I was referred to nuclear medicine and currently awaiting treatment in the next week but both my endocrinologist and the radiologist explained how they figure out if it’s spread is after radioactive iodine attaches to any thyroid cells (it includes the cancer that has spread as they originate as thyroid tissue cells), the total body scan will light up and this is used as a baseline. They repeat tue therapy and scan in 6 months. If there were any signals seen previously, they second treatment will be a higher dose. They keep repeating it till it’s gone and then monitor to see it it returns. Thyroids are not supposed to regenerate but there have been reports of the entire organ growing back. Best wishes to you.