Serum parathyroid hormone is not an accurate predictor of postthyroidectomy hypocalcemia in vitamin D-deficient patients: a pilot study.

Transient hypoparathyroidism resulting in temporary hypocalcemia is the most frequent complication of total thyroidectomy and affects up to one third of patients. The ability to accurately predict hypocalcemia after thyroidectomy allows timely intervention and facilitates early discharge of patients. The postoperative decline in serum parathyroid hormone (PTH) is currently regarded as the gold standard biochemical predictor of postthyroidectomy hypocalcemia. Although the value of PTH in predicting postthyroidectomy hypocalcemia has been extensively studied, its predictive accuracy in vitamin D– deficient patients is unclear. This is of particular importance because there is a high prevalence of vitamin D deficiency in patients with thyroid nodules, malignancy, and Graves disease, the major indications for thyroidectomy (1, 2 ). We retrospectively examined the value of serum PTH as a predictor of postthyroidectomy hypocalcemia in patients with and without vitamin D deficiency. We identified 74 consecutive patients who had undergone total/ completion thyroidectomy. Serum 25-hydroxyvitamin D concentration was measured preoperatively, and serum PTH and calcium concentrations were measured in all patients 8 –10 h after surgery. Serum calcium and albumin were measured by using standard automated assays (Abbott Architect, Abbott Diagnostics). PTH was measured by using an automated immunoassay (Abbott Architect, Abbott Diagnostics). The CV for this assay was 8% across the diagnostic range. Serum 25-hydroxyvitamin D was measured by using the automated Liaison assay (Diasorin) with a CV of 11% across the diagnostic range. A 25-hydroxyvitamin D concentration 25 nmol/L was used to indicate vitamin D deficiency, as suggested by the manufacturer’s reference range. The definition of temporary hypocalcemia after thyroidectomy in the literature varies between different studies. In one prospective study of hypocalcemia developing after thyroidectomy, patients were treated if they were symptomatic or had serum calcium 1.95 mmol/L (3). We used similar criteria and defined temporary hypocalcemia as a corrected serum calcium of 1.95 mmol/L or the presence of hypocalcaemic symptoms. Patients were divided into 2 groups according to their preoperative serum 25 hydroxyvitamin D concentrations. Of 58 patients with 25-hydroxyvitamin D concentrations 25 nmol/L, 23 patients developed hypocalcemia requiring treatment. Six of 16 patients with vitamin D deficiency developed hypocalcemia requiring treatment. ROC curves for serum PTH were created for both groups (Fig. 1). The ROC curve reflects the effectiveness of a test in correctly establishing the diagnosis. The area under the ROC curve for serum PTH to predict hypocalcemia postthyroidectomy in patients with a 25hydoxyvitamin D concentration 25 nmol/L was 0.93 (95% CI: 0.86 –1.00; P 0.0001). The area under the ROC curve for serum PTH in patients with vitamin D deficiency was 0.68 (95% CI: 0.39 – 0.97; P 0.23). Our results show that serum PTH is a good predictor of postthyroidectomy hypocalcemia in the absence of vitamin D deFig. 1. ROC curves demonstrating the accuracy of serum PTH in predicting postthyroidectomy hypocalcemia in patients with a serum vitamin D concentration >25 nmol/L and those with a serum vitamin D concentration <25 nmol/L. Letters to the Editor