I had a cat, Santa Fe, (rescue from the SPCA of Texas many years ago) who was diagnosed with hyperthyroidism. I elected to pay the $1500 for radioactive iodine treatment at a specialty vet clinic in north Dallas. She did gain appetite and put on weight, however, she went on to pass from heart disease.
I fostered a cat named Parker last year who also underwent the radioactive iodine treatment in New Mexico. After the treatment Parker's blood work began to show kidney issues. He continues to be medicated and resides as the "greeter" at Santa Fe Cats - a cat-only boarding facility run by Felines & Friends.
One of the challenges with hyperthyroidism is that it often masks underlying issues, such as heart disease and kidney function. Up until recently, not much was known about the disease in cats; it wasn't diagnosed until the late 70s by Dr. Mark Peterson.
What's even more interesting is what Dr. Peterson thinks is the cause of thyroid disease in cats: PBDEs (polybrominated diphenyl ethers) - flame retardants that were commonly used on household goods in the early 70s. It hasn't been until recently that the U.S. has begun to phase the use of these chemicals out.
It is thought that cats injest many more PBDEs because they walk on the floor, sit on furniture, then like their feet.
The New York Times released an in-depth article about this issue and I encourage you to read it here.