Radioactive iodine (I131) is injected into hyperthyroid cats. The iodine is taken up by the thyroid, which is destroyed by usually one injection. The radioactivity is then excreted by the cat in its urine and the owner must give the cat thyroid drugs for the balance of its life.
To answer your question, no, it is nothing like chemo. Usually the cats feel no ill effects at all. In fact, most regain their appetites a couple of days after treatment. Their thyroids make them much sicker than the I131.
The problem with this treatment is that it is cost prohibitive for many people and requires the cats stay at a radiation safe facility for about a week while the radiation decays to an acceptable level. (I131 has an incredibly short half life, too.) Their urine is very radioactive and this obviously poses a problem if people need to have their cats at home.
Very old kitties may do fine for the treatment, but die of stress or "a broken heart" while they are in the relative isolation of the radiocat facilities. Their immune systems crash from the stress of being away from home and no other reason. The phenomenon still amazes me.
Thank you for that info Darth.
Yes, our kittie wouldn't make it through that much time in isolation.