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Ask the Vet: Why Is My Cat Losing Weight?

Est. read time: 6 min.

Written by Justine Lee, DVM, DACVECC, DABT

Is your cat losing weight? If you’re even questioning this, then, yes, your cat is likely losing weight or muscle mass.

As a veterinarian and cat parent, I’m always on the look-out to make sure your cat is healthy. That’s because cats are so, so subtle with their clinical signs—they don’t show disease until it’s really severe. Why? It likely stems from descending from lions and big cats: they don’t want to show signs of illness for fear of another big cat taking over the pride. For this reason, it’s important to pick up on subtle signs such as weight loss in cats as early as possible

Top 5 causes of weight loss in cats

As a veterinarian, the top 5 causes of weight loss in cats that I end up seeing include:

  • Chronic kidney failure
  • Hyperthyroidism
  • Diabetes mellitus
  • Cancer
  • Osteoarthritis

Depending on the cause of the weight loss, some of these causes are very treatable, while others are less so. But in general, the sooner the cause for the weight loss is identified, the sooner we veterinarians can fix it… and the better the prognosis for your cat.

Chronic kidney failure

As was the case with my senior kitty (read on for the story), a cat losing weight may be experiencing chronic kidney failure. Chronic kidney failure occurs slowly, and cats may be able to compensate with this for months to years. Other than your cat losing weight, symptoms often include excessive drinking, excessive urination, and more. With appropriate diagnosis, nutrition, monitoring, and treatment, cats can potentially live with chronic kidney disease for years.

Hyperthyroidism

Hyperthyroidism occurs when the thyroid gland produces too much thyroid hormone. This results in an overstimulated, hyperactive metabolism. Hyperthyroidism is often seen in middle-aged to geriatric cats (around 8+ years of age). Along with gradual weight loss in cats, signs of hyperthyroidism include nervousness, excessive drinking and urination, frequent vomiting, and more. The sooner your vet diagnoses hyperthyroidism, the sooner we can minimize secondary life-threatening complications or damage to the body.

Diabetes mellitus

An estimated 1 in every 200 cats is thought to be affected by the endocrine disease diabetes mellitus. This is likely due to the growing prevalence of obesity. One of the common symptoms of diabetes in cats is weight loss (most commonly over the back), despite an overweight body condition. If your cat fails to respond to initial treatments such as oral medications, dietary changes, and “healthy” weight loss, twice-a-day insulin therapy is typically necessary.

Cancer

Cancer afflicts an estimated 30-40 percent of all cats, most commonly as lymphoma, skin cancer, and tumors originating in the mammary glands. Weight loss in cats is a common early symptom of cancer. Although the diagnosis can be initially devastating, some types of cancer are treatable if caught early. This is just another reason to bring your cat’s weight loss to your veterinarian's attention as soon as possible.

Osteoarthritis

Arthritis in cats is chronically undertreated. Signs of arthritis include changes in mobility, altered gait, being slower to jump or go up and down the stairs, and decreased activity and appetite. This is why weight loss in cats can be attributed to osteoarthritis. There are many options to treat arthritis in your cat, from simple environmental changes to medical therapy.

Identifying weight loss in cats early on

With my previous two senior cats, I made sure my Google calendar “to-do list” included a task to weigh both of them once a month. I weighed both myself and my cat on the digital scale and just subtracted my weight. I documented this in my Google calendar for several years. Seems neurotic, right?

Nope.

As my senior cat had chronic kidney failure, I had to check his weight and body condition carefully as a general guideline for his health. Thankfully, with careful monitoring (including monthly weighs and twice-a-year blood work screening), he lived with kidney disease for years (to 19.5 years of age, until he succumbed to mouth cancer). I also weighed my 13-year-old cat at the same time just to monitor her. She had always been consistently 11 pounds (for years), when over a period of 3 months, she dropped down to 10.3 pounds. Within weeks of her losing weight, she died acutely of cancer. 

So, that’s why I’m harping on the importance of monitoring your cat’s weight so carefully. If you notice your cat losing weight, you want to get to a veterinarian for a thorough physical examination, blood work, and a few other advanced diagnostics/tests. Yes, it may be expensive. But it may help save your cat before secondary side effects happen from untreated and undiagnosed medical problems.

How will my vet diagnose the issue?

Your veterinarian will want to do the following tests:

  • A complete blood count (CBC) to look at the red blood cell count (looking for dehydration or anemia) and white blood cell count (looking for underlying infection)

  • A chemistry panel to evaluate how high (or low) the blood glucose is and to look at the kidneys, liver, electrolytes, protein, and other bodily functions
  • A urinalysis (UA) to look for the presence of white blood cells, abnormal protein, bacteria, sugar, etc.

  • A urine culture to rule out a urinary tract infection

  • A T4 (thyroxine) to look at your cat’s thyroid gland

Depending on what these initial tests show, your veterinarian may need to do advanced tests such as:

  • X-rays of the chest and abdomen to rule out underlying medical problems like cancer, pneumonia, heart enlargement, bladder stones, etc.

  • An ultrasound of the abdomen to look at the architecture of the organs (like the pancreas) to rule out metabolic problems, fatty changes to the liver, cancer, etc.

When in doubt, as your cat ages, make sure to monitor your cat for any signs of weight loss. Unless the weight loss is intentional (like you put your cat on a diet), any cause of weight loss in cats always warrants an immediate veterinary visit. Again, the sooner you recognize your cat losing weight, and the sooner your veterinarian diagnoses it, the less long-term damage to your cat’s body and the longer he or she will live!

grey tabby cat getting pet; why is my cat losing weight?

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