thyroid radioactive treatment on a 18 year old cat?

I have been giving my cat the pills for the past couple of months. He is already up to 3 pills a day. 2 cans of food a day. He is losing lots of hair. Went to the vet and she say's it's a side effect of the pills. The vet said something about the thyroid radioactive treatment. Has anyone done it on a cat this old?