Clinical data: a 53-year-old woman was referred for an evaluation of a newly discovered nodule causing difficulties in swallowing. The patient noticed a lump in the left thyroid 4 months earlier.

Palpation: a very hard, not freely moveable nodule in the left lobe.

Functional state: euthyroidism (TSH-level 4.01 mIU/L).

Ultrasonography: a large nodule occupied almost the entire left thyroid. It was partly hypoechogenic, partly near echonormal, inhomogeneous.

Cytology: there were atypical, elongated tumor cells on the smear. The structures resembled that of an adenocarcinoma but neither papillary nor follicle formation was found.

We thought that it was metastatic carcinoma but the wash-out thyroglobulin level was 29.1 ng/mL. We gave the diagnosis of carcinoma. Secondary cancer has to be considered primarily.

Further evaluation of the patient found primary focus neither in the neck, lung and gastrointestinal tract, and thyroidectomy was performed.

Histopathology: primary mucoepidermoid cancer of the thyroid.