Clinical presentation: a 79-year-old man with a rapidly increasing nodule causing dysphagia and hoarseness.

Functional state: euthyroidism with TSH-level 3.73 mIU/L, FT4 16.0 pM/L.

Ultrasonography: the right lobe was intact, while the left lobe was replaced by a very large hypoechogenic mass. A vessel was identified within the nodule.

Cytological picture: no colloid in the background. Extremely cellular picture. The cells form neither follicles nor papillary fronds, they are arranged in loose syncytial structures and predominantly dissociated. The atypical cells are polygonal with eccentric nuclei. Note triangular forms specific for medullary cancer and several highly atypical, pleomorphic tumor cells.

Cytological diagnosis: medullary cancer.

Blood test for calcitonin: serum-level of calcitonin exceeded 585 pM/L (normal value: 0-3.36).

Histopathology: medullary cancer.