Clinical data: A 69-year-old woman was referred for evaluation of a thyroid nodule which was detected on evaluation performed because of 10 kg weight loss-up and diarrhea. Her complaints lasted for three months. Various gastroenterological examinations were performed but the cause for complaints was not revealed. The patient was diagnosed as having Hashimoto's thyroiditis for several years. She got no thyroid medication. The patient noticed a lump in the left side of the neck for 5 months. Otherwise, she had no complaints.

Palpation: There was a firm nodule in the left lobe.

Functional state: euthyroidism with TSH 1.86 mIU/L.

Ultrasonography. The thyroid was echonormal and contained several cystic areas and an echonormal nodule in the right lobe. There was a large, partly echonormal, partly hypoechogenic nodule in the left lobe. This lesion had irregular borders and presented an irregularly increased intranodular vascularization.

Cytology resulted in medullary carcinoma.

Serum calcitonin was above 5852 pM/L.

Total thyroidectomy was performed. Histopathology disclosed medullary carcinoma in the left lobe while hyperplastic nodules in the right thyroid.