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Medical Causes of Weight Loss

  • A variety of medical conditions can cause weight loss.
  • Weight loss is not a disease—it is a sign of an illness. Therefore, the treatment for weight loss depends on the underlying cause.
  • Regular physical examinations, routine wellness screening tests, and periodically weighing your pets at home can help with early detection of medical problems that can cause weight loss.

When Is Weight Loss a Cause for Concern?

Weight loss can result from decreased intake of calories, malnutrition (inappropriate diet), inadequate absorption or digestion of food (leading to malnutrition), or alterations in metabolism that make the body burn more calories than it is taking in. However, weight loss is not always an immediate cause for concern—it can be normal for pets to lose or gain small amounts of weight from time to time. For example, dogs may gain a little weight in the winter due to decreased activity and then lose those extra pounds when the weather warms up and activity increases. In fact, many pets fluctuate within a range of a few pounds on a regular basis.   

Determining when weight loss is a problem can be quite subjective, but the following criteria are causes for concern:

  • A pet is eating normal or increased amounts of quality food but is losing weight.
  • The cause of weight loss is unknown.
  • The amount of weight lost is significant (especially if it has occurred over a brief period of time).
  • Weight loss is accompanied by other signs of illness. 

What Medical Problems Can Cause Weight Loss?

Here are just a few of the medical conditions that can cause weight loss in dogs and cats. Some of these conditions are quite common and easily treatable, whereas others are life-threatening and require a long-term commitment to treatment or management:

  • Malnutrition due to poor diet or underfeeding
  • Anorexia (loss of appetite)
  • Intestinal parasites
  • Intestinal maldigestion (inability to digest food properly)
  • Intestinal malabsorption (inability to absorb nutrition properly)
  • Inflammatory bowel disease or other causes of chronic diarrhea
  • Thyroid disease (in cats)
  • Heartworm disease (in dogs)
  • Diabetes
  • Kidney failure
  • Heart failure
  • Liver disease
  • Cancer

How Are These Medical Problems Diagnosed?

Medical history and physical examination findings provide valuable information for your veterinarian. The medical history may include trying to determine what and how much the pet is eating, how long the weight loss has been occurring, and whether any other signs of illness have been observed. Physical examination findings may reveal evidence of underlying illness. For example, a cat with thyroid disease may have an increased heart rate and enlarged thyroid glands in the neck.

Initial diagnostic testing to begin looking into the cause of your pet’s weight loss may include blood work, such as a serum chemistry profile, complete blood cell count (CBC), and thyroid panel. Urinalysis and fecal testing can also be helpful early in the diagnostic process. Additional testing for specific diseases, such as cancer, may include taking radiographs (x-rays), performing ultrasound examinations of the chest or abdomen to look for irregularities in these areas, or taking biopsies (small tissue samples) from lymph nodes or other organs.

More targeted testing may be recommended based on the results of preliminary tests. 

How Is Weight Loss Treated?

Weight loss is not a disease—it is a sign of an illness. Therefore, the treatment for weight loss depends on the underlying cause. Fortunately, most conditions that cause weight loss are manageable or curable. Sometimes a diet change may be involved, but in other cases effective treatment of the underlying problem resolves the weight loss without altering the pet’s diet at all. If you suspect your pet may be losing weight, schedule an appointment with your veterinarian so that diagnostic testing can begin.

Can Medical Causes of Weight Loss Be Prevented?

Providing high-quality nutrition is a good way to help prevent weight loss in pets. It is also important to make sure (especially in a multipet household) that your pet is eating adequate amounts of food. Sometimes, one pet can “bully” another away from food, or one pet may eat significantly more than another. It may be necessary to feed pets in different rooms or feed cats on an elevated counter or platform to help prevent dogs in the house from stealing the cats’ food.

Some of the medical problems that cause weight loss cannot be prevented. However, regular physical examinations, routine wellness screening tests (including fecal and heartworm testing), and periodically weighing your pets at home (if possible) can help with early detection of  medical problems that can cause weight loss. In many cases, early diagnosis means better treatment options and improved quality of life for sick pets.