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This is Dr. Christian. Welcome to JustAnswer.
Thanks for the information. At her age I tend to think of several more common conditions that can cause the symptoms you've described:
Hyperthyroidism- This is caused by a tumor in the thyroid gland that causes overproduction of thyroid hormone. It causes a cat to have a very high metabolism. The tumor does not spread to other parts of the body. Symptoms that can occur are eating with weight loss, vomiting, increased thirst and urination, diarrhea, fast heart rate and heart diseases. It is a very treatable condition with medication or radiation treatment. Diagnosis is by bloodwork. Here's a link: http://www.petplace.com/cats/hyperthyroidism/page1.aspx
Diabetes- Diabetes is a lack of control of blood sugar. The blood sugar becomes very high. The symptoms are weight loss, increased urination and thirst, more prone to infections, occasional vomiting and eventually loss of appetite and weakness. Treatment is usually a diet change and insulin injections. It is diagnosed with bloodwork and sometimes a urinalysis. Here's a link: http://www.petplace.com/cats/diabetes-in-cats/page1.aspx
Exocrine Pancreatic Insufficiency (EPI)- The pancreas helps digest food. When it is not functioning well then the cat appears malnourished. It can be diagnosed with specific bloodwork and is very treatable with lifelong enzyme supplementation. Here's a link: http://www.petplace.com/cats/exocrine-pancreatic-insufficiency-in-cats-epi/page1.aspx
Inflammatory Bowel Disease (IBD)- Inflammation of the wall of the intestine will cause a decreased capacity to digest food leading to malnourishment. Symptoms can include diarrhea, weight loss and vomiting. This is sometimes caused by a food allergy. Diagnosis is usually made with an intestinal biopsy but many times treatment is prescribed to monitor for improvement. Treatment usually involves steroids, diet changes and sometimes antibiotics. Here's a link: http://www.petplace.com/cats/inflammatory-bowel-disease-in-cats/page1.aspx
Since she isn't urinating more than usual, diabetes and hyperthyroidism are less likely. It's definitely best to have her seen to determine whether any of these condition is is occurring. More than likely your vet will recommend some blood work. If you have more questions please reply back.
Basic blood work that would also check for thyroid problems would cost around 100 to 130$ in my area (Tennessee) but you're best bet would be to call your vet. Costs can vary throughout the country.
Canned pumpkin would be a better choice. The fiber is very good for constipation and safe. You can try a 1/2 to 1 teaspoon daily.