Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am concerned that your girl Dizzy is getting very thin even with a good appetite.
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 cancer.
In an older dog the more common diseases we see include diabetes and cancer, but primary intestinal disease is possible too, or a pancreas that is not producing enough digestive enzymes.
Ideally she should have had a complete blood count and biochemistry profile and thyroid profile checked as well as a urinalysis. These tests often give us the information that we need. It sounds like some or all of this has been done.
If those aren't diagnostic then it is time to move on to more specialized testing. I recommend checking folate and cobamalin 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.
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.