Unexpected arteriovenous malformation of the thyroid resulting in significant intraoperative blood loss

A 49-year-old morbidly obese woman with metastatic endometrial carcinoma was referred for evaluation of an incidentally identified large right thyroid nodule found on computed tomography performed for cancer evaluation. Ultrasound revealed a 9.7 cm solid isoechoic homogeneous right thyroid nodule. Fine needle aspiration was benign. Given size, resection was recommended following completion of chemotherapy and radiation. At the time of right thyroid lobectomy, extremely large vessels were encountered, and the procedure was complicated by estimated blood loss of 2 L. Final pathology revealed a large, benign adenomatous nodule and vascular features consistent with arteriovenous malformation (AVM). Unlike previously reported cases, the diagnosis of a thyroid AVM was not known preoperatively.