A Practical Mathematic Method to Predict and Manage Hypocalcemia After Parathyroidectomy and Thyroidectomy

Objective: Prediction and early intervention for hypocalcemia following parathyroidectomy and total thyroidectomy can decrease hospital cost and prevent severe hypocalcemia-related complications. This study aims to predict the severity of hypocalcemia after parathyroidectomy or thyroidectomy and to stratify patients into groups with different levels of risk for developing severe hypocalcemia, so that higher risk patients may be monitored more closely and receive earlier interventions. Methods: This was a retrospective cohort study of 100 patients with primary hyperparathyroidism who underwent parathyroidectomy as the primary treatment modality at a tertiary care hospital. Clinical information, including demographic information, perioperative PTH and calcium levels, vitamin D levels, weight of the pathologic glands removed, gland pathology, and re-admission rates, were retrieved. Statistical analysis was performed to analyze the association between collected variables and percentage of calcium drop following parathyroidectomy with statistical significant set at P-values <0.05. Results: Age, sex, and vitamin D level provided very minimal information to quantify risks of postoperative hypocalcemia. The percentage of decrease from preoperative PTH level to the lowest PTH level after the removal of the abnormal gland(s) is the most significant predicting factor for the severity of postoperative hypocalcemia. There is a mathematic regressional correlation between them. A formula was generated to quantify this linear relationship between them, and the nadir calcium can be calculated as Ca nadir = Ca preop * [ 1 − 0 . 35 * ( PTH preop − PTH intraop ) 2 PTH preop 2 ] , where Canadir = the lowest postoperative calcium level, and PTHintraop = PTH level 15 minutes after removal of the abnormal gland, with the value of R2 > 0.7. The formula has been tested primarily in our patient population with good reliability. Conclusions: The highest preoperative, lowest postoperative, and change in PTH level can help us reliably calculate the trend of postoperative calcium level. Decision to pursue early interventions can be made based on the calculated result from the formula we obtained.