6 Medical Reasons: Older Cat Losing Weight but Still Eating

When I see a patient who is an older cat losing weight but still eating, there is a short list of diseases I consider immediately. 

  • Hyperthyroidism
  • Diabetes
  • Renal Insufficiency
  • Geriatric wasting
  • Cancer
  • Poor Digestion

In short, a senior cat who is eating a normal or increased amount of food is either burning more calories or absorbing less of the food they take in. Older cats who are still eating but who are taking in less food than expected have a longer list of possible medical causes. 

Today I’ll concentrate on cats eating a normal or increased volume of food. 

Let’s go through each of these and talk about other symptoms of the disease, testing and treatment. The diseases are discussed in order of relative frequency in my experience.



  • Weight loss
  • Increased appetite
  • Poor hair coat
  • Changes in behavior (usually grouchier)

Hyperthyroidism is a very common endocrine disease in older cats. It happens when a small tumor develops on the thyroid gland and makes too much thyroid hormone. 

Too much thyroid hormone in the body turns most body functions into overdrive. That includes a cat’s energy burning and appetite. 

Most cats with hyperthyroidism act like they’re starving and eat a lot of food. Surprisingly, they still lose weight over a period of weeks. 

Hyperthyroidism is fairly easy to diagnose with blood tests. The most common treatment is an oral or transdermal medication that suppresses the production of thyroid hormone. The prognosis for properly treated hyperthyroid cats is good.

Diabetes Mellitus


  • Weight loss
  • Increased hunger
  • Increased thirst
  • Increased urination

Diabetes is another endocrine disease seen in senior cats. Overweight and obese cats get it more often but normal-weight cats can get it, too. 

Diabetes mellitus occurs in most cats due to insulin resistance. That means the cells don’t respond normally to the insulin the cat produces so the cells can’t let glucose in for fuel. 

With cells in a starved state, the cat starts to lose weight and becomes very hungry. Other symptoms include increased thirst and urination. 

Diabetes is diagnosed with blood and urine tests. Simple cases have a good prognosis with proper insulin therapy and diet. Some cats experience a reversal of the disease and go back to normal after a period of good treatment. 

Cats with advanced diabetes may require hospitalization and have a poorer prognosis.

Renal Insufficiency


  • Weight loss
  • Normal to decreased appetite
  • Normal to increased thirst
  • Normal to increased urination

Mild to moderate decreases in normal kidney function is called renal (kidney) insufficiency. Many, many older cats suffer from some degree of this kidney disease. 

Unlike full-blown renal failure, renal insufficiency may go on for years in cats. Poorly functioning kidneys don’t clean the blood the way they should, leading to a buildup of toxins. They also allow too much water to escape from the cat’s body in the form of urine. 

Renal insufficiency may cause a slower, more chronic form of weight loss. You might notice changes in thirst and urination, too. 

This disease is diagnosed with blood and urine tests but may require X-rays, ultrasound imaging and other tests to confirm it. Treatment consists of special cat food, possibly oral medication and fluid treatment. 

Close monitoring by you and your vet can help your cat avoid things like infections that could damage the kidneys more. 

Pastel tortie cat (older cat losing weight but still eating)

Geriatric Wasting 


  • Weight loss
  • Normal appetite
  • Normal thirst
  • Boney appearance

There is a “wasting” phenomenon in elderly animals and humans.(1) Medically called cachexia or sarcopenia, scientists don’t completely understand why senior cats lose body mass as they age. 

Even cats without other major diseases can lose weight and become very boney. A good diet with plenty of protein and fat can help prevent the progression of elderly cat wasting. 

Talk to your vet about the best food to maintain your aging cat’s body condition. Senior cat food is often lower in protein and calories, so it might not be ideal. 

Regular exercise will help keep muscles firing and maintain flexibility. 



  • Weight loss
  • Normal or decreased appetite
  • Other symptoms depend on the location/type of cancer

Many kinds of cancer affect geriatric cats from lymphoma to carcinoma and it can get into practically any organ system. 

We usually assume cats with cancer will have a poor appetite, but that’s not always the case. Especially when it’s early in the course of the disease. 

Cancerous tumors use a lot of energy and can lead to weight loss even if a cat is eating pretty normally. 

Diagnosing cancer is sometimes easy and sometimes difficult. Your vet will want to do blood tests, urine tests and X-rays to get started. Other tests like biopsies may be necessary to figure out exactly where and what kind of cancer is affecting your cat. 

Cat cancer may be treated with chemotherapy, radiation and/or surgery. Special cat food, medication and supplements can help slow the progression of some types of cancer. 

Poor Digestion


  • Weight loss
  • Increased, normal or decreased appetite
  • Most have other symptoms like vomiting, diarrhea or constipation
  • Outdoor cats are more likely to have parasites than indoor cats

There are a few medical problems that can cause poor digestion of food in cats. One of the most common is inflammatory bowel disease (IBD). 

The disease is called inflammatory because the walls of the intestines become inflamed with white blood cells. The underlying cause is not well understood but some scientists theorize it is a reaction to gut bacteria and food.(2) 

It is a challenge to definitively diagnose IBD in cats. An intestinal biopsy is considered the gold standard diagnostic, but blood tests, x-rays and ultrasound imaging can also be helpful. 

IBD can be treated with oral medication and a special diet. The long-term prognosis is pretty good for aggressively managed cases.

Another cause of poor digestion is intestinal parasites. These may include worms like roundworms and hookworms. Single-celled organisms that infect cats include Giardia and Tritrichomonas

Fecal testing for parasites is pretty accurate. Treatment consists of oral or injectable anti-parasitic drugs. The prognosis is good with proper treatment. 

What You Can Do When Your Older Cat Is Losing Weight

The most important thing is to get an appointment with your veterinarian. But there are a few things you can do while you’re waiting for your cat’s appointment…

Make some notes about your cat’s diet and supplements over the last few months. What other symptoms have you noticed? When does vomiting occur? How often does your cat have abnormal bowel movements? 

You should avoid making frequent changes to your cat’s diet because it could make things worse. In general, a high-quality pet probiotic such as Forti-Flora or Proviable-DC is safe to give your cat according to the label directions. 

If your cat does not have vomiting, diarrhea or constipation and has a good appetite, you can give them more food. Just increase the amount gradually because excessive calories could cause an upset stomach, too. 

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When Cat Weight Loss Is an Emergency

Since weight loss occurs over days and weeks, this symptom itself is rarely cause for an emergency visit to see a vet. However, many of the diseases that cause an older cat to lose weight can suddenly get worse and cause life-threatening symptoms. 

If your cat has trouble breathing, seizures, extreme lethargy/weakness, severe diarrhea, trouble urinating or significant bleeding you should take them to the veterinary clinic immediately.


The older a cat gets, the less surprising it is for them to lose weight. Losing muscle mass and fat is common in cats over 13 years of age. 

There are several diseases that cause weight loss in cats while increasing their appetite. Cats with milder, chronic diseases like kidney insufficiency may lose weight while maintaining a normal appetite. 

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  1. Peterson, M. E., & Little, S. E. (2018). Cachexia, sarcopenia and other forms of muscle wasting: common problems of senior and geriatric cats and of cats with endocrine disease. Animal Endocrine Clinic. Gerontology: an inside out perspective, 2018, 65.
  2. Simpson K: Feline IBD and Lymphoma: Pitfalls and Progress in Diagnosis and Management. Pacific Veterinary Conference 2017.