It can be very worrying when you notice your cat to start to lose weight, especially if they’re an older cat. Weight loss conjures up terrible thoughts and googling often makes things worse – a symptom as vague as weight loss can mean so many different things it’s bound to be a scary page of results.

We tend to notice problems in cats slowly. They’re masters at hiding signs of illness and disease and often live very independent lifestyles, some even going for several days without being seen. The fact that most cats toilet outdoors, unaccompanied, means that you may not notice symptoms such as diarrhoea or producing large volumes of urine. And since many cats have dried food down all day, changes in appetite can be hard to spot at first. This means that weight loss is often the first symptom that people notice in their cats, with other symptoms only coming after you start to keep a closer eye.

But what can cause a cat to lose weight?

Although there are lots of weird and wonderful things that can make cats lose weight, there are really only four that we vets see regularly. And when we’re presented with a new case, the first job is to rule these four out before we move onto less common things.

Renal Disease

Officially called ‘chronic renal insufficiency’ or ‘chronic kidney disease’, renal disease is a very common disease affecting 1 in 3 senior and geriatric cats. It is the end stage of a disease process where, due to small amounts of long-term damage and a lifetime of hard work, the kidneys become less able to function.

Instead of carefully filtering urine, retaining useful products such as protein, and getting rid of nasty chemicals, kidneys that are failing cannot effectively concentrate the urine or filter products.

The result is a cat who:

In addition to this, the body, losing protein in the urine, begins to digest muscles to get the protein it needs. Cats lose weight because of this combination: not wanting to eat and the destruction of protein stores in the muscle.

Other than drinking more, urinating more, eating less, and losing weight, other symptoms of kidney disease include:

  • vomiting
  • funny-smelling breath
  • a poor coat
  • pale gums
  • lethargy

It’s simple to test for renal failure – a urine sample and blood test are usually suggested, whilst blood pressure measurements can help to decide on an appropriate treatment course. For some kidney diseases, ultrasound scanning of the kidneys is also a sensible route to take. As the disease is a result of long-term damage, there’s no treatment available, but a special diet and some medications can help to reduce symptoms for a time.


Another extremely common disease, hyperthyroidism affects about 1 in 10 cats over the age of 10 years. In hyperthyroidism, the thyroid gland (which is found in the throat) causes the problem. This gland normally produces thyroid hormones, which control the body’s metabolic rate. In hyperthyroidism, the gland grows a tumour (95% of which are benign) which produces too much of this thyroid hormone and doesn’t respond to any of the body’s usual mechanisms to slow production.

With so much hormone floating around, the metabolic rate increases. This results in a cat who is always hungry but burns off all the food immediately, causing them to become skinnier and skinnier.

They may also:

  • be hyperactive
  • be more vocal
  • drink more
  • urinate more
  • have a poor haircoat
  • have vomiting and diarrhoea

A blood test is the best way to tell if a cat is hyperthyroid, although it’s sensible to check blood pressure, urine and even scan the heart, as heart disease often occurs in cats with hyperthyroidism. Treatment is with a special diet, surgery, daily medication or a one-off dose of radioactive iodine at a specialist clinic, and your vet can go through these options with you to choose the best for you and your cat.


Diabetes, or ‘diabetes mellitus’ is a lot rarer than the previous two diseases, but is still fairly common in the UK, affecting 1 in 200 cats. It’s also on the rise, making it a disease that most practices in Britain are seeing regularly.

In diabetes, the body becomes unable to control the amount of glucose (sugar) in the blood. Diabetes in cats is different to in dogs; cats suffer from Type 2 diabetes, where high levels of fat cause the body to become non-responsive to insulin. The cat is still making some insulin, but the body is unable to respond to it correctly.

Cats with diabetes:

  • Eat ravenously (but lose weight)
  • Or stop eating and start to vomit
  • Drink more and urinate more
  • May have a poor hair coat
  • May have weakness in their back legs

Testing for diabetes is, again, based on blood tests and urine tests. The level of glucose in the blood and urine is usually a good way to assess diabetes, and another blood test for fructosamine may be suggested to help with diagnosis and treatment planning. Treatment is with daily insulin injections, which most cats tolerate very well. Since obesity is one of the main causes of diabetes in cats, getting the cat’s weight under control may mean that they can have a reduced dose of insulin or even come off it altogether.


This is a bit of a weird catch-all header, as there are lots of different types of cancer in cats. However, all of them can cause weight loss. Once we, as vets, have ruled out the previous three diseases with urine samples and blood tests, we start to worry that some sort of cancer is causing the symptoms. Common cancers in cats include lymphoma (affecting the guts or kidneys) or leukaemia (which is often viral in cats), both of which affect blood cells and therefore are ‘invisible’ in the early stages. Squamous cell carcinomas, basal cell tumours and mast cell tumours occur in the form of lumps and bumps, so may be diagnosed earlier.

Some of these cancers leave clues on blood tests, but many will need more specific testing, such as sending biopsies to the lab or imaging such as x-rays and ultrasounds. Depending on the type of cancer, symptoms can include weight loss, diarrhoea, vomiting, respiratory disease, and lumps and bumps in various places. Treatment may be as simple as removing the lump but can also include medications such as chemotherapy or steroids.

The bottom line…

Although this list probably hasn’t raised your hopes much if your cat is losing weight, it’s important to remember that most conditions can be managed even if they can’t be treated – your cat can live for many more years and even regain the lost weight. Don’t forget to listen to and trust your vet; they’ll help you make the right decisions for you and your cat.

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