My 9 year old loving cat was diagnosed with Hyperthyroidism. My options are....?

The medication is methimazole, and with the proper dosage there are few side effects! It can cause nausea and diarrhea if the dosage is too high. What your vet will do is "guess" at her correct dose based on weight, and then retest her blood in a few weeks. At that point the vet will adjust her dosage if necessary. They do quite well on it - my elderly cat (he's 16) has been on it for 4 years now. His dosage was increased last year, and we've never had a problem. I had one foster cat who's dosage was too high - she had diarrhea which wen away just as soon as it was decreased. As long as the cat is easy (or fairly easy) to medicate, it's really not a big deal at all.