Here's a post I wrote ages ago on Duffy's trip through Radioactive Iodine Therapy
. Duffy was 14 years old when she had the treatment.
The clinic Duffy went to for her I-131 treatment is North West Nuclear Medicine their site is excellent for explaining the steps - click on "Cats & Owners"
The procedure basically is a very small injection via needle that targets only the thyroid gland & out of control cells. Actual treatment takes minutes after that the rest of the time in clinic is to allow the majority of the radiation to exit their body before going home.
The cats stay in-clinic for 7 days - it's required by law, there will be other cats in the same room but they are all kenneled separately so no direct contact between them. The cats are allowed out for playtime & they'll get pets & cuddles from the Drs & staff that are allowed in the treatment areas - you will not be allowed to visit but will likely get phone updates regularly. You can bring an old tshirt you've slept in for a few days & not washed to leave in Chico's kennel - one you don't mind throwing out as you will not get it back, same with a catnip pillow or favorite toy mouse etc you can bring them to the clinic but won't get it back because of the radiation.
After the 7 day in-clinic stay the cats go home, you must wear rubber gloves when cleaning the litter box as they will shed the I-131 for approx 14 days after initial treatment. They can play, eat & interact with everyone in the home as normal - it is recommended they not sleep in close contact with the family for the first week as there is still small amounts of radiation. You can cuddle & have him sleep on your lap etc usually they say for short periods of time, not hours & hours at a time.
In the link above for North West Nuclear Medicine under "Cats & Owners" go to "Documentation" and you will find Home Care Instructions so you can read those to see what after care entails before even taking Chico in.
There is actually more radiation in a human dental xray than what will remain in your cat after treatment. It honestly sounds worse/harder than it is.
Feel free to post any questions here or to me by pm