I doubt it would be a diet problem (except for this last diarrhea which happened right AFTER the Fancy Feast) because he has always eaten the same thing.

His decrease in appetite was BEFORE the steroid, so hopefully his appetite will return WITH the steroid (as he needs to gain weight).

Yes, the blood work was good except for a LOW thyroid number, and the vet said that this is puzzling. Cats don't get low thyroid.

I think he is not feeling good, just by his actions. Somoething that can't be described, but a mama knows.