Why is my Cat loosing weight?

Brief：
He is 9 and has a twin sister. They are inside cats always have been. Brought him to a Vet hospital and they ran tests. Nothing came back out of the o.. 

Unfortunetly for my cat he had to be put to sleep a couple days later because he was basically in a coma. The stress from going to the vet made it worse. He was an inside cat as well. My cat Bobby lost weight continuously in his last years but was 16 when he died. 9 years is not really too old for an indoor cat. Bobby ended up having hyperthyroidism and he already had inflammatory bowel disease but I know that they gave him some Nutri Cal vitamins that helped him to eat more and put on some weight a few times. You can get those from the vet or even in some pet stores. It's in a tube and you just rub it on their lips or paws and they lick it off. I would also consider a second opinion to see if something was missed. Good luck. maybe the cat dose not like it kind of food you by it, maybe it's having allirgiec reaction or it could be a bulimica cat i would get your cats teeth checked, and get a second opinon and change the food may be he is fed up with that you can also get tins of i d food from your vets which is soft and build your cat up you need go see a vet right away Hi, Does your cat drink a lot of water? my cat started to loose weight and wail and the vet found he had Thyroid trouble, he had his Thyroid Gland out. He was about 9yrs old when this happened and he was fine after it he lived to be 17yrs old. I hope you solve your problem soon, as i know they are part of your family and you worry over them. POOR YOU

9 YEARS IS A GOOD AGE THOUGH

I KNOW CAT'S CAN HAVE WEIGHT LOSS DUE TO A NUMBER OF REASONS