Serum thyroglobulin measurement and I-131 whole-body scintigraphy (WBS) are well-established methods for the detection of recurrence in the follow-up of patients with thyroid carcinoma. However, inconsistent results are observed frequently, and these two methods are not always able to detect recurrence. In some patients, serum thyroglobulin level is elevated but the WBS is negative, because the recurrent tumor is too small and below the sensitivity of the diagnostic scan, or there is a dissociation between thyroglobulin synthesis and the iodine trapping mechanism. In such cases, various nuclear imaging methods including TI-201, Tc-99m-sestamibi, and F-18-FDG PET can be used besides anatomical imaging methods. Among them, FDG PET localizes recurrent lesions in WBS-negative thyroid carcinoma with high accuracy. Several studies have suggested that empirical high-dose 1-131 therapy resulted in a high rate of visualization in post-therapy scans with evidence of subsequent improvement. An important question is when to operate on patients with recurrent tumor. We believe that surgical removal is the best means of treatment for patients with localized persistent tumor, despite the high-dose 1-131 therapy, with tumor in thyroid remnant, and with isolated recurrence in the lymph node, lung or bone. In addition, we recommend palliative resection of locally unresectable mass with subsequent treatment with high-dose 1-131 therapy. Before 1-131 therapy, the evaluation of sodium-iodide symporter expression in thyroid carcinoma can predict iodine uptake. Retinoic acid is known to induce redifferentiation, and to enhance 1-131 uptake in thyroid carcinoma. Retinoic acid therapy may represent an alternative approach before high-dose 1-131 therapy. (Korean J Nucl Med 200135:117-124)