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.. 

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 ordinary. I feed him good cat food and brush him and play with him but he is still loosing weight. This started last Decmber and he is now 9 pounds. He also has been whining for attention alot in the past 2 years but latley it has gotten worse. His sister is fine she is not having this problem. Help! I am not sure what I should do?

Its got cancer, best you shoot it Have his thyroid checked. Get him checked! Maybe it has cancer. its probably doing drugs and staying out all night.. oh wait you mean your cat sorry Have you tried giving him a different kind of food? Maybe he doesn't like the current kind. Since you have already gotten him checked out by one vet, try another just for a second opinion. You could always try giving him some human food, like chicken that has no seasoning or anything like that or tuna, or try hand feeding him. the first thing that comes to my mind is diabetes. did they test for that? monitor his water intake also. male cats famous for uti. this should have been one of the first things the vet looked for, though. worst case scenario, when a cat starts his / her descent they continually lose weight - FOR YEARS. the whining does lend to a health problem, though. when he whines for attention pet him, slowly. use that opportunity to relax him. check for bumps, anything out of the ordinary. I do not know why people do it but cats are not domesticated animals. Your cat just probably wants some freedom. I let my cat go where ever he wants to go. He knows that he has a maximum of 6 days to come back home otherwise i am going to get a dog. I tell him that and he understands it. She's not. She might be losing weight, though. I'd take the scrawny moggy back to the pet shop you bought it from and demand your wonga back !! Get a second opinion. Sounds like the cat is sick. Make sure the cat was tested for Feline Leukemia even though it is indoors. I had a cat once that was only indoor became sick and lost weight the same way. The first time I took her to the vet they said they did not see anything wrong. Several months later she became sicker then we found out she had Feline Leukemia. I was told that if you as a person are around other cats that have it or even if they rub up against your belongings then your cat gets near it your cat can get Feline Leukemia. First signs too look for are weight loss and whining, then their coat gets oily feeling, then if you pick up the skin on the back of the neck and it stays standing up instead of going back in place. So watch him and go get a second opinion. Get him checked again I would take the cat to another vet for a second opinion and another round of blood work. I had a cat that was 3 years old and started losing weight. I took him to the vet and had him tested for everything. He had diabetes and Feline Infectious Peridontis. There is no medcine for the FIP but there is a vaccine if you give it to them when they are younger, but the diabetes can be treated. If you can afford the insulin.