Watching your pet slim down after some healthy lifestyle changes can feel like a win—more energy, easier movement, and better overall wellness. In fact, with pet obesity on the rise, many pet parents are taking proactive steps to help their dogs and cats maintain a healthier weight.
But what if your dog starts losing weight without any changes to their routine? When weight loss is sudden or unexplained, it can leave you feeling worried and confused—especially if your pet seems otherwise healthy. Understanding the potential reasons behind this shift is the first step toward getting your dog the care they need.
“Often, weight loss is considered a good thing,” says Dr. Lois Palin, VMD, with Doylestown Veterinary Hospital. “We want to get our pets down to a healthier weight. But we want it to be controlled and intentional. Not caused by a disease process.”
Why Is My Dog Losing Weight?
According to Dr. Palin, some of the most common reasons for unexplained weight loss in pets are health-related, with digestive and gastrointestinal issues at the top of the list.
“Conditions like inflammatory bowel disease or exocrine pancreatic insufficiency hinder the absorption of nutrients in the GI tract,” Dr. Palin says. “They can cause vomiting and diarrhea while leading to nutrient loss and weight loss.”
Certain metabolic diseases may also be culprits. Diabetes, she notes, is a concept familiar to most pet owners and can cause animals to lose weight even when they are eating regularly and abundantly. Hyperthyroidism, too, is relatively common in older cats, leading to significant weight loss even with normal food intake.
“There are also some more unusual conditions, like one we occasionally see in dogs called Addison’s disease,” Dr. Palin explains. Also known as hypoadrenocorticism, this potentially serious hormonal disorder impacts the adrenal glands and hinders the production of steroid hormones like cortisol and aldosterone.
“In more general terms, kidney disease and liver disease may alter a dog’s appetite, cause nausea, and reduce the utilization of all the nutrients in their food,” Dr. Palin says. “We have also seen animals with heart disease that are losing weight.”
Stress and anxiety are frequently contributors, Dr. Palin says.
“Dogs that are stressed have a reduced appetite. Anxiety can come from any number of things: changes in the environment, different pets in the home, even the weather for some dogs,” she says. “Dental problems may also play a role. Not surprisingly, painful teeth and gums can make eating uncomfortable.”
Of course, cancer generally makes pets not feel well and may lead to weight loss. “Animals with cancer have nausea, they’re lethargic, and they don’t want to eat. There are also metabolic changes related to cancer that could lead to significant weight loss,” Dr. Palin says.
The Food Factor
Diet must always be considered when wondering, “Why is my dog losing weight?”
“Inadequate calorie intake can cause weight loss,” Dr. Palin says. “This happens when a dog is simply not getting enough food, or the food that’s provided doesn’t deliver enough calories to meet the dog’s needs. Poor quality or unpalatable food may be to blame. Dog food can go bad, especially if it’s stored incorrectly. And that, of course, would be unappetizing to a dog.”
The Problem with Parasites
Parasites are yet another medical reason that dogs may lose weight, particularly in severe cases.
Puppies, Dr. Palin says, can contract parasites directly from their mom. Adult dogs can pass them to one another, or through contact with contaminated stool or soil where stool has been.
“Believe it or not, tapeworms can come from fleas,” Dr. Palin says. “And roundworms have even been discovered in potting soil. So, even though it’s not common to see parasites in indoor cats, we recommend checking stool samples regularly, particularly if weight loss is noticed.”
Fortunately, today’s heartworm preventatives control most common intestinal parasites in dogs. “But not 100 percent,” Dr. Palin says. “That’s why we still recommend yearly checks, or even every six months, depending on your pet’s lifestyle. For dogs in daycare, we advocate for more frequent checks simply because they’ve been part of a group.”
When Should Pet Owners Be Concerned?
Significant weight loss is typically considered to be 10 percent or more of an animal’s body weight, says Dr. Palin. This might not seem to be much on the surface, but for a smaller animal, it’s substantial.
“If you’re a 10-pound cat and you lose one pound, that’s 10 percent,” she says. “Even though that doesn’t sound like a lot for us, if you’re that little pet, it’s huge.”
Rapid weight loss is always a reason to be concerned, particularly if accompanied by additional symptoms like changes in appetite, increased drinking and urination, lethargy, vomiting, diarrhea, or signs of pain.
“Weight loss in puppies, too, is always worrisome,” Dr. Palin says. “They’re growing. They shouldn’t be losing weight.”
Dr. Palin notes that some weight loss is to be expected in senior dogs and cats, but it could also indicate underlying issues and should be addressed, especially if other red flags are present.
“Coughing, difficulty breathing, changes in stool consistency or frequency. If there are new lumps or swelling. Even mobility issues are a reason to get your pet checked out.”
Seeking a Diagnosis
Determining just what is causing your pet’s unexplained weight loss starts with collecting a complete medical history.
“It’s important that pet parents know exactly what they’re feeding their dogs and cats and how much,” Dr. Palin says. “That includes treats and table foods.”
A physical exam will follow, she notes, most likely in conjunction with diagnostic testing.
“When a pet is losing weight, I think owners should expect that we’re going to need diagnostics to get to the root of the problem,” she continues. “So, it’s likely we’re going to recommend lab work, including bloodwork, a urinalysis, and a stool sample.”
More specialized testing, including GI profiles and specialized tests for endocrine diseases, may be ordered, along with radiographs and ultrasound studies.
What Pet Parents Can Do at Home
Pet parents can regularly monitor their pets’ weight at home.
“It’s always nice for pet owners to have a baseline on their own scale,” Dr. Palin says. “For cats and small dogs, you can weigh your pet while you’re holding it or in its carrier and then subtract your weight or the weight of the carrier.”
Pet parents are always welcome to utilize the scale at the veterinary office, she adds. “It’s a little easier and often more accurate.”
Weigh-ins should also always be conducted during routine veterinary exams or sick visits.
“If you are starting a weight loss program for your pets, you need a starting point. Then, we want to reassess that weight every 2-4 weeks,” Dr. Palin says. “If your pet is on a weight reduction diet, we don’t want them losing weight too quickly. We want it at a slow, steady rate.”
Common Misconceptions about Pet Weight
It’s normal to think, “My dog is getting older, it’s normal that they’re losing weight,” Dr. Palin says.
“A little bit of weight loss might be normal, but it can often be an indicator of something bigger going on.”
Some pet parents mistakenly believe they can manage a pet’s weight loss on their own by simply feeding them more or changing the diet.
“And maybe that’s going to be the answer,” says Dr. Palin. “But I certainly believe veterinary guidance is essential to rule out medical causes.”
