After radio-iodine treatment all cats will return to normal (“background radiation”) by 28 days.

Keep young children away from your cat at all times. Keep your cat in a room on their own the whole time, to which only designated carers should have access. Consider using door locks, if appropriate. If contact does occur to someone other than the carer, thoroughly clean for example the child’s hands (or wherever contact occurred). If a child and the cat are to be in adjacent rooms, please discuss with us what the wall is made of, and how far apart they will be.

Anyone of child-bearing age should have no contact with your cat or with your cat’s litter tray. This ensures that anyone who is or may be pregnant is kept away from radioactivity.

Do not spend more than a few minutes at a time in close proximity to your cat. It is safe to handle your cat but avoid close contact such as hugs or sitting on your lap. Your cat must not have access to your bedroom.

Always wash your hands thoroughly after handling your cat, and especially before you eat or drink.

https://www.hyperthyroidcatcentre.co.uk/cat-owners/radioiodine-treatment/after-radioiodine/