Abstract Thyroid tumors represent 1–4% of cancers in both dogs and humans. Most canine tumors are follicular (FTC) or medullary carcinomas (MTC), unlike humans, where only 10–15% are FTC and 2% are MTC, with BRAF/NRAS or RET mutations, respectively. Here, we conduct histological and molecular analyses of canine thyroid tumors. Transcriptionally, elevated ERBB2 expression characterizes FTC tumors, whereas MTC tumors show upregulated RET signaling.