Unilateral Multifocal Follicular Thyroid Carcinoma with Vascular Invasion and Primary Hepatic Metastasis in a Dog: First Documented Case

Simple Summary Thyroid cancer in dogs usually appears as a single mass on one or both sides of the neck and can sometimes be found as an ectopic form at the base of tongue or heart. In this case, a 14-year-old female Jindo dog was found to have three separate tumors growing only on the right side of the thyroid gland. The tumors were carefully removed through surgery, along with a nearby neck vein that the tumor had invaded. Some nodules in the liver were also removed and were confirmed to be the first place where the cancer had spread, even though no metastasis was detected in the lungs or regional lymph nodes. The dog recovered well after surgery and did not show any changes in thyroid hormone levels. This is the first known report describing multiple thyroid cancers on only one side of the gland in a dog. The findings from this case may help veterinarians recognize unusual forms of thyroid tumor, choose the best surgical plan, and understand how these tumors can spread in unexpected ways.