Why has my seventeen year old cat become very thin even though he eats well?

1. Age-related changes. As cats age, they tend to lose muscle mass and gain body fat. This is a normal process that is caused by a decrease in metabolism and an increase in the production of cortisol, a hormone that breaks down muscle tissue.

2. Dental issues. If your cat has difficulty eating due to dental problems, such as loose or broken teeth, gum disease, or oral tumors, he may not be getting the nutrients he needs.

3. Chronic diseases. Some chronic diseases, such as kidney disease, liver disease, and cancer, can cause weight loss even if your cat continues to eat well.

4. Parasites. Parasites, such as worms and fleas, can steal nutrients from your cat and cause weight loss.

5. Hyperthyroidism. This is a condition in which the thyroid gland produces too much thyroid hormone. Hyperthyroidism can cause weight loss, increased appetite, thirst, and urination, and diarrhea.

6. Diabetes mellitus. This is a condition in which the body does not produce enough insulin, a hormone that helps glucose (sugar) get from the blood into the cells. Diabetes mellitus can cause weight loss, increased appetite, thirst, and urination, and diabetic ketoacidosis (DKA), a serious condition that can lead to coma or death.

7. Medications. Some medications, such as corticosteroids and chemotherapy drugs, can cause weight loss as a side effect.

If your cat has suddenly lost weight, it is important to take him to the vet for an evaluation. The vet will be able to determine the cause of the weight loss and recommend treatment.