Medullary thyroid carcinoma in a patient with Hashimoto's thyroiditis diagnosed by calcitonin washout from a thyroid nodule.

Serum calcitonin is a tumor marker used in the diagnosis and follow-up of medullary thyroid carcinoma. Calcitonin washout evaluation is a new method used for suspicious thyroid nodules and lymph nodes. Limited clinical data are present about the efficacy of this method. A 61-year-old female patient with known Hashimoto's thyroditis and an 8-mm hypoechoic nodule was presented with one previously benign fine-needle aspiration cytology (FNAC). On referral to our department, she had a moderately high-serum calcitonin level, and we repeated the FNAC that was reported as nondiagnostic. We performed FNAC for the third time together with calcitonin washout evaluation from the thyroid nodule. The FNAC was again nondiagnostic, but the calcitonin washout level from the thyroid nodule was 152.569 pg/mL. Total thyroidectomy was performed, and the diagnosis was confirmed as medullary thyroid carcinoma. Calcitonin washout evaluation may be a useful method in the differential diagnosis of patients with thyroid nodules having moderately high-serum calcitonin levels.