When I had a kitty nearing the end of her life (diabetic), my wonderful vet told me to keep a journal each day and mark it a 'good' or 'bad' day. When the 'bad' days outnumber the 'good' days, it would be time to consider helping her. Turns out I didn't have to help her, she made the choice on her own and went back to spirit quietly in my arms.
Instead of asking the vet, ask your kitty. Sit quietly with him, love him, stroke him, whatever would be normal kitty time. Tell him you know he is not well and if he is ready to go, you understand and you are ready to let him leave his ill body. Just hold him in your gentle focus and he will let you know in your heart what the answer is.
If it will be a comfort to you, and has been to me with mine that have returned to spirit, kitties (all animals for that matter) are not afraid of dying. They are spiritual beings and have been in the pure Loving Light and they know that is where they will return. The hardest part for them is leaving their human and furry companions behind.
I have treated two hyperthyroid kitties, too, and I know it is hard. The illness makes all their body functions speed up which accelerates any other problems. One of mine also went into kidney failure and the other in cardiac failure--both about that 2 year mark from when they were first diagnosed.
Offering you loving thoughts during this difficult time.
Purrs to you and kitty.