We went the radioactive iodine route ourselves with Sophie. She was 14 at the time. I think it's worth noting that there is some risk that in treating the thyroid disorder, you worsen or create a renal problem. Plus, a hyperactive thyroid can make it difficult to even determine if the kidneys are functioning because the increased bloodflow caused by the hyperactive thyroid can mask kidney disease. So, it's really important to monitor kidney functioning closely when treating a hyperactive thyroid. And talk to your vet about a renal supportive diet.

With our cat Sophie, we decided to go with RAI, rather than a long-term use of Tapazole because she was very difficult to dose. In retrospect, we may have been better off using a transdermal with the hope of balancing her thyroid and kidney issues. While one of her kidneys was functioning just fine, her other was kaput. About seven months after RAI, she experienced renal failure with the remaining kidney. She didn't respond very well to further interventions, and we ended up having to put her to sleep at that point.