The aim of this study is to assess the response of functioning lung metastases of thyroid origin to radioactive iodine-131 therapy out of one hundred fifty patients with metastatic differentiated thyroid cancer, who were assessed to evaluate the response of metastatic lung lesions to RAI-131 therapy. The peak age of onset was between 39-57 years, 64 patients (62.1%) were females compared to 39 males (37.9%). 51 patients (49.5%) had papillary type while 52 patients (50.5%) had follicular type. Among the lung metastases which were detected in 103 patients , 62 patients had only lung, 32 patients had lung and bone and 9 patients had multiple sites, most of these metastases were at initial presentation or detected after the first therapy dose. Complete response was detected in 23 (22.4%) patients, partial response in 33 patients (32%), 36 patients (35%) were in stable state, & 11 patients (10.6%) showed evidence of disease progression. The overall survival from the detection of metastases was 95% at 5 years & 81% at 10 years. Conclusion: Radioiodine therapy is still the most effective systemic therapy to treat patients with metastatic lung lesions From differentiated thyroid carcinoma (DTC) with favorable outcome and over all 5 as well as 10 year survival rate.