It does usually only take one treatment (at least 90%, some facilities say 95% or more) of the time, with about 5% of cats requiring a second dose. It seems to be more effective when done early in the disease. The longer the cat is on methimazole, the more likely he/she will need a second or a higher dose. Occasionally cats will have a sore throat that resolves within a few days. Some cats will stop eating. This is not common, but it does happen occasionally, so it's good to be prepared to assist feed just in case. Thyroid storm is becoming more well known (http://www.avmi.net/NewFiles/Hyperth...inks/074.html)
. It happens when all the dying thyroid cells release T4 into the blood stream, ramping up the heart and breathing rates. Some vets will have the cat on heart medications before, during, and after treatment to help protect their hearts in case of thyroid storm. Cats with extensive kidney failure aren't candidates for the procedure, but cats with mild kidney disease may still be able to have it done. There are a lot of old rumors and information floating around about keeping a CRF cat slightly hyperthyroid to protect his/her kidneys. In the long run, however, this causes permanent damage to the heart and is not usually considered to be a good idea.
None of these things are very common. Methimazole has side effects as well, including itchiness, vomitting, etc. in the short term, and liver problems and/or anemia in the long term. Also, I-131 typically cures the disease whereas methimazole treats the symptoms. As a result, organ damage, particularly to the heart and kidneys, may continue to occur when the cats are kept on medication for long periods of time. Cats treated with I-131 have approximately twice the survival rate over cats treated with just medication (http://www.avmi.net/NewFiles/Hyperth...inks/071.html)