Why is my cat losing so much weight?

A 13-year-old cat's age is equal to 69 human years. Older cats are usually a bit bonier and may have some extra skin hanging, or protruding shoulder blades or hips. If your vet can find no internal or external reasons for your cat's weight loss and since he is still eating well, his loss of weight can probably be attributed to the fact that he is getting older.

You might want to add a couple of drops of Pet-Tinic liquid vitamins to his water dish. That's easier than trying to drop it in his mouth because that stuff will stain. Your vet probably has some, or he will order some for you. A 4-fluid-ounce bottle costs about $15, and that will be plenty to last for a long time.

A senior cat I once had would drop a few pounds during the summer because he was more active then, and pick the weight back up in the winter.

Vets ASAP.

Maybe cancer? My first cat Mitten had cancer at that age and went down to half his weight when I took him in - he was always a 14-15 lbs cat (not fat) and he only weighed about 6-7 lbs when I put him down. He was a walking skeleton and could not keep food down or it would go right thru him and he couldn't make it to the litter pan in time.

Geriatric cats quite often lose a lot of weight / muscle mass.

If the bloodwork showed the internal organs working well then it's just a senior cat thing. Did the vet also check the cat's mouth for dental problems because if the cat's mouth hurts they can't eat and they starve!

I would recommend a whelping pad (for dogs and cats to give birth on) and wrap it in a pillowcase. My 19 year old cat slept on it for several years from October through March. It gives out just a small amount of heat 24/7 and is great for old bones.

http://www.2ndchance.info/oldcat.htm

http://pets.webmd.com/cats/guide/aging-cat-behavior

Cats often lose weight when they get older,they may still be healthy,ask the vet if it is ok.

my old cat lost a bunch of weight like that too till I noticed he was having trouble moving around, due to arthritis in his hips. I started him on a glucosamin and chondroitin supplement made for cats and then gave him treats and fed him in ways where he didn't have to get up and go to the food. I had also noticed he no longer crouched when he peed, which is what actually tipped me off. After a couple of weeks on the supplement, he seemed to have less pain and started crouching again when he peed.
I also noticed that my old man cat ate better in the summer(when it was warm) which is also when MY arthritis is better so I found ways to keep him warmer...bought a cushion that reflected his body heat back to him, got him a heating pad for cats,etc. Warm and supplemented, he ate much better and regained some of his lost weight.
Did your vet also check his mouth? Maybe he has a bad tooth?

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There is a number of reasons and only a vet can diagnose. My cat Herbie is a golden oldie she is currently being tested for hypothyrodism and kidney issues because she has lost a lot of weight too. But losing weight in old age is common too. Put her on a senior food, how is her teeth? If she has bad teeth this could be another reason for weight loss. I think you should seek a second opinion. I would.