I see she is taking thyroid meds. Tapizole can cause inppetence or she may have abscessed teeth.
Weight loss is usually related to hyperthyroid. The cats eats great but the excess thyroid hormone burns up all the calories.
She may have any food that does not contain onion or garlic powder.
It is better if she does not eat dry food. Canned will ensure she gets adequate fluid. Cats her age can become dehydrated more easily. A dehydrated cat is not motivated to eat.
She should also be encouraged to drink as much as possible to keep the kidneys and bladder flushed. You can give her one daily drop of unsweetened (only) cranberry juice. That will reduce the PH of her urine and make her bladder inhospitable to UTI bacteria.
Although overweight can cause diabetes and other health problems, it is very dangerous for a cat to suddenly lose weight. That is because a cats liver becomes accustomed to a certain caloric intake. If that amount is cut (for any reason) the liver will think it is starving and grab protein and fat from the cats muscles and organs. This can cause liver and organ failure, muscle wasting and lead to a weakening of the immunity and a disorder called Hepatic lipidosis:
Feed her only soft foods like canned or baby food with no onion or garlic.Try to feed every two hours as that is the best way for her body to metabolize the calories.Add water to the canned food to make it easier to lick up..
Cats do not have whims like people. Whenever they stop eating they are either ill or in pain. If she has no other symptoms like fever, constipation, infection, etc then here is something going on that is difficult to ascertain - but it exists nevertheless. Sometimes a cat can have an abscessed tooth or root that is in the gum.They can ingest a piece of plastic that in not perceivable in an xray. They can be mildly dehydrated or have a metabolic or organic imbalance that is too slight to read on a test. She may also feel nauseated.
If your cat stops eating and is not diabetic begin by giving her two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When she feels hydrated she will lick up the gruel on her own.
Older cats are often calcium deficient - she can have cows milk if she has hard stool which is another cause of inappetite.
A tsp of plain yogurt contains aciophilus and will firm the stool by adding good bacteria.
She can have a daily vitamin suppliment but is it more improtant to get her eating.
So to review:
Canned cat food with water added.
Boiled chicken, fish
baby food with no onion
tuna and the water.
Please let me know if you have more questions and how your baby is doing.. I will always be here for both of you.
Please print all the links for future reference even if they do not apply now.
I hope she feels better very soon,