I assume she is on a medication for the thyroid disease? Usually once the medication builds in the system the weight will slowly come back. Many new age vets reccomend the raw food diet for animals with hypothyroidism. All animals are more apt to eat the foods they would naturally eat. Maybe do some research on it. I have one dog who was very sickly until we started the raw food diet but within weeks of starting this diet she started showing signs of improvement. I would tlk with her vet and see what they suggest. You may need to seek out a younger modern vet to think outside the box. We see a vet that even does natural remedies for animals. This may be a good option for a cat with low appetite.