Adenocarcinoma originating from presumed liver ectopic thyroid in a cat

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.