You need to be on the Low Iodine Diet. If you go to thyca.org and there is the low iodine cookbook pdf
on the left hand side. Remember, it's not necessarily a no salt diet. You can have salt, just the non-iodized kind. And no sea salt.
I was told to go on it a week before my treatment, which ended up being almost two weeks, because my treatment dose was given late in the week on the second week. I then had to stay on it for 48 hours afterwards.
Hope that helps!