Our objective was to determine prevalence of incidental thyroid pathology during surgical treatment of primary hyperparathyroidism and to compare sensitivity and specificity of surgical neck exploration with histologic study of resected thyroid gland to confirm unsuspected pathologies. This was prospective cohort study performed at the Specialty Hospital's Department of Endocrine Surgery at the Mexican Institute of Social Security's (IMSS's) Centro Médico de Occidente in Guadalajara, Jalisco State between 1995 and 2003. All were submitted to bilateral neck exploration under general anesthesia. According to the surgeon's criteria, any suspected thyroid anomaly was resected for transoperative frozen sections as well as definite histopathologic study. Preoperatively from a total of 44 cases of hyperparathyroidism, only four cases with thyroid anomalies were detected with neck ultrasound and/or computer tomography (CT) scan; additionally, 13 patients had thyroid anomalies found during neck exploration according to surgeon criteria. All were biopsied. Final study revealed thyroid disease in 16 cases; three had well-differentiated thyroid carcinoma and the remaining 13 patients had different benign entities. In one case, the thyroid specimen was considered normal. Sensitivity and specificity of surgeon criteria were 100 and 96%, respectively, to establish thyroid disease during neck exploration for another purpose. Association of hyperparathyroidism and benign and malignant thyroid disease is a common phenomenon. Independently of the ability of preoperative imaging procedures to detect unsuspected findings, we suggest exploration neck bilaterally and resection of any suspicious lesion.