A 23-year-old male was referred for suspected thyroid enlargement. The serum PTH level of the patient was 81.4 pg/mL and Tc-99m MIBI was negative, but US found a cystic nodule behind the lower pole of the left lobe of the thyroid. The aspirate fluid PTH level of the patient was 302 pg/mL. The patient was followed-up with serum PTH determination and US examination for 5 years; no significant change was found. MIBI = sestamibi, PTH = intact parathyroid hormone, US = ultrasound.