Selenium suppresses autoimmune destruction of thyrocytes and decreases titers of serum TPOAb in AIT patients. Older 4 clinical trials approved the efficacy of the daily dose of 200micg. It's believed that Se saturates the deficient stores of GPX so GPX saves the thyrocytes against to oxidative stresses. Although less than 70 micg/d is sufficient to maximize GPX activity, none of the authors tested the doses less than 200 micg/d. Our hypothesis was that If 100 micg/d can not suppress the TPOAb titers,it means autoimmune destruction can not be blocked by saturation of deficient stores of GPX solely and the mechanism of action requires more than repletion of deficient stores. It's important not only to estimate the optimal dose but to understand the mechanism of action. High dose therapy may also suppress TPOAb levels in Se-non-deficient AIT patients, if it is so, Se therapy may becomes the solely treatment modality which can suppress the autoimmunity in more than 400 million AIT patients. Because there've been no way to suppress autoimmune war and replacement of LT4 had been the only treatment modality for palliation. An other independent part of the study is to test the effect of Se in adolescent AIT patients.

Inclusion Criteria:

• Clinically approved AIT patients who do not use any medication other than LT4 to keep TSH in the lower half of normal range.

Exclusion Criteria:

• Any kind of drug use other than LT4 or any kind of known pathology which may effect GIS absorption.