Fattening up my cat

My cat has hyperthyroidism and lost a pound in the last year. She has medicine but for a bit she was getting good at hiding from me when she heard the pills. I'm employing a ton of new tricks now to get her taking the meds every day twice a day.

But she's going to have a lot of difficulty gaining the weight back because of her condition. She's not really a "treats" cat but I feed her wet food every morning. I was thinking of trying her on kitten food because it's higher in fat and protein.

Can anyone think of a reason that would be bad? Or another idea to help her with the weight?