FionaFiat... Radioactive iodine in sufficient amounts, basically, "kills" the thyroid, so it no longer produces hormones.
I've had the thyroid uptake scan and it's nothing to worry about. It's a much smaller amount of radioactive iodine and doesn't damage the thyroid (so they say)... All they do for the scan is give you the iodine pill, wait a few hours, do a scan to see how much of the iodine was taken up by the thyroid, then wait longer to see how much more of the iodine was taken up...
This is called a nuclear medicine scan. If they're going to give you radioactive iodine to destroy your thyroid, I can't see why they'd do the nuclear medicine scan, but if they do it, there's nothing to worry about - i.e. no pain, etc...
The radioactive iodine that destroys the thyroid will leave you with hypothyroidism and thus, dependent on replacement thyroid hormones for the rest of your life. That's the equivalent to those of us, including myself, whose thyroid has been destroyed by the autoimmune condition called Hashimoto's... It's a matter of taking a daily pill and making sure that Free T4 and Free T3 (the actual thyroid hormones) are kept within a range that's good for your body, not just a range determined by a lab or the NHS...
Radio iodine is nothing to worry about. You often only need a blood test first. You should be advised to stop Carbimazol 7 days before treatment, and resume it 7 days after treatment. They give you lots of advice about being radioactive for a few days. Limiting your contact with children example. I have had it twice, as it did not work for me. (Rare for it not to work!) Nothing to worry about at all, just taking a tablet under control conditions.