Abstract : An adult beagle dog was presented with a cervical mass detected by palpation and computed tomography.Fine needle aspirates revealed numerous epithelial cells with plasmacytoid appearance and frequent naked nuclei.Histologically, the mass consists of multiple packets of neoplastic cells and extensive areas of necrosis and fibrosis.Neoplastic cells were also found in submandibular lymph nodes. Immunohistochemistry showed that neoplastic cellswere positive for calcitonin and negative for thyroglobulin. Based on these findings, the cervical mass was diagnosedas thyroid C-cell carcinoma. Almost one year after the surgical excision, the dog remains healthy without any symptomof recurrence or metastasis. Keywords : calcitonin, C-cell carcinoma, dog, metastasis, thyroid Tumors of thyroid gland are relatively common andaccounts for 1.2 to 3.8% of all canine tumors [9]. In dogsmalignant thyroid tumors are more common than benigntumors and can arise from either follicular or parafollicularcell lineages [6]. Generally, canine thyroid C-cell carcino-mas are more encapsulated and less invasive than thyroidadenocarcinomas. Compared to thyroid adenocarcinomas,canine thyroid C-cell carcinomas carry a more favorableprognosis. Because prognosis of thyroid tumors in the dogs isdifferent for each subtype, determination of their subtype isimportant [5]. The present report describes the clinical, mor-phologic and immunohistochemical features of a thyroid C-cell carcinoma occurred in a beagle dog.An 11-year-old female beagle dog weighing 9.8 kg waspresented to the Irion Animal Hospital, Seoul, South Koreafor several-year history of intermittent facial nerve paralysis.On physical exam, the dog was overweight with a body scoreof 4/5. Palpation revealed a mass on neck region. Computedtomography (CT) was performed in order to characterize theanatomic location of the cervical mass in association withsurrounding tissue. CT revealed a heterogeneous, peripher-ally contrast-enhanced, ellipsoidal and lobulated mass, 42.3mm × 13.7 mm × 10.0 mm in size, adjacent to the commoncarotid artery and the trachea (Fig. 1). Radiography showedbilateral degenerative arthritis in stifle joints. Complete bloodcounts, serum chemistry, and urinalysis were carried out andthe results were unremarkable except calcium level in theupper margin (11.8 mg/dL, reference interval: 7.9~12.0 mg/dL). Total T4, free T4 and thyroid stimulating hormone lev-