Experimental 1 : Intraoperative remifentanil infusion at 0.2 ㎍/㎏/min and MgSO4 30 mg/kg IV at the induction followed by intraoperative infusion at 10 mg/kg/hr Drug : remifentanil, MgSO4 Experimental 2 : Intraoperative remifentanil infusion at 0.2 ㎍/㎏/min and normal saline Drug : remifentanil Active comparator : Intraoperative remifentanil infusion at 0.05 ㎍/㎏/min and normal saline Drug : remifentanil

Patients undergoing thyroidectomy will be randomly assigned to one of three groups. Remifentanil will be infused intraoperatively at 0.05 ㎍/㎏/min (group Ⅰ) or 0.2 ㎍/㎏/min (group Ⅱ & Ⅲ). Patients in group Ⅲ will be received 30 ㎎/㎏ MgSO4 at the induction of general anesthesia followed by a maintenance infusion of 10 ㎎/㎏/hr intraoperatively until skin closure.