Dog Losing Weight – Symptoms and Causes

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In today’s world where over 50% of dogs are overweight, the thought of having your dog lose a few pounds is quite appealing. Lean dogs seem to live longer, have healthy immune systems, and enjoy running and playing outside.

However, there’s a big difference between lean dogs, and those that are unexpectedly losing significant amounts of weight. While staying relatively lean is typically healthy for dogs and promotes longevity, rapid weight loss can signify more serious problems for your dog.

Interestingly enough, dogs’ bodies are made up of about two-thirds water, so oftentimes, sudden weight loss can be a sign of dehydration rather than a loss of fat in your dog’s body. Should this be the case, keeping your dog well hydrated will be an important step in figuring out the underlying cause in your dog’s decrease in weight.

What is Weight Loss in Dogs?

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Unhealthy weight loss in dogs is considered a sudden decrease in weight accompanied by weakness. Typically, if your dog is losing weight at an abnormal rate, unaccompanied by extreme physical activity, there is an underlying illness responsible. You’ll likely see other signs and symptoms, such as low energy levels, weakened immune systems, lethargy, and dull coats that grow in patches.

These signs and symptoms can be explained by a series of illnesses, some more serious than others, so you want to seek medical advice from your vet as soon as you notice your dog losing a significant amount of weight.

Occasional & Less Serious Causes of Dog Weight Loss

While there are more serious illnesses that can result in canine weight loss, there are also common conditions that can cause the same effects. Most commonly, changes in your dog’s environment can alter his food intake. For example, changing your dog’s food or his exercise schedule can both cause weight loss in your dog. In addition, as your dog gets older, he may start to lose some weight naturally.

However, the important thing to recognize with these causes is that they should not be producing a significant amount of weight loss. In these instances, it should be an easy and painless process to get your dog to gain back the weight with just a few extra calories. However, if your dog has lost more than a few pounds, you may need to look into the more medical reasons for weight loss.

Causes of Chronic Weight Loss in Dogs

If your dog is losing more than 10% of his body weight at a rapid and dramatic pace, there is likely a more serious underlying reason for weight loss. Here are a few different possibilities for rapid weight loss in dogs.

Heartworms or Intestinal Worms

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Heartworms and Intestinal worms both cause weight loss in dogs that affect the body differently. Intestinal worms feed off of your dog’s nutrients, which cause him to lose weight at a rapid pace. Common types of intestinal worms in dogs include tapeworms, roundworms, and hookworms and require a medical diagnosis from your vet to be properly treated.

Heartworms are a very serious condition if left untreated. The worms can be very aggressive and affect areas of the heart, liver, and kidneys, causing these organs to overwork themselves to a point of exhaustion. There are a series of tests that can be run to diagnose heartworms, and once diagnosed, the recovery period is typically two months.

Liver Disease

Liver disease can be brought on by a number of reasons including untreated heartworms, genetic predisposition, diabetes, or overuse of antibiotics. Other symptoms of liver disease in dogs other than weight loss include vomiting and diarrhea, increased thirst, frequent urination, yellowish eyes, and blood in the stool. These symptoms can be severe, however once diagnosed, liver disease is manageable and treatable.

Kidney Disease in Older Dogs

Another cause of rapid weight loss in dogs can be because of kidney disease, or kidney failure. Kidney disease is usually brought on by old age – a dog’s kidneys simply wearing out. Kidneys act as the body’s filtration system, so when an older dog begins to develop kidney problems, they are unable to filter out waste from the blood. Signs you can look out for are increased water consumption and an increase in urination. Other clinical signs can include weight loss, depression, vomiting, diarrhea and loss of appetite.

Thyroid Problems in Dogs

An excess of the thyroid hormone in dogs can also cause weight loss in dogs. Excessive amounts of thyroid hormones in a dog is called hyperthyroidism. When this hormone secreted by the thyroid gland increases, metabolism increases as well. More symptoms other than your dog losing weight are; increased appetite, increased anxiety, diarrhea and hyperactivity.


Dogs with cancer can lose weight for several reasons. Typically, cancer demands significant nutrients from your dog’s body, which causes the body to lose its fat and muscle tissue. Specifically, cancer starts to produce tumor necrosis factor (TNF), which resets the body’s thermostat so that cells consume more energy than normal. TNF makes dogs feel sick, to the point that a dog has no appetite and begins losing weight.

There are several other conditions that can cause your dog to rapidly lose weight so it is important to not self-diagnose. The best way to identify and treat cases of weight loss is to monitor your pet’s health and well-being on a daily basis. Taking note of extreme physical changes along with other signs and symptoms will help your veterinarian better diagnose the reason for your dog’s sudden weight change.

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