Coexisting thyroid nodules are the most common cause of false positive localization by radioscintigraphy in the preoperative evaluation for MIPS in patients with primary hyperparathyroidism (pHPT). This false positive finding can prompt full neck exploration in the setting of an unanticipated and incompletely evaluated thyroid nodule. Therefore, we are studying prospectively the routine use of preoperative thyroid US in patients with pHPT to determine the prevalence of concurrent thyroid disease and to assess how frequently this added information could alter the surgical plan.Twenty-four patients with biochemically confirmed pHPT were evaluated with thyroid US after localizing (99m)Tc-sestamibi scintigraphy prior to parathyroid operation.Of the 24 patients, 38% (n = 9) had their operations altered from a planned MIPS or four-gland exploration due to coexisting thyroid nodule(s). Of these, 33% (n = 3) had underlying thyroid malignancy (all papillary thyroid cancer) requiring thyroidectomy in addition to parathyroidectomy. All but one patient had parathyroid adenoma as the cause of pHPT.The routine use of preoperative thyroid US in patients with pHPT undergoing parathyroid surgery may aid in the timely diagnosis and treatment of coexisting thyroid disease. This added information secured before operation may avoid difficult intraoperative decision dilemmas and prevent the increased morbidity associated with a second neck exploration. A large scale prospective study is on-going.