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Weight loss can be due to simple problems of feeding and nutrition, or can be due to a variety of medical conditions that result in poor digestion, decreased absorption of nutrients, or loss of nutrients from the body.
Dietary problems. Weight loss occurs when a cat’s diet does not contain enough energy to meet the body’s needs. This could mean the cat just is not getting enough to eat or is eating a poor-quality food, but it could also mean that the cat has unusually high energy requirements such as rapid growth, pregnancy, or intense physical activity. For example, young active kittens need extra energy and specific nutrients for growth and development; they could easily be underweight if fed an adult diet only. Any pet fed a low-quality diet could be at risk for weight loss due to incomplete nutrition or poorly digestible ingredients. Your veterinarian can give you guidelines about the best food for your cat and how much to feed.
Disorders and medical conditions. These include difficulties chewing and swallowing food, diseases such as hyperthyroidism, parasites, infectious diseases, cancer, and disorders of the kidney, heart, liver, pancreas, or intestines.
Finding the cause of a cat’s weight loss usually starts with a complete history and physical examination. A cat’s history of illness includes details about the quantity and quality of food being fed, changes in appetite and activity, changes in thirst or urination, and other signs of illness such as vomiting or diarrhea, and so on.
"Finding the cause of a cat’s weight loss usually starts with a
complete history and physical examination."
Physical examination involves checking the entire cat, listening to the heart and lungs with a stethoscope, and palpating the abdomen (gently squeezing or prodding the abdomen with the fingertips to identify abnormalities inside the body). A complete physical examination may give clues about the cause of the weight loss; for example a kitten with a “pot-bellied” appearance may have intestinal parasites; an elderly cat with a mass in the neck region could have thyroid disease.
The cause of the weight loss may not be clear on physical examination and your veterinarian may recommend doing screening tests. These are a series of simple tests that provide information about the overall health of your cat and may provide further clues about the underlying problem. In a cat with weight loss, the most common screening tests would include complete blood count (CBC), serum biochemistry profile, urinalysis, parasite testing, and evaluation of thyroid hormones. Based on the results of these screening tests, additional specific tests may be recommended.
A) Complete blood count. This is a simple blood test that provides information about the different cell types in blood. These include red blood cells, which carry oxygen to the tissues, white blood cells, which fight infection and respond to inflammation, and platelets, which help the blood to clot.
The CBC provides details about the number, size, and shape of the various cell types, and identifies the presence of abnormal cells in circulation. See handout “Complete Blood Count” for further information.
In a cat with weight loss, examples of changes seen on a CBC could include:
B) Serum biochemistry. This is the chemical analysis of serum, which is the pale yellow liquid part of the blood that remains after the cells and clotting factors have been removed. There are many substances in serum, including proteins, enzymes, fats, sugars, hormones, electrolytes, etc.
"Measuring the levels of the various substances in the blood provides information about the health of the body’s organs and tissues such as the liver, kidney, and pancreas, and helps to detect diabetes."
Measuring the levels of the various substances in the blood provides information about the health of the body’s organs and tissues such as the liver, kidney, and pancreas, and helps to detect diabetes. See handout “Serum Biochemistry” for further information.
Some examples of changes on a serum biochemistry profile that might help explain weight loss in a cat could include:
C) Urinalysis is a simple test that analyzes the physical and chemical characteristics of urine. It measures how well the kidneys are working, identifies inflammation and infection in the urinary system, and helps to detect diabetes and other metabolic disturbances.
Urinalysis is important in any sick animal and is necessary for the proper interpretation of the serum biochemistry profile, especially in a cat that has kidney disease or diabetes. See handout “Urinalysis” for further information.
In a cat with weight loss, examples of changes seen on urinalysis could include:
D) Parasite tests. Having intestinal parasites, or “worms,” is a common cause for weight loss, especially in kittens. Testing a fresh stool sample for parasite eggs is an important screening test, and a simple fecal flotation is often the first test done. This involves taking a small sample of fresh stool and mixing it with a solution that causes the parasite eggs to float to the top of the sample. The eggs are collected and examined under the microscope to determine which parasites are present and how many there might be. See handout “Fecal Flotation” for further information. There are many other tests for parasitism and your veterinarian may recommend additional testing.
E) Serum thyroxine (total T4). This test is used to diagnose hyperthyroidism in cats (see handout "Hyperthyroidism in Cats" for further information). Hyperthyroidism is a common disorder in older cats and is caused by an overactive thyroid gland. The gland produces excessive amounts of thyroid hormones, which substantially increases the body’s metabolic rate and leads to weight loss. Most cases of hyperthyroidism can be diagnosed with a single blood test that measures the level of total thyroxine (T4) in the blood stream. Affected cats typically have markedly elevated levels of T4 in their blood.
The need for additional testing will depend on the history, physical examination, and the results of the initial screening tests. Given the many causes of weight loss, there is an equally long list of possible tests. A few of the more common specialized tests would include:
