Purpose: To evaluate the predictive value of day zero post-operative parathyroid hormone (PTH) levels in determining long-term cure for primary hyperparathyroidism (pHPT) following parathyroidectomy. Methodology: This multicentre, retrospective diagnostic accuracy study utilised data from a single surgeon, recorded in a prospectively maintained surgical database. Patients who underwent parathyroidectomy for pHPT were included, with exclusions for secondary or tertiary causes and incomplete follow-up. Day zero post-operative PTH levels were the key predictor variable, while cure at six months, defined by normocalcaemia, was the outcome. Diagnostic parameters including sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy were calculated for an optimal PTH cut-off determined using a receiver operating characteristic (ROC) curve and Youden’s index. Results: Out of 291 patients included, 278 (95.5%) were cured at 6 months. An optimal PTH cut-off of <3pmol/L was identified, correlating with a true positive rate of 81.3% and a false positive rate of 0%. Patients with day zero PTH <3 (77.7% of the cohort) were all cured at six months. The PTH <3pmol/L cut-off demonstrated a sensitivity of 81.3%, specificity of 100%, PPV of 100%, NPV of 20%, and an accuracy of 82.1%. Conclusion: Day zero post-operative PTH level <3pmol/L is a reliable predictor of cure for pHPT following parathyroidectomy. A PTH level of <3pmol/L is an effective cut-off to identify patients who will be cured, potentially reducing the need for prolonged biochemical monitoring.