HELP! My cat has hyperthyroidism and dimentia?

My cat, God love him, is 17. He developed hyperthyroidism about 1.5 yrs. ago. Started on 1/2 a pill methimizole AM & PM. Then new tests were run at his check up and its 1 pill AM & PM. The vet seems to think he's too old for surgery or radioactive iodine and not very helpful. Now he is randomly pooping everywhere. No kidney problems, clean boxes throughout the house. He just is insatiable in regards to food, esp. if I am eating, I think he just forgets he ate. I have never had a cat this long - does he have kitty alzheimers? I hate to put him down but is 17 kind of too old for the other treatments - I don't think the pills are cutting it.