First examination (first row of images)

Clinical presentation. A 37-year-old woman was referred for evaluation of a nodular goiter discovered on screening.

Palpation: an elastic nodule in the right lobe.

Result of blood tests: TSH 1.09 mIU/L.

Ultrasonography. The thyroid was echonormal. There was a central-type cyst in the ventral part of the right lobe. The lesion had halo and perinodular blood flow.

3 mL brown fluid was aspirated. Aspiration cytology resulted in benign cystic lesion.

Second examination 4 years later (second and third rows of images)

Clinical presentation. The patient came to routine follow-up visit. She noticed a continuous increase in nodule size and nodule caused some problems is recumbent position.

Palpation: an elastic nodule in the right lobe.

Result of blood tests: TSH 1.81 mIU/L.

Ultrasonography. The presentation remained unchanged except for two conditions. Firstly, the nodule increased significantly. Secondly, the cyst seemed to be a pure one.

After aspirating 20 mL brown fluid, the presentation of the nodule has changed, it became evident that this is a central-type cyst.

Suggestion: ethanol sclerotherapy.

Comment. This case illustrates the challenge of defining a pure cyst. Most seemingly pure thyroid cystic lesions larger than 2 cm are in fact not pure cysts but the fluid compresses the solid part. The real subtye of a seemingly pure cyst can be determined only after the removal of cystic fluid. In other subtypes of cysts, i.e. central, spongiform and peripheral types, the categorization is not influenced by removal of cystic content.