My cat is losing weight: the causes, especially if it eats a lot, and which tests to ask for
Many owners only notice their cat has lost weight once it has dropped a fifth of its body weight, because the coat hides it and the changes are slow. In cats, losing weight for no reason is almost always a sign of illness, and a big appetite is no reassurance: sometimes it means quite the opposite.
Why weight matters so much in cats
A cat is a small animal: a 4 kg (8.8 lb) cat that loses 400 g (14 oz) has lost ten per cent of its weight, the equivalent of a 70 kg (154 lb) person losing 7 kg (15 lb). Those 400 g do not show in the mirror or the face, and under the fur they go unnoticed. That is why feline vets insist on weighing cats at home regularly, once a month for adults and more often for older or sick cats. A sustained loss of five per cent or more without any change in diet already warrants a vet visit. Many chronic feline diseases, from kidney to thyroid problems, show up on the scales first, months before anything else.
How to weigh it and assess its body
The simplest way is to weigh yourself holding the cat and then without it, and subtract, although bathroom scales are unreliable with small weights; baby scales or large kitchen scales with a box work better. Always note the date and weight. Also learn to assess its body condition: run your hands along its ribs, which you should be able to feel under a thin layer of fat, like the back of your hand. If they feel like the knuckles of a fist, and you can also see the spine or hip bones sticking out, it is thin. Muscle mass counts too: older cats lose muscle along the back and thighs before they lose fat. The ideal weight tool explains the scale.
Losing weight despite eating a lot
If your cat is eating more than ever, asking for food all the time and still losing weight, there are three main suspects. Hyperthyroidism, the most common hormonal disease in cats over ten, which speeds up the metabolism and often comes with restlessness, vomiting, yowling and an unkempt coat. Diabetes, which causes hunger and great thirst with lots of urine, and which in cats is linked to previous obesity. And digestive problems that prevent food being absorbed properly, such as inflammatory bowel disease, low-grade intestinal lymphoma or exocrine pancreatic insufficiency, which usually cause vomiting or abnormal faeces. Intestinal parasites can be the cause in kittens or cats that hunt.
Losing weight because it eats little
If the cat is eating less, the question is why. Mouth pain is very common: periodontal disease, tooth resorption or gingivostomatitis make it eat less or only soft food. Chronic kidney disease takes away appetite and causes nausea. Nausea from any cause, arthritis pain that makes it visit the food bowl less if it is up high, the stress of a change at home, another cat stealing its food or keeping it away, or a change of food it does not like all count too. And cancer, more common with age, can cause weight loss with or without loss of appetite. Remember that a cat that stops eating altogether for more than a day needs a vet, because of the risk of hepatic lipidosis.
By age: what is most likely
In kittens and young cats, think of parasites, infections, digestive problems, too little food for their growth and, in at-risk areas or groups, FeLV, FIV or feline infectious peritonitis. In middle-aged adults, diabetes (especially if they were obese), inflammatory bowel disease, dental problems and stress. In cats over about ten, the classic trio is hyperthyroidism, chronic kidney disease and diabetes, joined by tumours, joint pain and age-related muscle loss. An older cat often has more than one problem at once, and one can mask another: hyperthyroidism, for example, can hide kidney disease.
When it is urgent
See the vet the same day if the cat has stopped eating for more than twenty-four hours, vomits repeatedly, has yellowish gums or whites of the eyes, is very subdued or dehydrated, suddenly drinks and urinates a huge amount, or is breathing oddly. Book an appointment within a few days if you notice it has gradually lost weight, if it eats a lot and loses weight, if it has lost muscle along its back or if its weight has dropped by five per cent or more in a few months. Bring any weight records you have and a list of what it eats each day: that information is enormously valuable to the vet.
Which tests the vet will ask for
After the examination, which includes the mouth, the thyroid (sometimes it can be felt to be enlarged), the abdomen, the heart and blood pressure, the basic step is a full blood panel with haematology and biochemistry, a thyroid hormone (T4) measurement in cats over about seven or eight, a full urinalysis and an FeLV and FIV test if not already done. If there are digestive signs, faecal tests and investigations such as an abdominal ultrasound are added, sometimes with vitamin B12 levels and pancreatic enzyme tests. If everything comes back normal and the cat keeps losing weight, it may be necessary to go further with endoscopy or intestinal biopsies.
What it costs in Spain in 2026
As a guide: consultation, €40 to €60; full blood tests with T4, €100 to €200; urinalysis, €30 to €60; blood pressure, €20 to €40; FeLV and FIV test, €30 to €55; abdominal ultrasound, €70 to €150; gastrointestinal panel with B12 and pancreatic enzymes, €60 to €150. A complete initial work-up for an older cat losing weight usually comes to between €250 and €450. An endoscopy with intestinal biopsies, including anaesthetic and laboratory work, €500 to €1,200. After that, treatment depends on the cause: hyperthyroidism or diabetes has very different monthly costs from a dental problem that is solved in a single procedure.
What not to do
Do not simply increase its portion to “make up for it” without knowing why it is losing weight: a cat with hyperthyroidism or diabetes will keep losing weight, and you will be delaying the diagnosis. Do not put the weight loss down to age: healthy older cats have no reason to lose weight, and if they do, there is almost always a cause that can be treated or at least eased. Do not keep changing its food in search of one it likes if the cause is pain or nausea. And do not give it appetite stimulants or supplements you have read about online without asking: some exist in veterinary medicine, but you first need to know what is going on.
Prevention: the scales as a check-up
Prevention here means early detection. Weigh it once a month and write it down, or ask your clinic to weigh it at every visit and give you the figure. Annual check-ups for adults and every six months from about eleven, with blood tests, urinalysis and blood pressure. Check its mouth and breath. Watch whether it eats what you put down and how much it leaves, especially if you have several cats that eat together. And watch for changes in thirst: a cat that suddenly drinks much more is usually warning you of something. Many of these diseases, caught in time, have treatments that give back years of good life.
Losing weight for no reason is almost always a sign of illness in a cat, and eating a lot is no reassurance: it may be hyperthyroidism or diabetes. Weigh it every month, see the vet if it loses five per cent or more, and ask for blood tests with T4 and a urinalysis if it is over seven or eight.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.