A permanent hypoparathyroidism is a problematic complication of total thyroidectomy. In this study, we investigated its incidence and how to avoid it at the time of completion total thyroidectomy after hemithyroidectomy. Eight of the 154 patients who underwent completion total thyroidectomy as the second surgery (5%) after hemithyroidectomy (two-surgery group) showed a permanent hypothyroidism. Patients without parathyroid autotransplantation either at initial or second surgery were more likely to show a permanent hypoparathyroidism. In the subset of 74 patients in two-surgery group, who underwent bilateral central dissection, 6 (8%) had a permanent hypoparathyroidism. The incidence was higher than those in control group who underwent total thyroidectomy with bilateral central dissection at one time, which was 2%. However, all 6 patients showing a permanent hypoparathyroidsm underwent bilateral central dissection in initial surgery and none of the patients who underwent bilateral central dissection in twice had a permanent hypoparathyroidism. Taken together, we can conclude that 1) in initial surgery of hemithyroidectomy, we have to carefully search the parathyroid glands and if dissected, they should retrieved and autotransplanted to save the patients from a permanent hypoparathyroidism when they undergo second surgery in future, and 2) hemithyroidectomy with bilateral central dissection significantly increases the risk of permanent hypoparathyroidism and only ipsilateral dissection is better when we do not perform total thyroidectomy.