Weight Loss in Older Cats: Common Causes and When to See the Vet

Weight loss in an older cat is most often an early sign of illness rather than a normal part of ageing, and the common causes are an overactive thyroid, kidney disease, diabetes, dental pain, gut disease, and cancer. Some very old cats do thin out gradually, but because weight loss is the first visible sign of almost every common senior condition, it deserves a vet check before anyone puts it down to age.

The difficulty is that it happens slowly. You see your cat every day, so a cat who has lost a tenth of their weight over three months can look exactly the same to you as they did in the summer. This guide covers how to spot it, what usually causes it, and what the vet is likely to do.

Is weight loss ever just old age?

Sometimes, but only once everything else has been ruled out. The Cornell Feline Health Center notes that many cats put weight on as they age, while others become too thin, and that age-related changes often begin somewhere between 7 and 10 years old¹. Very elderly cats can lose some muscle and digest their food a little less efficiently, which shows as a bonier back and hips.

The trouble is that the same picture, a cat who feels lighter when you pick them up and whose spine you can suddenly feel, is also how hyperthyroidism, kidney disease, and diabetes first show themselves. Cornell lists hyperthyroidism, high blood pressure, diabetes, inflammatory bowel disease, and cancer among the conditions that become more common as cats get older¹. That is why the AAFP’s senior care guidelines put weight and body condition at the heart of every senior check², and why our guide to senior cat vet check-ups recommends a visit every 6 months from about 11 years.

How to tell your cat is losing weight

The reliable way is a number: weigh your cat every 2 to 4 weeks on the same scales and write it down. Your eyes and hands are poor judges of slow change, and long or thick coats hide a great deal.

The bathroom scales method works for most cats. Weigh yourself, weigh yourself holding the cat, and subtract one from the other. Use the same scales, the same time of day, and ideally before a meal. Between weigh-ins, run your hands along your cat now and then and notice:

• The spine and hips. In a healthy adult you should be able to feel the backbone under a thin covering. Sharp, prominent bones along the back are a sign of muscle or fat loss.
• The ribs. Easy to feel but not see in a cat at a healthy weight; visible or very sharp ribs suggest they are too thin.
• Muscle over the shoulders and back legs. Muscle wasting often shows here first in senior cats, even when the belly still feels soft.
• The coat. A coat that has gone dull, greasy, or unkempt often travels with weight loss.

There is no single cut-off, and any steady, unexplained loss is worth a vet check. Even a tenth of a 4kg cat’s weight is just 400g, about the weight of a tin of soup, which is why the scales catch it months before the mirror does.

Weight loss with a big appetite

A senior cat that is eating more than ever and still losing weight most often has an overactive thyroid, and less often diabetes or a problem absorbing food from the gut.

Hyperthyroidism is a common disease that mostly affects middle-aged and older cats, and its most common signs are weight loss, a bigger appetite, and more drinking and urinating³. Many cats also become restless or noisy, vomit or have diarrhoea, and let their coat go. It is diagnosed with a blood test that measures the thyroid hormone T4, and the outlook with treatment is generally good. Cornell describes radioactive iodine as the treatment of choice where it is available, curing about 95 percent of cases within three months; daily medication, surgery, and an iodine-restricted diet are the alternatives³. Our guide to hyperthyroidism in cats goes through each option.

Diabetes usually shows as weight loss with more thirst and more urine, often in a cat who was overweight to begin with. Some diabetic cats also walk flat on their back legs. The diabetes guide covers the signs and the treatment.

Gut disease, including inflammatory bowel disease and some intestinal lymphomas, can stop food being absorbed properly, so a cat eats well and still loses condition, sometimes with vomiting or loose stools.

Weight loss with a poor appetite

A senior cat that is eating less and losing weight is more likely to have kidney disease, dental pain, a gut problem, or cancer, and needs checking sooner, because cats cope badly with going short of food.

• Chronic kidney disease is one of the most common illnesses in older cats. It typically brings more thirst, a fading appetite, gradual weight loss, and sometimes nausea and vomiting⁴. Our guide to kidney disease in cats explains the stages.
• Dental disease is extremely common in older cats and can make eating painful enough that they eat less without ever looking obviously sore¹. Signs include dropping food, chewing on one side, drooling, and walking away from the bowl after a few mouthfuls.
• A fading sense of smell may make food less appealing to some elderly cats, which is one reason warming wet food slightly is often suggested.
• Cancer becomes more common with age and can cause weight loss with or without a change in appetite.

A senior cat eating almost nothing for more than a day, or who has lost weight and is now vomiting, hiding, or very quiet, should be seen promptly rather than at the next routine appointment.

What the vet will check

Expect a physical examination, a blood panel, a urine test, and a blood pressure reading, which between them find the cause in most cats. The vet will weigh your cat and compare it with earlier records, look in the mouth, feel the neck for an enlarged thyroid gland, and feel the belly and kidneys.

The blood tests cover kidney values, blood glucose, and a total T4 for the thyroid. Cornell notes that most hyperthyroid cats have a raised T4, but a small number do not and need further testing³. A urine sample shows how well the kidneys are concentrating and whether there is glucose present. Blood pressure matters because high blood pressure travels with both thyroid and kidney disease. If all of that comes back clear, the next step is usually an abdominal ultrasound or more specific tests for gut disease.

Take your weight notes with you. A list of dates and weights tells the vet far more than “I think she’s got thinner”.

Supporting a thin senior cat at home

Feed the cause, not just the cat: once the vet has a diagnosis, the right diet and treatment usually bring the weight back far better than extra food alone. While you wait for that appointment, a few things help:

• Offer small, frequent meals rather than two large ones. Many older cats manage more this way.
• Warm wet food to around body temperature to boost the smell.
• Do not switch foods suddenly. If a change is needed, make it gradually over about a week, unless your vet advises otherwise⁵.
• Do not start a prescription or special diet without the vet, because a kidney diet and a thyroid diet, for example, are very different.

Our guide to feeding a senior cat has more on choosing food for an older cat.

A note from Hannah

I missed Marmalade’s weight loss for longer than I like to admit. He was a big lad, so when he started to feel a bit less solid I told myself it suited him. It was only when the vet weighed him at his routine check and read out a number almost 600g lower than the year before that I took it seriously, and his bloods showed the kidney disease that we then managed for the rest of his life. After that I bought a cheap set of baby scales and weighed every cat in the house on the first of each month, with the numbers in the notebook by the bowls. It takes two minutes, and it has caught a drop in two cats since, both well before I could have seen it.

This guide is general information, not advice about your own cat. If your older cat is losing weight, speak to your own vet: they can examine your cat, run the right tests, and find the cause.