Abstract
CASE SUMMARY: Ectopic thyroid tissue is rarely reported in dogs and cats in its prediaphragmatic location and has never been described in the liver. A 15-year-old spayed female domestic shorthair cat was diagnosed by ultrasound with a heterogeneous hypoechoic nodular area in the liver at the periphery of the quadrate lobe. A generic diagnosis of carcinoma was made after ultrasound-guided fine-needle aspiration and cytological examination. The patient underwent staging by CT scan and subsequently underwent hepatic lobectomy. Histologically, a diagnosis of thyroid adenocarcinoma was made, confirmed immunohistochemically using positive thyroglobulin staining; the tumour was suspected to be of metastatic origin. CT scans excluded primary thyroid involvement; in addition, lesions at other sites were not detected. Therefore, a final diagnosis of thyroid adenocarcinoma arising from ectopic thyroid tissue in the liver was made. The cat recovered uneventfully from surgery. RELEVANCE AND NOVEL INFORMATION: This report describes an unusual case of an adenocarcinoma originating from presumed thyroid ectopic tissue within the liver of a cat. Ectopic thyroid tissue has been rarely reported in both dogs and cats and, to the authors' knowledge, it has never been described in the liver of a cat.