was hypothesized due to radioiodine resistant tissue in the form of thyroid adenocarcinoma. Bilateral thyroidectomy was then performed and histopathologic examination revealed benign thyroid hyperplasia (right) and adenocarcinoma (left). The cat endured several post-operative complications following surgery that required intense medical management before discharge. This paper will discuss the pathophysiology of hyperthyroidism, clinical signs, and complications uncommonly encountered after treatment with radioiodine.