Preoperative parathyroid hormone concentration predicts postoperative hypocalcemia severity in dogs undergoing parathyroidectomy

Objective: To evaluate preoperative parathyroid hormone (PTH) concentration as a predictor of moderate to severe postoperative hypocalcemia following parathyroidectomy in dogs with primary hyperparathyroidism. Methods: Retrospective analysis of dogs undergoing parathyroidectomy for primary hyperparathyroidism and excluding dogs receiving prophylactic calcitriol. Preoperative serum PTH measurement was stratified into groups by multiples of the assay-specific upper limit of normal (ULN). Whole blood ionized calcium (iCa) nadir was used to classify postoperative outcome, with moderate to severe hypocalcemia defined as iCa < 1.00 mmol/L. Results: 92 dogs were included. Moderate to severe hypocalcemia occurred in 36 dogs (39.1%). A graded relationship was identified between PTH strata and moderate to severe hypocalcemia: 9 of 47 dogs (19.1%) with normal PTH, 14 of 27 (51.9%) with > 1 to 2 times ULN, 5 of 10 (50.0%) with > 2 to 3 times ULN, and 8 of 8 (100%) with > 3 times ULN. A PTH cutoff of 1.05 times ULN yielded an area under the curve of 0.76 (95% CI, 0.65 to 0.86), with 72.2% sensitivity and 71.4% specificity. Median time to overall calcium nadir was 65.8 hours (range, 3.7 to 19,794.1), with 40 of 92 dogs (43.5%) reaching nadir after 72 hours. Late nadirs were common in dogs with moderate to severe hypocalcemia (25 of 36 [69.4%]) versus normocalcemia (10 of 44 [22.7%]). Conclusions: Preoperative PTH concentration stratified by ULN multiples demonstrates a graded relationship with postoperative hypocalcemia. Clinical Relevance: PTH-based stratification could identify high-risk patients warranting prophylactic therapeutics and extended monitoring.