I would be happy to try to help answer your question regarding Dusty. One common reason that cats eat well but lose weight is a malabsorption or maldigestion issue. There is a blood panel including a folate, cobalamin, TLI, and PLI level that is sent to Texas A&M Veterinary University. This test will help rule out a maldigestion/malabsorption disorder. If there is evidence of this on the blood panel, antibiotics, a food
change, or a food supplement may be recommended.
Another concern would be GI lymphoma, a very common form of cancer in cats that is not seen on routine blood work. This is a very treatable disease and most cats live several years when the disease is treated (treatment is typically prednisolone sometimes combined with an oral treatment called Leukeran). GI lymphoma is diagnosed by an ultrasound examination (thickening of bowel loops is found) and sometimes a biopsy is recomended (although if this is highly suspicious on ultrasound, most vets will begin treating without performing an invasive biopsy). Inflammatory bowel disease is very similar to GI lymphoma and looks similar on ultrasound exam. Fortunately it is also treaed the same as GI lymphoma and prognosis is good with treatment. If your clinic cannot perform ultrasound, x-rays of the abdomen may give some insight as to whether either of these diseases might be a factor. Attempting a trial of prednisolone is not a bad idea if your veterinarian is suspicious of GI lymphoma or inflammatory bowel disease.
Lastly make sure the blood work that they performed includes kidney
values and a glucose value. It is not a bad idea to re-test the thyroid in 2-3 weeks as well, as sometims we get falsely low results.
I hope this information helps,