Low serum iPTH at the end of surgery is the earliest predictor of postoperative hypocalcemia after total thyroidectomy

BACKGROUND: The most frequent complication of total thyroidectomy remains hypocalcemia due to low postoperative levels of serum intact parathyroid hormone (iPTH). The purpose of this study was to investigate the role of decreased iPTH at the end of surgery in predicting hypocalcemia. In addition, we examined the percentage decrease of iPTH as potential indicator of hypocalcemia. METHODS: We retrospectively collected the data of patients who underwent total thyroidectomy for benign and malignant diseases at our institution between 2010 and 2022. The iPTH level was measured before and at the end of surgery, and serum calcium levels on the first postoperative day. Demographic, clinical, and biochemical characteristics of patients with low iPTH were compared with patients with normal iPTH levels using ANOVA for continuous variables and χ2-tests for categorical variables. Multivariable logistic regression analysis evaluated the association of iPTH at the end of surgery and the relative reduction of iPTH with postoperative hypocalcemia. RESULTS: The mean age of the 607 patients in this study was 55.6 years, and the female-to-male ratio was 5:1. Goiter was the most common indication for surgery (N = 382, 62.9%), followed by Graves’ disease (N = 135, 22.2%). The mean preoperative iPTH was 49.0 pg/ml, while the mean postoperative iPTH was 29.3 pg/ml. A total of 197 patients (32.5%) had an iPTH level below normal, 77 patients (39%), had iPTH levels of 10–15.0 pg/ml and 120 patients (61%) of < 10.0 pg/ml at the end of surgery. Among all patients, 124 (20.4%) developed hypocalcemia on the first postoperative day. The mean percentage of decrease of iPTH was highest among patients with iPTH < 10 pg/ml (76.9%, p < 0.01); this group of patients had also the highest rate of postoperative hypocalcemia on day one (45.0% vs. 26.0% vs 12.2%, p < 0.01). CONCLUSIONS: Measurement of iPTH at the end of total thyroidectomy predicts patients who are at risk for postoperative hypocalcemia. The combination of low serum iPTH with a decrease in iPTH level of ≥ 50% may improve prediction of hypocalcemia compared to iPTH levels alone allowing for early calcium substitution in these patients at high risk of developing postoperative hypocalcemia.