Radioactive iodine therapy has been used in the treatment of thyroid cancer for more than half a century.While the concept may be simple, the risk of secondary cancers such as leukemia can be avoided only with a clear understanding of the underlying mechanism and following safety guidelines.Yale-New Haven Hospital has one of the highest volumes of thyroidectomy in the United States, which resulted in a very busy practice for radioiodine ablation and therapy for metastases.In this talk, I will review basic radiobiology and share the considerations in establishing our protocol,including the advantages and disadvantages ofa dosimetry protocol.