Heck we don't even take them in every year! For one thing, Rascal (the oldest one) is near impossible to catch. She reads our minds the day before her appointment and hides. When she hides, she is not to be found. Don't know how many appointments I've cancelled because we didn't know where she was. For another they are 100% indoor cats, no way they are going to catch any of the diseases cats are vaccinated for annually. We only take them in for the required rabies shot. Twice a year might be reasonable for an animal that has known health issues. If, like Rascal, they have declined in some manner - time for the vet to check.
We have medicine we give Rascal twice a day in her food. Started Dec 1 and have an appointment for a blood draw next Friday so they can see if she is responding and that there is no impact on other organs. BUT ... assuming all is going in the right direction we are going to do the radioactive iodine therapy. Not cheap but the medicine, after shipping costs, is $65 a month. She doesn't have to live many years to come out ahead with the one time iodine treatment.