Relatively New Cancer

Usually the procedure for radioactive iodine scans is:
Stop thyroid hormones - 3-6 weeks prior - sometimes they will give you cytomel, a short acting hormone; this needs to be stopped 14 days prior to the scan and/or treatments;
- at 14 days prior, begin the low iodine diet - there is lots of info on this here:
http://www.thyca.org/ThyCa Cookbook 011804.pdf
And you may find this very helpful, too:
http://www.thyca.org/newly_diagnosed.htm

More information can also be located through either the online forum or local support groups in your area:
http://www.thyca.org/support.htm and here is a very busy list group - sometimes 100 plus messages a day, with over 3000 of us:
http://health.groups.yahoo.com/group/Thyca/

About a week, up to a few days before, or even sometimes the day of the dose, we have bloodwork done, to ensure the TSH is high enough, to measure our thyroglobulin level, and ensure we are not pregnant before swallowing the radioactive iodine.

Most docs like our TSH to be above at least 30 for scans, and some prefer above 50 for treatment doses.

My hospital usually gives me the dose on Tuesday, then does the scan on Friday. We continue the low iodine diet for about 24-48 hours after taking the dose, to ensure the best uptake in thyroid like tissues.

If you might be going forward with a (higher) treatment dose following the scan, you do need to remain on the low iodine diet until the treatment dose is completed, too.

There is some good info on procedures at one US hospital here - this is primarily for treatment doses, though - scan doses are usually very small - 2-5 mCi's, while our treatment doses can range from 29.9 up to 200mCi's:
http://www.cancer.dartmouth.edu/radonc/thyroid/index.shtml

Hope this helps a bit. There are a bunch of links about these things on my personal webpage here - just search under member name "Rustifox".