Thirty-eight patients with metastatic well-differentiated thyroid carcinoma treated with 200mCi $^{131}I$ were studied. There were false negative serum thyroglobulin values during TSH suppression or at anti-thyroglobulin antibody(+) and discrepancies between findings of whole body scan and serum thyroglobulin level. After one to five cycles of 200mCi $^{131}I$ therapy, complete remission and partial remission were achieved at 5.3% and 57.9%, respectively. We concluded that all of serum thyroglobulin, TSH, anti-thyroglobulin antibody, $^{131}I$ or $^{123}I$ whole body scan were necessary in follow up of metastatic well-differentiated thyroid carcinoma. Also, if there was no response after repetitive 200mCi $^{131}I$ therapy, higher doses of $^{131}I$ therapy should be considered.