Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for Pricnches (although I'm not sure if this is her name or if this was a typo).
I can actually think of several different reasons why a cat this age might be losing weight and yet has a good appetite:
1. Normal since some cats tend to lose weight as they age.
2. Hyperthyroid Disease which means her thyroid is in overdrive and her metabolic rate has increased.
3. Kidney disease . While many of these patients will be drinking more water, not all of them do especially in the early stages of the condition.
4. Diabetes will often have weight loss in the face of an increased appetite although, again, most of these patients are also drinking more water and urinating more frequently.
5. Internal parasites or "worms" might cause weight loss if she goes outside but, in general, it wouldn't cause a noticeable amout.
Of course, some of these conditions might be considered “better” than others. For example, we can treat diabetes with insulin and/or diet. We have very effective treatments for Hyperthyroid Disease and many of these cats can do very well for long periods of time.
It can take up to 75% of kidney tissue to be damaged before changes are seen in the blood so this is not as good a disease to have; treatment is primarily focused on preventing further damage.
It's possible that a thorough physical exam might pinpoint a problem (such as an enlarged thyroid nodule) but usually bloodwork is needed to confirm or definitively diagnose once of the conditions I mentioned above. If everything looks normal and she's otherwise acting pretty much like her normal self, then I might assume that her weight loss is secondary to aging changes; however, this is primarily a diagnosis of excluding other possibilities if this makes sense.
I suppose cancer should be included on the list given her age but usually these patients demonstrate other signs such as a decreased appetite or vomiting/diarrhea in addition to weight loss. The latter also tends to be more sudden rather than gradual.
I hope this helps to provide possible explanations for the symptoms that you're seeing. Deb