Purpose: To evaluate the effectiveness of radioiodine treatment for metastatic thyroid carcinoma, we reviewed results of radioactive iodine treatment in patients with functional lung or bone metastases. Materials and Methods: Of 760 patients who were treated for differentiated thyroid cancer between 1984 and 1998, we detected pulmonary metastases and bone metastases in 76 patients (10.0%) and 20 patients (2.6%), respectively. Among them, we could evaluate the effectiveness of I-131 therapy in 53 patients with lung metastases and 15 patients with bone metastases. Results: Of 53 patients who received I-131 therapy with a mean cumulative dose of 26.2 GBq (1.1-84.4 GBq) for pulmonary metastases, metastatic lung lesions completely resolved in 19 patients (35.8%) and improved in 22 patients (41.5%). In 13 of 19 patients with complete remission of pulmonary metastases, the total accumulated dose of I-131 was less than 18.5 GBq. We found 43 sites of metastatic bone lesions in 15 patients with bone metastases. Of 29 lesions which received I-131 therapy, metastatic lesions improved in 14 sites (48.3%), but did not change or progress in 15 sites (51.7%) despite the I-131 therapy. Three lesions were completely cured with a combination treatment of surgery(¡¾ external radiotherapy) and I-131 therapy, and the other 11 lesions improved. Conclusion: Radioactive iodine treatment gives favorable results for pulmonary metastases. However, for bone metastases, there might be a need to use combination therapy including I-131 and surgery or external irradiation. (Korean J Nucl Med 2000;34:107-118)