Hypocalcemia is the most frequent complication after total thyroidectomy. Parathyroid hormone (PTH) measurement has been proposed as an early predictor of this condition. Total thyroidectomy was performed in 39 patients. Hypocalcemia was present in 15 cases (38%). Patients undergoing hemithyroidectomy (n = 13) were considered control subjects not developing hypocalcemia. PTH was measured before surgery and 10 minutes after resection of the gland using a rapid (15 minutes) chemiluminescent immunometric assay. Patients developing hypocalcemia had lower calcium and postresection PTH levels and higher PTH decline than patients not developing hypocalcemia (P < .0001). PTH decline (cutoff value, 62.5%) had the better sensitivity (93.3%) for predicting hypocalcemia, allowing for a fairly safe early discharge. However, the best overall results corresponded to the combination of postresection PTH level (< or = 18 pg/mL [< or = 1.9 pmol/L]) and PTH decline (>62.5%), with a sensitivity of 90% and a specificity of 97.9%. Perioperative PTH measures can accurately predict hypocalcemia after thyroidectomy, granting the laboratory a key role in the immediate decision about calcium supplementation for patients at risk.