American Society of Anesthesiologists physical status I-II adults undergoing elective thyroidectomy under general anesthesia are recruited and randomly allocated to receive dexmedetomidine iv infusion of 0.2μg·kg-1·h-1 (Group D1, n = 100), 0.5μg·kg-1·h-1 (Group D2, n = 100), 0.7μg·kg-1·h-1 (Group D3, n = 100) or saline (Group C, n = 100). Primary outcome is the incidence of coughing during emergence. Second outcomes include severity of coughing, which is assessed using a four-point scale; Ramsay score, Bruggrmann comfort scale score, VAS pain score are recorded. The respiratory rate (RR), heart rate (HR), and mean arterial pressure are also recorded.