목적: The aim of this study was to evaluate the prevalence, risk factors, and clinical characteristics of hypothyroidism following hemithyroidectomy, thus paving the way for appropriate patient care and surveillance after hemithyroidectomy. 방법:We included 406 patients undergoing hemithyroidectomy from August 2004 to February 2011, and their medical records were retrospectively reviewed. The prevalence, type of hypothyroidism at diagnosis, lag time to development of hypothyroidism, and risk factors of development of hypothyroidism were investigated. In addition, clinical course of treated and untreated hypothyroidism were assessed with approximate 5-year follow-up period. 결과:During 56.4 months of follow-up period, postoperative hypothyroidism developed in 226 patients, with an overall prevalence of 55.8%. Of those, 222 patients showed subclinical hypothyroidism at the time of diagnosis. Mean lag time to diagnosis of hypothyroidism after surgery was 4.3 months; 218 (96.5%) patients were diagnosed within 21 months. In uni- and multivariate analysis, TSH > 1.97 mIU/L and coexistence of Hashimoto’s thyroiditis were verified as independent risk factors of development of hypothyroidism. In 222 subclinical hypothyroidism patients, 149 (67.1%) patients spontaneously recovered from subclinical hypothyroidism. 결론:The present study with relatively long-term surveillance for thyroid function showed that prevalence of hypothyroidism after hemithyroidectomy was much higher than expected, reflecting its actual incidence better than the previous. Therefore, regular thyroid function measurement is recommended at least for 21 months after surgery in patients undergoing hemithyroidectomy, especially with TSH > 1.97 mIU/L and coexistence of Hashimoto’s thyroiditis. In addition, levothyroxine supplement for hypothyroidism should be determined in more prudent way, because thyroid function status was sponstaneously normalized in approximately two thirds of patients with subclinical hypothyroidism.