The timing of parathyroid hormone measurement defines the cut-off values to accurately predict postoperative hypocalcemia: a prospective study

Serum intact parathyroid hormone (iPTH) level is an early marker of post-thyroidectomy hypocalcemia. However, lack of methodological standardization to define timing and cut-off points of iPTH measurement limit its clinical applicability. Here, we evaluated the relationship between two distinct postoperative time sampling and iPTH accuracy on predicting hypocalcemia. iPTH was measured within 4 h after surgery (iPTH 4 h) and on the morning of the first postoperative day (iPTH 1st PO). Hypocalcemia was defined by levels of total calcium corrected by serum albumin ≤ 8.0 mg/dL and/or by the presence of symptoms. The most accurate iPTH cut-off point for hypocalcemia prediction was established from a ROC curve comparing both time-points. The study included 101 patients. The mean age was 52.4 ± 12.9 years, 93 were women (92.1%) and 69 patients underwent total thyroidectomy (68.3%). Hypocalcemia occurred in 25 patients (24.8%), of whom 12 were symptomatic. Total thyroidectomy, longer duration of surgery, surgical complications related to parathyroid glands and lower levels of iPTH 4 h and iPTH 1st PO were associated with postoperative hypocalcaemia (all P < 0.05). Using the ROC curve, the optimal cut-off points were 19.55 pg/mL and 14.35 pg/mL for iPTH 4 h and iPTH 1st PO, respectively. The comparison of the AUC showed no significant difference between these two points of evaluation (0.935 vs. 0.940; P = 0.415). Serum iPTH levels measured within 4 h or on the first morning after surgery are predictors of postoperative hypocalcemia. Notably, both time-points have the same accuracy to predict postoperative hypocalcemia (with different cutoff points).