Background: Minimally invasive parathyroidectomy for primary hyperparathyroidism depends on accurate preoperative imaging. Cervical ultrasound is commonly used to localize parathyroid adenomas, but can lead to discovery of concomitant thyroid gland pathology requiring modification of the operative approach. How the identification of incidental thyroid lesions affects patient management is unclear. Study Design: A prospective database of patients undergoing parathyroidectomy was analyzed for thyroid pathology discovered by ultrasound. Lesions were biopsied if indicated, and operative management was adjusted accordingly. Clinical data were correlated with operative decision-making. Results: Between July 2002 and November 2009, 310 patients with primary hyperparathyroidism underwent ultrasound. Concomitant thyroid pathology was noted in 89 (29%) patients. Thirty-seven patients (42% of pathology) underwent fine-needle aspiration of a thyroid nodule. Thirteen patients (4% of all patients) underwent a thyroid operation not related to parathyroid disease: 9 thyroid lobectomies for presumably benign nodules and 4 total thyroidectomies for malignancy. Two were for confirmed papillary thyroid cancer, and the other 2 were for an indeterminate biopsy that later proved to be papillary thyroid cancer. One lobectomy discovered microscopic papillary thyroid cancer independent of the biopsied nodule. In total, 5 (2% of all patients) malignancies were discovered. Conclusions: Twenty-nine percent of patients with primary hyperparathyroidism had concomitant thyroid pathology on ultrasound. Forty-two percent of these patients underwent biopsy, and 2% had malignant pathology. Routine use of ultrasound in patients with primary hyperparathyroidism leads to discovery of unrecognized thyroid pathology and cancer. © 2010 by the American College of Surgeons.