Feline hyperthyroidism can be treated medically, surgically, or with radioactive iodine. In this case study, a 9-year-old spayed cat presented to a primary care veterinarian. Emaciated and tachycardic, it had a large palpable goiter on the right side of its neck. It was aggressive, and blood sampling was not possible. The cat was treated with 15 mg of carbimazole once daily for presumptive hyperthyroidism with a plan to evaluate suitability for radioactive iodine treatment. On day 72, the cat presented with acute onset of left pelvic-limb lameness. On day 77, necrosis of the tail tip was reported and carbimazole was discontinued. By day 83, the lameness had progressed and the left digits II to IV were firm, cold, and discolored with deep pain absent. There was no sensation in the distal part of the tail. Although ultrasonography of the kidneys on day 70 was unremarkable, repeat examination revealed multiple renal infarcts. Mid-femoral amputation and tail amputation were performed. Histological examination of the tissue revealed the vascular lumens partially to fully occluded by mature fibrin thrombi; vasculitis was present. The cat recovered and was eventually treated with radioactive iodine.