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I think I can help you out with your question.
There are a few common causes for weight loss with appetite.
First is hyperthyroid. Cats get benign tumors on their thyroid glad that causes it to become overactive. It can be diagnosed with blood work. Symptoms are a big appetite but weight loss. They can vomit frequently and seem very active. You can treat hyperthyroidism with medication, or cured with radioactive iodine treatment. Here is some further information http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=516
Second is diabetes. Symptoms are drinking more and urinating more, increased appetite with weight loss. Sever symptoms can be muscle neoropithy in the hind legs causing them to walk funny. It too is diagnosed with blood work. It can be controlled with diet and insulin. Here is more information about diabetes. http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=631
Third is an intestinal disease, like inflammatory bowel disease, or irritable bowel syndrome. It can cause cat not to be able to absorb the food they do eat, causing weight loss. It is harder to diagnose because there are no direct testing. Normally diagnosed by process of illuminating other easily diagnosed diseases. Diet, steroid therapy and vitamins are common treatment. Here is more information http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=598
Lastly, you always have to add kidney insufficiency to the list when there is weight loss. Normally cats appetite isn't great. There also can be vomiting and increased urination. It would be picked up with routine blood work.
I would take your kitty to the vet as soon as you can. The soon we can stop the weight loss, the better. Take a look at the website, and let me know if you have any other questions!