Cat losing weight but still eating? Causes and what to tell your vet

Your cat eats well, but the weight is going. The spine feels bonier. The old collar is loose. It seems odd. If a cat eats, how can it be thin? The answer is that several common illnesses do exactly this. This guide shows the causes, how to track weight at home, and what to tell your vet.

This page gives general information. It is not medical advice. Only a vet can find the cause.

Three common causes in older cats

An overactive thyroid. The Cornell Feline Health Center lists weight loss, increased appetite, and increased thirst and urine as the most common signs of hyperthyroidism. Vomiting, diarrhea, restlessness and a greasy coat can also occur. It mostly affects middle-aged and older cats.

Diabetes. Cornell says the two signs owners most often see at home are weight loss despite a good appetite, and more thirst and urine. A vet finds diabetes by showing persistently high glucose in the blood and urine.

Kidney disease. Cornell lists weight loss, less appetite, more thirst and more urine among the signs of chronic kidney disease (CKD). The 2026 iCatCare guidelines say appetite often goes up and down in CKD. Early on, a cat may look well. Our guide to the signs of kidney disease in cats lists the other signs.

These illnesses can look alike, and some cats have more than one. A vet runs blood and urine tests to tell them apart.

Why weight loss is easy to miss

You see your cat every day. A loss of a little each week is hard to see. The 2026 iCatCare guidelines on CKD say weight loss can go unnoticed by caregivers, or be put down to age.

They also give a number. In a study of over 500 cats, the median weight loss was 8.9% of body weight in the 12 months before CKD was found. The loss had begun 3 years before, and it sped up after the diagnosis. The guidelines say unintended weight loss, or a fall in body or muscle condition, should be reported to your vet.

How to track weight at home

The 2026 guidelines give home weight checks as an example of how caregivers can take part in monitoring. Ask your vet how to do it for your cat, and how often. Then:

• Use the same scale. Do not switch between scales. Each scale reads a little differently.
• Use the same time of day. For example, before breakfast.
• Write every number with the date. One number says little. A line of numbers shows the trend.
• Note the scale type. A baby scale, a kitchen scale and a bathroom scale give different readings.
• Look and feel too. Look from above and the side. Ask your vet to show you how to feel the ribs, spine and hips. Muscle loss can hide behind a steady weight.

Some owners weigh themselves with and without the cat. This works with a calm cat. A cat that wriggles may need a carrier on a kitchen scale, with the carrier weight taken off. If the weight has changed, ask your vet to confirm it on the clinic scale.

What else to write down

Weight is one line. Your vet also wants:

• Appetite. More, less or the same. Fast eating. Food left.
• Thirst and urine. A bowl that empties faster. Bigger or more frequent clumps.
• Vomiting or diarrhea. The date and what you saw.
• Energy and coat. More sleep. Less play. A rough or greasy coat.
• Changes at home. New food, a new medicine, a new pet, a stressful week.

Our guide to why a cat drinks so much water explains the thirst line in more detail.

What not to do

• Do not change the food on your own. Find the cause first. Your vet decides the food and any medicine.
• Do not give a supplement or a home remedy unless your vet says so.
• Do not wait. Weight loss in an older cat is a reason to book an exam now, not in three months.

When to call a vet now

Call a vet or an emergency clinic now if your cat:

• Strains in the litter tray and passes little or no urine.
• Cannot keep water down, or vomits again and again.
• Is very weak, collapses, or cannot stand.
• Has not eaten for a day or more.

If you are not sure, call.

Bring the record to the visit

A short summary helps your vet see the trend in one minute. Our free vet visit summary helps you write down what you have seen.

If your vet finds kidney disease, the Cat Kidney (CKD) Care Log has a home weight log, a body and muscle check, a daily log and a weekly cat check, so every number stays in one place. It records only. It gives no diagnosis, diet or dose. See every page of the Cat Kidney (CKD) Care Log.

If your cat’s illness is advanced, read how to know when to put your cat down.

If your cat has stopped eating, read cat not eating but acting normal.

If your older cat also jumps less, read older cat not jumping anymore.