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BACKGROUND/OBJECTIVE The purpose of this study is to assess outcomes and the detailed clinicopathological features of differentiated thyroid carcinoma (DTC), including the relation of Hashimoto's thyroiditis and postoperative pathological features in child under 18 years old. METHODS We reviewed patients with DTC under 18 years old (pediatric DTC patients) seen during recent 16 years. The clinicopathological features and outcomes of pediatric DTC were analyzed by comparison with patients of 19-20 years old or 21-44 years old. RESULTS Sixty four children with DTC [median age 16 years (range, 5-18)] were studied. The ratio of female to male was 5:1, but no difference was found by comparison with adult of 21-44 years old. No difference was found in multifocality, but DTC in child showed lager tumor size (P < 0.001), higher rate of extrathyroidal extension (P = 0.017), more local or pulmonary metastasis (P < 0.001, P < 0.001 respectively) than adult thyroid carcinoma. High rate of Hashimoto's thyroiditis (19/43) without influence on pathological features was found in patients under 18 years old. No differences, except for distant metastasis, were found by comparison of clinicopathological features between patients under 18 years old and 19-20 years old. Pediatric patients possessed highest rates of persistent/recurrent disease, though only one child died. CONCLUSION Pediatric DTC has more aggressive behavior characterized by a high rate of extrathyroidal extension, local and pulmonary metastasis. Pediatric DTC has low mortality, but active treatments are needed for the high risk of persistent or recurrent diseases. Hashimoto's thyroiditis may be associated with the pathogenesis or mechanism of pediatric DTC.