Why Is My Old Cat Losing Weight? 7 Common Causes

You pick up your fourteen-year-old cat and she feels lighter. The spine is sharper under your hand than it used to be, the hips stick out a little, and yet she is eating, using the tray, and behaving normally. It is tempting to call it old age. It almost never is.

Why “It's Just Old Age” Is the Wrong Answer

Old cats do change shape. Muscle along the back and hind legs thins out with age, so a fifteen-year-old can feel bonier than she did at eight even while her weight holds steady. That part is normal.

What is not normal is the scale going down. Cats are small enough that the losses are small too — half a kilo on a 4.5 kg cat is more than 10% of her body weight, and that is the kind of loss a person can miss for months when they see the cat every day. Vets take unexplained weight loss seriously in any cat over 10 precisely because the causes behind it are common, serious and, in most cases, treatable when they are caught early.

The Seven Usual Causes

Appetite is the single most useful thing you can tell your vet, because it splits the list roughly in half. Find the row that matches what you are seeing:

Two of these can be present at once — hyperthyroidism and kidney disease frequently travel together in older cats, and treating one can unmask the other. That is a vet's job, not a guessing game at home.

Eating Well and Still Losing: Start Here

Hyperthyroidism is the first thing most vets think of. An overactive thyroid gland pushes the whole metabolism into overdrive, so the cat burns through everything it eats. It affects roughly one in ten cats over 10, and the picture is distinctive: a hungry, restless, often vocal cat that keeps getting thinner, sometimes with vomiting and a coat that has stopped looking cared for. A single blood test usually finds it, and it is treatable — with medication, a special diet, surgery or radioactive iodine.

Diabetes is the other big one where the bowl empties but the cat shrinks. The giveaways are thirst and urine: a cat that is suddenly draining water bowls and producing heavy, frequent clumps in the tray. Some cats also develop a distinctive dropped-hock walk on the back legs. It is diagnosed with blood and urine tests and managed well in many cats once treatment starts.

Intestinal disease — inflammatory bowel disease, or low-grade intestinal lymphoma — is the slower one. Food goes in but is poorly absorbed, often with intermittent vomiting or soft stools, and the weight comes off over months rather than weeks.

Eating Less: Kidneys, Teeth and Pain

Chronic kidney disease affects roughly a third of cats over 10. It builds slowly, and nausea is a large part of why affected cats go off their food — they feel queasy rather than uninterested. Alongside the weight you will usually notice more drinking, more urine, a duller coat and less energy. It cannot be cured, but diet, fluids and medication can add good years, which is the whole reason for testing early.

Dental disease deserves more suspicion than it gets. Cats do not stop eating because their mouth hurts; they eat differently. They approach the bowl, take a few mouthfuls, chew on one side, drop food, then leave. Owners read that as fussiness. Bad breath and a red line along the gums are the clues to look for — and a dental treatment often brings back a cat people had written off as old.

Arthritis can do it too, in a more ordinary way: a stiff cat may simply not want to crouch over a low bowl, or cannot get to a food bowl upstairs any more. Raising the bowl and moving food to the floor the cat actually lives on sometimes fixes a mysterious appetite problem overnight.

Weigh Your Cat at Home

You cannot judge this by eye, and neither can anyone else who sees the cat daily. Use a bathroom scale: weigh yourself, then weigh yourself holding the cat, and subtract. Do it on the same day each month and write the number down.

A 5% drop — 250 g in a 5 kg cat — is the point at which most vets want to know. Bring the list of numbers with you, because a trend across three months tells your vet far more than one weight on the day.

Between weigh-ins, use your hands. Ribs should be easy to feel with a thin covering over them; the spine and hip bones should be felt, not seen as ridges. If the shoulder blades and hips have become visible, the loss is already significant.

What the Vet Visit Looks Like

For an older cat losing weight, the standard work-up is a physical exam including a proper look in the mouth, blood work with a thyroid level, a urine test and blood pressure. Blood and urine together are what separate kidney disease from diabetes from an overactive thyroid — three conditions that can look identical from the sofa.

If those come back clear, imaging usually comes next, since intestinal disease and tumours often need an ultrasound to find. Ask for copies of the results either way: a kidney value that has crept up over two years tells a story that no single test can.

Helping While You Wait for Answers

• Warm the food slightly.Body-temperature food smells stronger, and smell drives a cat's appetite far more than taste.
• Offer smaller meals more often rather than one big bowl.
• Use a wide, shallow dish at a comfortable height so whiskers and stiff joints are not part of the problem.
• Feed away from other pets and away from the litter tray.
• Track what actually gets eaten for a few days — not what you put down. Your vet will ask.

For portion sizes while you sort this out, our cat food calorie calculator gives a daily target, and the senior cat care guide covers the rest of the routine that keeps older cats steady.

The Bottom Line

A senior cat losing weight is telling you something, and in most cases it is something a blood test can name. Hyperthyroidism, kidney disease, diabetes and dental pain cover the large majority of cases, and every one of them is more manageable early than late. Weigh your cat monthly, note whether the appetite is up or down, and take both numbers to your vet rather than waiting to see if it settles. Check where your cat sits in human years with the Cat Age Calculator. 🐱