Malignant thyroid carcinomas are relatively common in dogs. The majority of tumors are unilateral and nonfunctional. Before deciding on treatment options, it is important to determine whether the tumor is freely moveable or fixed and invasive into adjacent tissues. Thyroidectomy is recommended for unilateral, mobile thyroid carcinomas. Radiation therapy or radioactive iodine therapy are recommended for dogs with invasive or bilateral thyroid carcinomas. The role of adjunctive chemotherapy is poorly defined, but should be considered in dogs with high-risk tumors, such as large or bilateral thyroid carcinomas. The prognosis is good following surgical treatment of mobile thyroid tumors and irradiation of fixed thyroid carcinomas, with median survival times greater than 3 years.