Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your cat.
Even though he's doing well on his medication, has a T4 been checked recently?
A t4 is a blood test which is usually high in Hyperthyroid cats (the problem I assume he has) and the value that we typically use to diagnose this condition in cats.
Once medication is started, a recheck of this value is suggested about 4-6 weeks to make sure that the patient is receiving the correct dose. Sometimes it has to be increased or decreased.
Thanks for the answers to my questions and the additional information.
In general, most owners have just the opposite problem with their cats....getting the weight off rather than putting it on!!
Usually I'm not in a great rush to get a thin cat to gain weight since from a longevity perspective, it's better to be a little on the thinner side than to carry even a small amount of extra weight (similar to humans in that respect). Usually once the underlying disease process which caused the weight loss in the first place is resolved or corrected (which it can be in this case), most cats will reach their appropriate body weight.
So, I don't want to fatten these cats up too quickly or much at all.
There are prescription diets which contain a higher amount of fat which can help put weight on: Hill's a/d or Critical care/Recovery diets are fine for the short-term but I wouldn't want to use them for the long term.
Another option would be to research foods on the following link (http://catinfo.org/docs/FoodChartPublic9-22-12.pdf) and select those with a higher fat and carbohydrate content in them. This link only provides percentages for canned or semi-moist foods but dry food tends to have more carbohydrates which can also lead to weight gain for some cats.
I hope this helps. Deb
You're more than welcome.
From what you've said, she sounds quite healthy even though she may not be at quite the weight you'd want her to be. Perhaps it won't take much to get a little bit more weight on her with just a few modifications in her diet.
Best of luck with her. Deb