Preparation. Some diagnostic centers ask patients to undergo a 2 week low-iodine diet prior to a thyroid uptake and scan. This is because the study uses radioactive iodine (i-123 or low i-131) for the test. The iodine contained in food (seafood, table salt, etc.) may compete with the iodine used for the study, and thus results may be biased.Ideally, for the study your thyroid gland should be "hungry" for iodine.
Unclear reference. If you mean why do you have to be on a low iodine diet before a nuclear thyroid scan, the reason is that ingesting a large amount of iodine from seafood could saturate the thyroid transport channels and interfere with the test. If you're asking what the diet is i imagine you would need to avoid using iodized salt, eating seafood and processed foods supplemented with iodine.