Thyroid tumors in dogs account for 1% to 4% of all tumors, and in necropsy studies, 30% to 50% of thyroid tumors are benign adenomas. Thyroid adenomas are usually small, noninvasive, and without clinical signs; not surprisingly, therefore, 90% of thyroid tumors detected clinically are malignant.Thyroid carcinomas in dogs can be large; poorly encapsulated; and extend locally into or around the trachea, esophagus, and muscles of the neck. Additionally, they can be highly vascular and invade local blood vessels. About 25% to 47% of thyroid carcinomas are bilateral. Because of the presence of ectopic thyroid tissue, ectopic thyroid tumors can develop, particularly in dogs. In one study of 544 dogs, 13% had an ectopic tumor:, 5% had a tumor in the sublingual area only, 3% had a tumor at the eutopicsite (cervical area) and cranial mediastinum, 2.6% had a tumor in the cranial mediastinum only, 1.7% had a tumor in the sublingual area and eutopic site, 0.4% had a tumor at the sublingual, cranial mediastinum and eutopic sites, and 0.2% had a tumor atthe sublingual and mediastinum sites.
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