Assuming that this radio iodine scan is done after removal of the whole thyroid gland, it can detect only areas of cancer spread which take up radio iodine. That means only those areas of thyroid cancer
spread are detected. Again, those areas which do not take up radio iodine (like less differentiated varieties of thyroid cancer) are not picked up.
In an intact thyroid, these scans DO NOT pick up metastasis (areas of spread)
In a patient who has undergone total removal of thyroid gland, it picks up areas of spread as described above
Radio iodine scan is limited for some varieties of thyroid cancer only (papillary and follicular cancers). Hence, these scans are not meant for other varieties of thyroid cancer and for other cancers