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Why is my cat losing weight?

There are many reasons for a cat to lose weight. Most of the time there is a medical reason for weight loss in cats.

Reasons for older cats to lose weight.

  • Affects older cats - usually older than 10 years.
  • These cats have a good appetite. Often they will have a ravenous appetite.
  • Hyperthyroidism is diagnosed by a simple blood test.
  • Hyperthyroidism can be treated with medication or with radioiodide therapy.
  • You can read more about this condition by following this link to hyperthyroidism in cats.

Kidney Disease:
old cat
Older cat with her neck shaved for blood collection. Photo: Mel B. Flickr
  • Causes an increase in the amount of water a cat is drinking and an increase in how much urine is produced.
  • Can come on very gradually.
  • Usually causes very slow weight loss.
  • When kidney disease first starts, it is called renal insufficiency. At this point most cats will have a good appetite. As kidney disease gets worse we call it renal failure. At this point, appetite decreases and we may have vomiting as well.
  • Is diagnosed with blood and urine tests.
  • While there is no cure, there are many things that we can do to extend the cat's life.

  • There are many different types of cancer, and often they can be difficult to diagnose.
  • Cats with cancer will often lose weight very quickly.
  • Most of the time when a cat has cancer their appetite will decrease.
  • While we have treatments for some types of cat cancers, for many of them we don't have cures.


This is discussed below, in the section about reasons why young cats lose weight.

Inflammatory Bowel Disease:
  • IBD can cause a cat to have chronic diarrhea. This can result in weight loss.
  • Many cats with IBD have chronic vomiting. In fact, some cats with IBD have no diarrhea at all, but vomit very often.
  • IBD can be difficult to diagnose. We usually do tests to rule out other conditions first. The best test for IBD is to do intestinal biopsies.
  • Many cats with IBD can be treated with small doses of steorids such as prednisone.

Reasons for a young cat to lose weight.

  • Diabetes causes a great increase in thirst and urination.
  • It can cause significant weight loss.
  • Some cats with diabetes are very hungry.
  • If left untreated, diabetes can cause a cat to walk on their hocks of their back legs. This is called diabetic polyneuropathy.
  • Left untreated for long enough, cats can get a serious condition called diabetic ketoacidosis and can die.
  • Diabetes can be treated, but almost always needs insulin injections.
sick kitty
Sick Kitty - by Pete Markham, Flickr
  • When a young cat is not able to gain weight it may be because of a serious virus such as Feline Leukemia, FIV, or FIP.
  • Feline Leukemia and FIV can be diagnosed with a blood test. FIP is a little more difficult to diagnose.
  • Unfortunately there is no cure for any of these viruses.

Not being spayed or neutered:
  • A cat that is not spayed or neutered burns a lot of calories and often will not gain weight well.

Foreign object ingestion
  • If a young cat is repeatedly vomiting and losing weight it may be because he or she has swallowed a foreign object.
  • Many foreign objects can be seen on xray, but not all.
  • Often a surgery is needed to remove the object.
  • If the object moves into the intestines then this can be life threatening.

Weight loss myths

The following conditions are NOT normally a significant cause of weight loss in pets.

  • Tapeworms: Many people think that if a cat is losing weight they must have tapeworms. While tapeworms are gross, and need to be treated, they rarely get to the point where they cause significant weight loss.
  • Going outside more: I often have clients tell me that their pets have lost weight because they are now outside more. While they may lose some weight by going outside, there should not be a dramatic weight loss.

Still need help?

While these are the most common reasons for weight loss, there are other possible reasons as well. If you would like to consult with me about what could be going on with your cat, you can click here to Ask Dr. Marie a question.

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Dr. MarieDr. Marie is a veterinarian who practices in a busy animal hospital in Ottawa, Ontario. She created Ask A Vet Question as a resource for good, accurate veterinary advice online. Dr. Marie treats dogs, cats, hamsters, guinea pigs, and rats. She has been a vet since 1999.

Is an online vet visit just as good as a trip to your veterinarian? No! But, many times, asking an online veterinarian a question can help save you money. While Dr. Marie can't officially diagnose your pet or prescribe medications, she can often advise you on whether a vet visit is necessary. You can also ask Dr. Marie for a second opinion on your pet's condition.