Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am concerned that Tara is getting very thin even with a good appetite.
While kidney failure can lead to weight loss as they lose blood proteins through the kidney, it also usually causes loss of appetite, and weight loss that way. Cats with only kidney failure don't eat well. I think she may have kidney disease, but that's likely not the only health problem she has.
When we see weight loss with an apparently normal or increased appetite we need to worry about disease processes that either don't allow her to digest and/or absorb the food she is taking (such as diabetes, liver disease, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as hyperthyroidism or cancer.
In an older cat the more common diseases we see include hyperthyroidism (a tumor of the thyroid gland that overproduces thyroid hormones) or if she were drinking more and urinating a tremendous amount, diabetes. Sometimes early on with diabetes we only see weight loss.
Both diabetes and hyperthyroidism are treatable or at least manageable.
Hyperthyroidism is controlled with daily oral medication (Tapazole, also called methimazole) or a very special diet called y/d both of which must be done for the rest of her life or treatment with radioactive iodine to selectively kill tumor cells which is usually a one time therapy. There are pros and cons to each therapy which your veterinarian can discuss with you after getting a firm diagnosis.
Hyperthyroidism is not painful, but elevated thyroid hormones can make them feel edgy or nervous and thus cranky, increase the heart rate and interfere with sleep, like drinking way too much coffee or energy drinks. Long term if left untreated it can lead to heart disease, hypertension, possible eye damage as well as kidney and liver damage. But if caught and treated many of these things can be reversed.
Diabetes is controlled with diet and insulin therapy.
Has she had any testing done to achieve a diagnosis?
Ideally she should have a complete blood count and biochemistry profile with T-4 checked as well as a urinalysis. These tests often give us the information that we need.
If those aren't diagnostic then it is time to move on to more specialized testing. I recommend checking vitamin B levels to look for signs of intestinal disease, a test for pancreatic insufficiency called a TLI, and an abdominal ultrasound to look for signs of changes in the walls of the intestines as well as evaluating her organs. She may need an endoscopy to collect biopsies of her intestines if they look abnormal.
If she has any evidence of dental disease then she may need some radiographs of her mouth and tooth roots, although dental disease alone rarely leads to weight loss.
In the meantime you might try feeding her a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian and make sure she has access to plenty of fresh, clean water.
Please let me know if you have any further questions.