Successful Long-Term Survival Following Thyroid Storm Induced by Radioactive Iodine Therapy in a Dog with Thyroid Carcinoma

An 8-year-old castrated male Pomeranian with a non-resectable functional thyroid carcinoma and concurrent myxomatous mitral valve disease was referred for radioactive-iodine therapy. Due to clinical thyrotoxicosis at referral and concurrent cardiac disease, the radioiodine dose was selected conservatively at the lower end of the reported therapeutic range. Despite a conservative radioactive iodine dose, the dog developed acute thyrotoxic decompensation consistent with thyroid storm (manifesting as anxiety, diarrhea, hyperthermia, hypersalivation, and marked tachycardia) within hours of treatment. Propranolol and butorphanol administration led to rapid clinical stabilization. Before the second radioactive iodine therapy, methimazole and propranolol were used for subsequent management, effectively controlling thyrotoxicosis risk and enabling a higher radioiodine dose. Serum thyroxine normalized within 1 month after the second treatment, and the dog maintained complete clinical remission thereafter. Radioactive iodine therapy served as definitive therapy to prevent recurrent life-threatening thyrotoxicosis, resulting in a euthyroid state and long-term survival. This case describes the first documented case of a dog with thyroid carcinoma developing probable thyroid storm associated with radioiodine treatment and subsequently achieving a favorable prognosis.