Causes of Weight Loss in Cats

Weight loss isn’t a disease in itself; rather it is a sign of an underlying problem. Weight loss can be quite subtle as the cat’s coat can make it difficult to see when this is occurring.

Your cat’s weight should be checked at least once a year, during his annual visit to the veterinarian. He will then have a baseline to go on. Unfortunately, many pet owners put weight loss in senior cats down to old age, but it is extremely common for cats to have underlying medical causes. Weight loss may include loss of fluids (usually due to vomiting and/or diarrhea), loss of fat, loss of muscle mass or a combination of all three; it can further be divided into the following:

  • Impaired intake – poor appetite, inadequate diet.
  • Malabsorption – An impairment of food absorption such as a lack of digestive enzymes.
  • Excess nutrient losses – Vomiting, diarrhea, intestinal parasites.
  • Changed nutritional requirements – Organ dysfunction, hyperthyroidism or pregnancy.

The medical term for weight loss due to disease is cachexia.

How much should a cat weigh?

Cats come in all shapes and sizes, so it isn’t possible to give you a guideline on how much a cat should weigh. Instead of focusing on a number, veterinarians go by body condition.

Once the body condition has been assessed as underweight, normal or overweight, it can help to regularly weigh the cat to make sure the weight remains stable.

How to check if a cat is underweight

I have lost two cats to diseases that caused weight loss (kidney disease and cancer), and both times it happened right before my eyes. Because of their coat, weight loss had been masked. This is very unscientific, but both times I didn’t realise weight loss had occurred until I picked up the cats and noticed a huge difference in how they usually would weigh.

Physical signs a cat is underweight:

  • The cat feels very bony. The ribs on an underweight cat are easy to feel when you run your hands along the sides, and the spine can easily be felt when running your hands along the back.
  • The hind legs appear bony when significant weight loss has occurred.
  • There is a significant pinching in behind the ribs which should occur in all cats, but it is much more apparent in underweight cats.
  • There is a noticeable loss of muscle mass. Even slender breeds of cat should have good muscle definition, particularly around the shoulders.

Take a look at the chart at the end of the article for further information on determining if your cat is a healthy weight.


There are many possible reasons which may be further split into the following:

  • Acute (sudden)
  • Chronic (slow and progressive)
  • Other may refer to a condition that may be acute or chronic depending on the underlying cause.

Acute causes of weight loss:

  • Acute kidney disease: Resulting in decreased function leading to toxins building up in the body.
  • Infection – Bacterial, protozoal and viral infections can lead to acute weight loss due to anorexia and/or vomiting and diarrhea.

Chronic causes of weight loss:

  • Chronic kidney disease: Disease of the kidneys resulting in decreased function, which causes toxins to build up in the cat’s body.
  • Addison’s disease: An endocrine disorder caused by the adrenal glands not producing enough hormones.
  • Diabetes mellitus: A very common endocrine disorder caused by not enough insulin being produced by the pancreas, or insulin resistance, resulting in cells not taking in enough glucose.
  • Exocrine pancreatic insufficiency: Disorder caused by the pancreas not producing enough pancreatic enzymes to digest food.
  • Feline immunodeficiency virus: Viral infection, similar to HIV in humans.
  • Hyperthyroidism: A very common cause of weight loss in middle-aged to senior cats. Hyperthyroidism is almost always caused by a benign tumour of the thyroid gland which secretes excess hormones.
  • Hypertrophic cardiomyopathy: Thickening of the left ventricular wall of the heart. It may be primary or secondary, brought on by medical conditions such as hyperthyroidism or high blood pressure.
  • Inflammatory bowel disease: Inflammation of the intestinal tract with inflammatory cells.
  • Liver disease: A decreased function of the liver.

Other causes of weight loss:

  • Cancer: A malignant tumour. Cancer is seen more commonly in middle-aged to senior cats and is a common cause of weight loss.
  • Dental problems: Stomatitis, dental abscess etc., resulting in a reluctance to eat.
  • Dietary: Insufficient calorie intake (malnutrition) which may be due to poor quality food, inadequate amount of food, fussy eater, dominant cat (or dog) who eats the majority of the food.
  • Feline infectious anemia (hemobartonellosis): Caused by an unusual type of bacteria that attach themselves to the wall of red blood cells, destroying them in the process.
  • Feline leukemia virus: FeLV is a viral infection caused by reovirus, which is in the same family as the feline immunodeficiency virus. It is an oncovirus, meaning it can cause cancer. It also suppresses the immune system.
  • Gastric ulcer: Open sores develop in the lining of the stomach, due to increased production of stomach acid, certain medications, parasites, stress, Helicobacter pylori and systemic disease.
  • Glomerulonephritis: A disease of the kidneys caused by the inflammation of the nephrons.
  • Heartworm: Parasitic worm infection of the heart and lungs.
  • Pancreatitis: Inflammation of the pancreas.
  • Parasitic worms: Heavy infestation with hookworm, roundworm or tapeworm can lead to weight loss due to loss of nutrients.
  • Pregnancy and lactation: Both use up a lot of the female’s nutritional resources. If adjustments aren’t made to her diet to compensate for this, she will lose weight.
  • Stress or depression: Some possible causes include moving house, loss of a companion, new pet/person in the house, hospitalisation, being boarded).

This list is by no means complete; there are also many other possible causes of weight loss in cats.


Weight loss can be further split into polyphagic which means a progressive decrease in body weight in the presence of an increased appetite. Hyperthyroidism is a common cause of polyphagic weight loss, particularly in cats over eight years of age.

Weight loss is due to a decreased appetite, increased activity (which leads to more calories being burned), decreased access to food, quality of food, pregnancy, lactation etc.

Other signs related to weight loss may include the following:


Your veterinarian will perform a complete physical examination of the cat and obtain a medical history, including the cat’s age and other symptoms you may have noticed.

Hyperthyroidism and diabetes are common causes in older cats and may be tested first. Tests your veterinarian will commonly perform include:

  • Baseline tests: Biochemical profile, complete blood count, and urinalysis to evaluate the overall health of your cat and the organs, these tests may reveal an infection, kidney function, liver function, anemia, calcium levels, magnesium levels which can all paint an overall picture of your cat’s health.
  • Bile acid test: To evaluate liver function.
  • Abdominal x-ray or ultrasound: To evaluate the organs, look for tumours or blockages.
  • T3 and T4 blood tests: To detect elevated levels of the hormones T3 and T4 are performed. Some cats with hyperthyroidism may show normal levels of these hormones in their blood test. If this is the case, then a T3 suppression test may be performed. This involves taking a blood test to check the levels of T3 and T4, 7 oral doses of the thyroid hormone T3 and a blood test after the hormone was given. In a healthy cat, the level of T4 will drop, in a cat with hyperthyroidism the T4 levels will stay the same or increase slightly.
  • Fecal studies: To look for parasites.


The goal of treatment is to manage the underlying cause (if there is one) and provide supportive care and may include nutritional support such as appetite stimulants or feeding tubes in a cat with anorexia.

Full details on most of these medical conditions can be found in articles relating to the condition (linked above).

Systemic disorders

  • Kidney failure: Switch to a low protein diet, phosphorous binders, fluids to treat dehydration.
  • Addison’s disease: Lifelong replacement of missing adrenal hormones.
  • Hyperthyroidism: Radioactive iodine to destroy the tumour, surgery to remove it or a prescription diet that is low in iodine.
  • Hypertrophic cardiomyopathy: There is no cure for this condition, treatment is aimed at relieving symptoms including beta-blockers to assist with contraction of the heart muscle, blood-thinning drugs and restricting activity.
  • Diabetes: Switching to a low protein diet, insulin injections may be required if dietary management is unable to bring the condition under control.
  • Pancreatitis: Find and treat the underlying cause, if possible. Painkillers to relieve discomfort, anti-nausea medication, antibiotics if there is an infection and supportive care.
  • Exocrine pancreatic deficiency: Pancreatic enzyme extract, feeding a high protein diet. Antibiotics are sometimes prescribed to treat bacterial overgrowth in the small intestine.
  • Liver disease: Nutritional support, IV fluids, anti-nausea medication, corticosteroids may be useful in some cases.
  • Inflammatory bowel disease: Dietary management with a highly digestible diet, corticosteroids to reduce inflammation, immunosuppressive drugs, sometimes antibiotics will be prescribed.
  • Glomerulonephritis: Diuretics to remove fluid excess, low sodium/high protein diet, medications to treat high blood pressure, steroids to reduce inflammation.
  • Cancer: Treatment will depend on the type of cancer involved and if it has spread. If it is possible, surgical removal of the tumour, chemotherapy or radiotherapy may also be given.

Infections and parasites

  • Infections – Antibiotics for cats with a bacterial infection. Protozoal infections can be difficult to treat with medications although in some cases antibiotics will be administered. Supportive care will also be necessary which may include fluids to treat dehydration as well as nutritional support.
  • FIV and FeLV – These conditions cannot be treated; supportive care is offered, which may include keeping your cat in a stress-free environment, antibiotics to treat infections that may occur, keeping your pet parasite free, feeding a high-quality diet — regular checkups with your veterinarian.
  • Parasitic worms – Hookworms, roundworms, and tapeworms can be easily treated with anti-worming medications. Heartworms are more difficult to treat. Mild cases may require the use of an adulticide to kill the heartworms which come with risks and is generally only used in cats displaying symptoms of heartworm disease.


  • Pregnancy and lactating cats have higher nutritional needs than other adult cats and as such, need to be fed more.
  • Dietary – Feed a high-quality, balanced diet that meets the cat’s nutritional needs. Cats should be fed from separate bowls, in separate areas if necessary as some cats can hog the food bowl.
  • Stress – Finding the cause of stress and reducing it. Avoid changes in routine, schedule daily play sessions, manage inter-cat conflict, always give all cats a safe place to go on their own with different areas, perches, and beds.

How much food should a cat eat?

According to DVM, the best way to calculate your cat’s daily requirements is to use this simple formula. This calculates your cat’s RER (resting energy requirements) which is the amount of energy (in calories) expended in a day without any activity. RER is similar to basal metabolic requirements in cats.

Once you have calculated your cat’s RER, the formula then shows how to calculate your cat’s DER (daily energy requirements) which is several calories your cat should eat.

Calculate your cat’s resting energy requirements: 30 x your cat’s body weight in kilos (1 kg = 2.2 lbs) + 70.So, let’s look at some examples below.

  • Average cat who weighs 5 kg: 30 x 5 kg + 70 = 220 is your cat’s RER
  • Kitten who weighs 1 kg: 30 x 1 kg + 70 = 100 is your cat’s RER
  • Obese cat who weighs 6.5 kg: 30 x 6.5 kg + 70 = 265 is your cat’s RER
  • Underweight cat who weighs 3.5 kg: 30 x 3.5 + 70 = 175 is your cat’s RER

Now you have your RER (resting energy requirements), you need to calculate your cat’s DER (daily energy requirements). To do this, you take your cat’s resting energy requirements and then apply the following formula.

Calculate your cat’s daily energy requirements:

Growing kittens RER x 2.5
Normal desexed adult (maintenance) RER x 1.2
Intact adult RER x 1.4
Obese prone RER x 1
Weight loss RER x 0.8
Weight gain RER x 1.8

Now to calculate the DER (daily energy requirements):


  • A 5 kg cat who is on a maintenance diet and doesn’t need to gain or lose weight. 220 RER x 1.2 = 264 calories per day.
  • A 1 kg kitten who is growing. 100 RER x 2.5 = 250 calories per day.
  • A 6.5 kg obese cat who needs to lose weight. 265 RER x .8 = 251 calories per day. As your cat loses weight, his RER will drop, and you should adjust this formula. The same goes for kittens, a kitten’s weight will gradually increase, and therefore you will need to recalculate his daily energy requirements weekly.

A pregnant and lactating female should be allowed to eat as much food as she likes to stay healthy. Pregnancy and lactation use a considerable amount of the female’s resources, and she should be fed to accommodate these extra needs.

Follow-up care

Feed your cat a good quality, high-calorie diet to help with weight gain and check his weight once a week.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio