I had thyroid cancer and no longer have a thyroid, so I'll well versed in all things thyroid. Actually, technically I still have it; I have microscopic cancer cells there that we're watching, so now what I have is classified as "cancer as a chronic condition."
Disclaimer that I have not had this specific procedure done (because there's no thyroid to check the function on) but to kill off my thyroid, they used radioactive iodine, which is what they'll give you, but a very small dose so it doesn't damage your thyroid. The thyroid is a sponge for iodine, which is why they're giving you RAI. It basically makes your thyroid light up when it takes it, and that tells them how well your thyroid is working. This isn't a painful test and the RAI itself doesn't have side effects that would make you feel bad.
The only thing I will say is be very vigilant about dental health for a while after this test because the radiation can get into your saliva. So make sure you are brushing twice a day, flossing, etc. I'd even do it three times a day for one to two days.
But otherwise, you'll be fine.
If you want to PM me, feel free. I have been dealing with this for 6.5 years now, so I am a wealth of knowledge on thyroids, tests, meds, etc.