A 44-year-old man with a 1-month history of fever visited the department of infectious disease. His temperature was 38.4°C and non-movable, non-tender mass with firm consistency was palpated on right thyroid gland. Laboratory tests showed elevated levels of C-reactive protein (15.6 mg/dL) and erythrocyte sedimentation rate (85 mm/hr), but normal range of white blood cell count (9,750/mm³), thyroid stimulation hormone (1.67 mIU/L), free thyroxine (12.28 pmol/L), and thyroglobulin (5.30 ng/mL). Blood and urine cultures were sterile. Positron emission tomography-computed tomography showed a high fluorodeoxyglucose uptake on the lower pole of the right thyroid gland (Fig. 1A). Thyroid ultrasound revealed 3.3 × 2.5 × 2.3 cm sized hypoechoic, heterogeneous mass with irregular margin on right lobe (Fig. 1B) and its cytology contained some atypical cell clusters with prominent nucleoli. Within core needle biopsy, the solid nests of polygonal cells were separated by fibrous bands with many lymphocytes infiltration (Fig. 2A). The tumor cells had either ill-defined cell border with vesicular and prominent nucleoli, or had distinct cell border with focal squamoid feature (Fig. 2B and 2C). The immunohistochemistry staining showed diffuse positivity for CD117, CD5, p63 (Fig. 2D-2F), and negative for TTF1, calcitonin, and synaptophysin. The patient was diagnosed as intrathyroidal thymic carcinoma (ITC). Total thyroidectomy and adjuvant radiotherapy (60 Gray/30 fraction) was performed. After the operation, he was no more feverish.