I wish your vet could meet my vet. Mac is a senior kitizen (15) and he takes those thyroid pills, too. The last time he was checked, his kidney were pretty good. That might have changed, but for now, he's doing pretty giood for an old guy!
Anyway, when I asked my vet what kind of diet he should be on she told me that at his age, anything he wanted and would eat. She said that older cats especially should eat regularly and if they won't eat the "good-for-them" food, then feed them what they like. Grandma is more likely to live longer if she eats "the bad stuff" regularly than if she eats nothing at all! When our kitties get a certain age, it is sometimes best just to love them and keep them happy.