Clinical data: A 40-year-old woman was referred for evaluation of a thyroid cyst which was discovered by chance on thyroid ultrasound examination. The patient has been treated for hypothyroidism for more than five years.

Palpation: no abnormality.

Laboratory tests: TSH 1.67 mIU/L, aTPO 259 U/mL.

Ultrasonography. The thyroid was moderately hypoechoic and has several more hypoechoic areas corresponding to the underlying Hashimoto's thyroiditis. There was a cystic lesion under the lower pole of the right lobe. This was an almost complete cyst.

5 mL watery fluid was aspirated. There were no cells on the smear.

Additional tests: serum-parathormone 28.0 pg/mL (normal value 12-88), parathormone content of the aspirated fluid 302 pg/mL.

Suggestion: continuation of the replacement therapy. TSH in a year, ultrasound in two years.