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To identify the risk of clinical hypocalcemia in the first hours after thyroidectomy.Prospective observational study.Single-institution tertiary hospital in Madrid, Spain.A total of 123 patients who underwent total or completion thyroidectomy between June 2010 and March 2012 were included. Pre- and postoperative intact parathyroid hormone (iPTH) levels were obtained. Patients remain hospitalized a minimum of 48 hours until blood calcium stabilized. Calcium and/or vitamin D supplements were prescribed only when signs or symptoms of hypocalcemia developed. Receiver operating characteristic curve analysis was employed to evaluate the postoperative iPTH level and the pre- to postoperative decrease in iPTH levels. Two cutoff values were determined to stratify the risk of developing clinical hypocalcemia into 3 groups.The areas under the receiver operating characteristic curve were 0.991 for the postoperative iPTH and 0.998 for the decrease in iPTH. An iPTH decrease of 80% had 100% sensitivity to detect patients who developed clinical hypocalcemia, while a postoperative iPTH <3 pg/mL had 100% specificity. Thus, patients with an iPTH decrease ≤80% are at a very low risk of clinical hypocalcemia, and patients with a postoperative iPTH <3 pg/mL are at a very high risk. Patients with an iPTH decrease >80% and a postoperative iPTH ≥3 pg/mL are at intermediate risk. No significant correlation was found between the time when the sample was obtained and iPTH values.This study stablishes a very accurate test to stratify the risk of clinical postthyroidectomy hypocalcemia based on pre- and postoperative iPTH levels.