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Why is my old cat losing weight?

Weight loss is one of the commonest symptoms we see in older cats, but it can be a warning sign of a number of common conditions. These include the “Big Four” chronic diseases of cats, plus two other conditions that are perhaps a little underdiagnosed. In this blog, we’re going to take a quick look at these six common conditions.

Informative image: old cat

The “Big Four”...


An overactive thyroid gland is a common cause of weight loss in the older cat. The thyroid gland makes the hormone thyroxine, which controls how fast the cat’s metabolism runs. Too much thyroxine means their metabolic rate skyrockets, resulting in weight loss, hyperactivity, ravenous hunger, and often ferocious behavioural changes. If it goes on too long, it can cause heart failure or blood clots, and is almost always fatal if untreated. The condition can easily be diagnosed with a blood test. There are several treatment options, including medication (as tablets or a liquid), surgery, or radiation therapy.

Chronic Kidney Failure

As a cat ages, their kidneys naturally become less and less efficient. Eventually, if they live long enough, they are no longer able to filter the blood effectively. This results in a build-up of toxic products in the blood (e.g. urea, creatinine and phosphate) and excessive losses of minerals (especially potassium) and protein. The result is weight loss, increased thirst, increased urination, and chronic dehydration. In most cases, kidney failure is diagnosed from a blood test, although urine tests may also be useful. There is no cure for the condition, but medication (usually ACE inhibitors) and a special diet can often give them a good quality of life for months or even years, if they are diagnosed early.

Diabetes Mellitus

In diabetes, the body stops processing sugars as it should, and although the blood sugar level climbs higher and higher, the cells can’t use it - either because the pancreas doesn’t produce enough insulin, or because the tissues don’t respond normally to it. The liver therefore produces ketones to keep the cells alive - but too many ketone chemicals in the blood leads to “ketoacidosis”, causing shock, collapse and death. Occasionally, especially in overweight cats, early and aggressive treatment can lead to diabetes going into remission (i.e. not causing any problems), but the vast majority of cats need twice daily insulin injections, and regular blood glucose monitoring at home.


Just like humans, older cats are at risk of tumours. In many cases, the first symptom is weight loss, because the tumour is “eating” all the cat’s energy reserves. Many tumours can be surgically removed, and this is curative in a lot of cases. Others have already spread by the time of diagnosis, although for some of these (e.g. lymphoma) there are chemotherapy treatments that maintain quality of life and extend lifespan significantly. However, the earlier it’s caught, the better the cat’s chances!

Underdiagnosed killers?


This is a condition characterised by inflammation of the pancreas, a small gland in the abdomen that produces digestive juices and also insulin (see above). It is probably massively underdiagnosed - recent research showed that over 40% of apparently normal cats have some degree of pancreatitis! The condition can be quite dramatic, with abdominal pain, vomiting, collapse and shock; but in cats, loss of appetite and weight loss are more common. Once diagnosed, intensive care treatment is often needed, but the condition can be treated effectively - if the cat hasn’t declined too far.

Dental Disease

Incredibly common, some studies suggest that over 70% of older cats have dental disease. If a cat’s mouth hurts, they’ll struggle to eat - but what normally happens is that they cope, and cope, and cope… until one day it’s too much and they stop eating. However, most of the time, the warning signs were there well before the pain got that bad, and they’ve been eating slowly and losing weight for weeks or months. The earlier we can get to them, the better the chance that surgery to remove the diseased teeth will be successful.

If your cat is losing weight, make an appointment to get them checked out by one of our vets as soon as possible - most of these conditions are manageable if caught early!