OBJECTIVE: There is no clear cut-off value of serum parathyroid hormone (PTH) or calcium in which patients are at risk for hypocalcemia after total thyroidectomy. We evaluated the usefulness of serum calcium and PTH concentration measurements after total thyroidectomy in predicting late-occurring hypocalcemia. DESIGN: A prospective, single-center, non-randomized longitudinal cohort study of 143 patients undergoing thyroidectomy between August 2019 and December 2019 with serum calcium and PTH levels sampled 1 h after surgery and on the first and fifth postoperative day. Hypocalcemia was defined as serum calcium levels < 2.14 mmol/L regardless of clinical symptoms. Normal PTH range was 1.6-6.9 pmol/L. MEASUREMENTS: The primary outcome measure was presence of hypocalcemia on the first and fifth postoperative day, analyzed by a logistic regression model. The PTH cut-off value for prediction of hypocalcemia was identified using a ROC curve comparing all three time points using the Youden J index. RESULTS: Out of 143 patients, 52 (36.4%) had hypocalcemia on the fifth postoperative day. Advanced age, concomitant neck dissection and serum PTH levels < 2.9 pmol/L 1 h after surgery and on the first postoperative surgery day were associated with a high risk of hypocalcemia on the first and fifth postoperative day and need for higher doses of calcium supplements (P < 0.0001, AUC 0.748, 95% CI 0.669-0.817, with 76.92% sensitivity and 71.43% specificity). CONCLUSION: Serum PTH level measured immediately postoperatively and on the first postoperative day is a reliable predictor of postoperative hypocalcemia with important clinical implications.