Clinical data: a 64-year old man was referred for evaluation of a rapidly increasing mass in the left side of the neck. The lesion was first noticed 4 weeks before the present investigation.

Palpation: the left thyroid was extremely enlarged, stone-hard.

Functional state: hypothyroidism (TSH 17.1 mIU/L).

Ultrasonography: the left thyroid and the isthmus were infiltrated by multiple hypoechogenic nodules. The horizontal diameter of the thyroid was 8 cm, while the longitudinal diameter exceeded 20 cm.

Cytological diagnosis: anaplastic carcinoma.

Histopathology: anaplastic carcinoma.

Comment: the hypothyroidism might cause differential diagnostic problems. The clinical presentation is unequivocal, such tumor must be either anaplastic cancer or MALT-lymphoma. The latter evolves in Hashimoto's thyroiditis and great proportion of the patients have hypothyroidism at the time of diagnosis. Nevertheless, patient with anaplastic cancer may have also hypothyroidism caused by the destruction of the tumor. The frequency of hypothyroidism is around 10-15% in the case of anaplastic cancer. If FNAC is not decisive, the sonographic pattern may be of help: the presence of necrotic foci is a frequent finding in the case of anaplastic cancer but it is a very rare phenomenon in the case of a MALT-type lymphoma.