Clinical presentation: A 23-year-old woman was referred for an evaluation of signs of hyperthyroidism.

Palpation: no abnormality.

Functional state: euthyroidism with TSH 1.00 mIU/L.

Ultrasonography revealed a deeply hypoechogenic nodule in the left lobe. The nodule did not display a halo sign. The presence of perinodular blood flow was doubtful.

Cytology resulted in atypical follicular tumor.

Histopathological diagnosis: microfollicular adenoma.

Comment. The nodule should be classified as an EU-TIRADS 5 lesion due to the deeply hypoechoic pattern.