SUN-359 - Early Postoperative PTH Cutoffs to Predict Chronic Hypoparathyroidism After Thyroid Surgery

Background: Early postoperative parathyroid hormone (PTH) is used to stratify risk of post-thyroidectomy hypoparathyroidism, but thresholds and sampling windows vary. The performance of the commonly cited 1.05 pmol/L 24-48 hours post-op was tested to assess it as a valid cut off measure for predicting long-term hypoparathyroidism.

Objective: To evaluate whether early 24 to 48 hours postoperative PTH ≤1.05 pmol/L predicts long-term hypoparathyroidism. A secondary exploratory objective, to compare PTH level of 1.05 pmol/L performance with alternative thresholds, and to explore factors increasing the risk of hypoparathyroidism.

Methods: Single-center registry-based analysis of all patients receiving thyroid surgery between September 2021 until October 2025 was conducted. A total of 175 patients were included. PTH level at ≤1.05 pmol/L was considered as a cutoff to predict chronic hypoparathyroidism. PTH and calcium studies were obtained at the 6 weeks intervals. Patients with PTH level of < 1.6 pmol/L (1.6 to 6.9) with hypocalcemia and/or patients requiring active vitamin D or calcium supplements are considered hypoparathyroid. The same parameters were used for chronic hypoparathyroidism. Alternative PTH cutoffs (≤0.6, ≤1.1, or ≤1.3 pmol/L) were tested, and diagnostic performance was assessed using sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV).

Results: A total of 175 patients had surgery. 15 patients had PTH level ≤1.05 pmol/L at 24-48 hours of which 6 patients recovered at 6 weeks intervals and continued to be normocalcemic long-term. For early PTH ≤1.05 pmol/L, sensitivity was 80.0% and specificity was 18.2% , with PPV 40% and NPV 100% . When considering a PTH level of ≤0.6 pmol/L, 12 patients were hypoparathyroid of which 3 patients recovered at 6 weeks. Sensitivity 66.7%, specificity was 45.5%, PPV 40.0%, NPV 71.4%. Whereas Higher cutoffs (≤1.1 and ≤1.3) reduced specificity and PPV without improving sensitivity. All patients had total thyroidectomy. 7/15 had neck dissection non-significant (NS). FNA-positive/ AUS (4+1/15) did not increase the risk of hypoparathyroidism (NS).

Conclusion: In this cohort, early PTH ≤1.05 pmol/L is a favorable cutoff to detect patients at risk for early post operative hypoparathyroidism. For prognostic measures for chronic hypoparathyroidism we suggest using 0.6 pmol/L. Larger studies are needed to validate optimal cutoffs and factors across practice settings.

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