- Optional fine needle aspirate (Day 15 only) Low Iodine Diet: You will begin a low iodine diet on Day 15, in preparation for Day 23 when you will receive radioactive iodine (131I). The study team will provide you with instructions on this diet. The low iodine diet will be continued until the whole body scan is complete. For those participants who will receive a therapeutic dose of radioactive iodine on Day 37, the low iodine diet will be continued until the Day 42 whole body scan is complete.