BACKGROUND: Post-operative hypoparathyroidism is a complication in patients who undergo thyroid surgery. Our study aimed to evaluate the incidence and causes of post-operative transient and permanent hypoparathyroidism in patients undergoing thyroid surgery. MATERIALS AND METHODS: The data of 933 consecutive patients who underwent total thyroidectomy in a single center were retrospectively evaluated. The rate of post-operative hypoparathyroidism, clinicopathological features, and laboratory parameters during the post-operative first day, first month, and first year of patients with and without hypoparathyroidism were analyzed. Patients with hypoparathyroidism were classified as transient or permanent cases. RESULTS: The incidence of post-operative hypoparathyroidism was 22.7%, including transient (20.6%) and permanent (2.1%). In multivariable analysis, independent predictors of permanent hypoparathyroidism were as follows: surgery due to malignant thyroid disease, tumor multifocality, and pre-operative vitamin-D deficiency (VDD) (p<0.001, 0.047, and 0.002, respectively). During the post-operative first month, the mean serum PTH levels were found to be 7.58 pg/mL, and they remained low on the post-operative first year in patients with permanent hypoparathyroidism. CONCLUSION: Surgery due to thyroid malignancy and VDD should be considered risk factors for permanent hypoparathyroidism in patients who undergo thyroid surgery. The post-operative first month is important in the prediction of permanent hypoparathyroidism.