cat weight loss

Question
I have a black dometic about 12yrs old. I went from feeding him 1/3 wet food a day to 2cans & he's not gaining any weight back from having fleas this summer. I gave him worm tablets a few fimes. I paid $25 for 3 tablets-praziquantel. Other than that, he's normal in every other way,& has a very good appatite. So I was wondering what could be wrong with him-why can't I get him to gain weight? Thanks for any help you can offer--Maria  P.S.-
he also eats hard food too.

Answer
There is a condition referred to as the "Typical Old Cat Syndrome" (TOCS) where you can pour the calories to the cat and they don't gain weight.  They even lose weight in many cases.  But before just writing this off to TOCS, a complete blood panel including a thyroid test should be done.  Some older cats will develop tumors on their thyroids and that can cause weight loss although that is usually associated with vomiting food.