Go for the blood work. I have one kitty with hyperthyroid, one with diabeties, and one young and overweight (Bonnie, Herald, and Calypso respectively). Three cats, three different diets. I get my exercise keeping Herald (diabetic cat) from eating out of Bonnie's bowl.

I'll have to try Iams for Bonnie. She doesn't seem to like Science Diet anymore, but tries to steal Herald's SD prescription food all the time. I try to tell her that she can have the high calorie food, doesn't need low fat!

Hope your kitty gets to feeling better.