Preoperative Serum Calcium Levels as an Indicator of Postthyroidectomy Hypocalcemia

Background: Our group has previously demonstrated that serum calcium levels measured in conjunction with parathyroid hormone (PTH) levels early in the postoperative period can help identify and prophylactically treat patients at significant risk for postthyroidectomy hypocalcemia. This study evaluated whether preoperative serum calcium levels can similarly be used as a reliable indicator of a patient's risk for developing postoperative hypocalcemia. Materials and Methods: This was a retrospective review of 1000 consecutive total thyroidectomy patients (2004-2008), with multiple exclusion criteria considered. Postoperative hypocalcemia was defined as total serum calcium of <= 1.90 mmol/L up to 1 month following surgery, a PTH <= 8 ng/L, or signs and symptoms of hypocalcemia. Laboratory values were drawn at various intervals according to our institution's protocol. Results: The pertinent data were analyzed for 247 of 1000 patients and indicated that patients with a preoperative corrected calcium level below 2.27 mmol/L had a postthyroidectomy hypocalcemia rate of 63%, whereas those with a calcium level above 2.27 mmol/L experienced hypocalcemia 24% of the time (p < .0001*). This threshold calcium value yielded a specificity of 93% and a likelihood ratio of 4.2. Conclusion: Our data suggest that preoperative serum calcium levels may correlate with the development of postoperative hypocalcemia. A serum calcium level of 2.27 mmol/L is an important threshold separating patients with an elevated risk of hypocalcemia from those who will likely remain normocalcemic. These data are relevant and useful clinically in identifying patients at risk for hypocalcemia. Current studies are investigating whether patients below our critical threshold of 2.27 mmol/L would benefit from early prophylactic supplementation.