Can we use ionized calcium in the evaluation of post-thyroidectomy hypocalcemia?

The postoperative hypocalcaemia has the higher incidence as complications related to thyroidectomy.

From 1 June 2006 to 30 June 2008, we examined 492 patients operated on consecutively in our unit with a total thyroidectomy for thyroid disease. We evaluated the values of ionized calcium in all the cases, matching these with the preoperative and postoperative values of serum calcium. The pre- and postoperative (24 hours after treatment) data for ionized calcium and serum calcium were examined statistically with the Student's t-test; results with a P-value <0.05 were considered to be statistically significant.

Two-hundred-and-twenty-three of the 492 patients (45.2%) treated with total thyroidectomy had preoperative values of ionized calcium lower than 1.13 mmol/L (normal values 1.13-1.32 mmol/L), while the ionized calcium values were lower than 1.10 mmol/L in 154 of the 223 patients. The mean value of ionized calcium in all 223 cases was 1.04+/-0.07 mmol/L. The mean serum calcium value in these patients was 9.13+/-0.291 mg/dL (normal values 8.3-10.5 mg/dL). In the other 259 cases, the values of ionized calcium and serum calcium were 1.21+/-0.03 mmol/L and 9.1+/-0.29 mg/dL, respectively. In 75 cases on 223 with symptomatic hypocalcemia, the mean value of ionized calcium was 0.88+/-0.05 mmol/L, while, in the remaining 148 cases, the mean value was equal to 0.97+/-0.08 mmol/L (P<0.001). We compared this, in both groups, with the values of postoperative serum calcium; in the 75 cases with clinical hypocalcemia, the value of serum calcium was 7.32+/-0.35 mg/dL, while the value was equal to 8.4+/-0.34 mg/dL in the other cases (P<0.001).

The values of ionized calcium must not be used as marker of hypocalcemia but must be seen as a diagnostic aid linked to others laboratory values, such as serum calcium.