Postoperative tetany in patients with Graves' disease: a risk factor analysis

OBJECTIVE There is little information regarding the clinical risk factors for postoperative tetany in patients with Graves' disease, We analysed the risk factors responsible for postoperative tetany by univariate and multivariate analysis in thyroidectomized patients with Graves' disease, and we discuss the mechanisms of hypocalcaemia and tetany after surgery. PATIENTS The subjects were 1742 consecutive patients with Graves' disease who underwent subtotal thyroidectomy between 1992 and 1994. RESULTS Univariate analysis of 15 possible risk factors demonstrated that 10 were significant: sex, required operation time, blood loss, preoperative serum calcium level, estimated weight of thyroid remnant, size of goitre, amount of excised thyroid tissue, serum alkaline phosphatase concentration, age at operation and TSH-binding inhibitory immunoglobulin (TBII). In multivariate analysis in female patients, the risk factors were: preoperative lower serum calcium level, younger age, higher serum alkaline phosphatase concentration, larger size of goitre and higher value of TBII, in order of decreasing probability of significance. CONCLUSION These results, combined with previous observations in which a significant decline in serum parathyroid hormone level was found in patients with Graves' disease who suffered postoperative tetany, suggest that postoperative tetany may occur during the period of bone restoration due to antithyroid drug therapy and be due to continuation of a calcium flux into bone concomitant with transient hypoparathyroidism induced by surgery. Calcium supplements during the preoperative period and/or after surgery may be appropriate for patients who are anticipated to develop postoperative tetany based on these risk factors.