Purpose <p>Intraoperative parathyroid hormone (PTH) measurement is a beneficial tool in the surgical management of primary hyperparathyroidism. The expected degree of intraoperative PTH reduction, which guides surgical decision-making, determines the sensitivity and specificity of this test. While stricter criteria may enhance diagnostic accuracy, an optimal threshold has not been conclusively established. The aim of this study was to identify the PTH reduction threshold that provides the highest sensitivity and specificity for achieving biochemical cure.</p> Patients and methods <p>A retrospective analysis was conducted on 141 patients who underwent parathyroidectomy for primary hyperparathyroidism, focusing on the intraoperative drop in PTH and surgical success. A receiver operating characteristic analysis was performed to identify the optimal threshold that balances sensitivity and specificity in predicting biochemical cure.</p> Results <p>The mean percentage reduction at the end of surgery was 73.93% (SD ± 16.54%) with an overall cure rate of 94%. The area under the curve was 0.73 for a 50% PTH reduction, 0.77 for a 60% reduction, and 0.68 for a 70% reduction.</p> Conclusion <p>The optimal balance between sensitivity and specificity was achieved with a 60% intraoperative PTH reduction. Stricter criteria increase sensitivity but may also raise the risk of surgical overtreatment.</p>