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

A 14-year-old spayed female Jindo dog presented with a firm, non-painful right-sided cervical mass. Computed tomography identified three distinct, separate masses thought to be arising from the right thyroid lobe; the largest measured 66.6 mm × 42.0 mm × 37.6 mm with an estimated volume of 56 cm³ and showed invasion into the right internal jugular vein. Multiple hepatic nodules were detected without evidence of pulmonary metastasis and regional lymph node involvement. Right thyroidectomy with resection of the invaded vein and partial liver lobectomy were performed. The histologic results confirmed all three masses as follicular-compact thyroid carcinomas, and the hepatic lesion as metastatic thyroid carcinoma. The dog recovered uneventfully, remained euthyroid, and showed no local recurrence over a 5-month follow-up. In human medicine, multifocality is common in papillary thyroid carcinoma and is associated with a high rate of recurrence. This report documents the first canine case of multifocal thyroid carcinoma, featuring macroscopic vascular invasion and an uncommon metastatic pattern in which the liver was affected in the absence of detectable pulmonary lesion. The presence of multifocal disease within a single canine thyroid lobe necessitates comprehensive cross-sectional imaging, meticulous surgical planning with vascular considerations, and long-term monitoring to optimize the prognosis of this carcinoma.