Symptom guide

Why Is My Cat Losing Weight? Causes and When to Worry

Unexplained weight loss in cats is a sign to see a vet. Common causes include hyperthyroidism, diabetes, kidney disease, dental pain and intestinal disease.

Updated October 2026 · 7 min read · By the Hushku editorial team · 3 sources

See a vet within 24 hours. Book a vet visit within a week; sooner if not eating

Short answer

Unexplained weight loss in a cat should always be checked by a vet. If your cat is losing weight while eating more, hyperthyroidism and diabetes are the most likely causes. If it's eating less, think of dental pain, kidney disease, intestinal disease or cancer. Weigh your cat monthly: losing 10% of body weight, just half a kilo (about 1 lb) in a 5 kg cat, is significant.

Key facts

Weight loss with a big appetite suggests hyperthyroidism or diabetes.
Losing 10% of body weight, just 500 g in a 5 kg cat, is significant.
Kidney disease, dental pain and intestinal disease are other common causes.
Weigh your cat monthly; senior cats need checks every 6 months.

This guide is general information, not a diagnosis. If your pet seems unwell, or you're worried, call your vet.

Overview

Cats hide illness well, and weight loss is often the first, or only, sign that something is wrong. Because it happens gradually, and fur hides the change, owners often don't notice until a cat feels bony when stroked.

Is your cat eating more or less? This is the most useful question for your vet.

Losing weight while eating normally or more suggests the body is burning or losing calories faster than they're taken in: hyperthyroidism, diabetes, or poor absorption from the gut.
Losing weight while eating less suggests something is making your cat feel unwell or making eating painful: dental disease, kidney disease, nausea, intestinal disease or cancer.

Weight loss is especially common in senior cats, and the most common causes are treatable or manageable if they're caught early.

Go to a vet now if you see

Not eating at all for more than 24 hours
Vomiting repeatedly, weakness or collapse
Heavy drinking with vomiting or lethargy (possible diabetic ketoacidosis)
Yellow gums, eyes or skin (jaundice)

Common causes

1. Hyperthyroidism
Serious
The classic sign is weight loss despite a huge appetite, often with restlessness, yowling, vomiting and drinking more. Common in cats over 10.
2. Diabetes
Serious
Weight loss with a good appetite and increased thirst and urination. Overweight, middle-aged male cats are most at risk.
3. Chronic kidney disease
Serious
Weight and muscle loss, drinking more, poor appetite and vomiting. Very common in older cats.
4. Dental disease and mouth pain
Needs a vet
Dropping food, chewing on one side, drooling or preferring soft food.
5. Inflammatory bowel disease and intestinal lymphoma
Serious
Weight loss with vomiting, diarrhea or changes in appetite. These can look alike and need tests to tell apart.
6. Parasites
Needs a vet
Worms can cause weight loss, especially in kittens and outdoor cats.
7. Stress, competition or food access
Usually mild
Another cat guarding the food, a new feeding station or a change in routine can mean a cat simply isn't getting enough.

What you can do at home

Weigh your cat on a baby or kitchen scale, or weigh yourself holding your cat and subtract your weight.
Feel along the spine and ribs every few weeks; prominent bones mean your cat is too thin.
Watch at meal times to see whether your cat is eating, struggling to chew, or being pushed away by another cat.
Note any changes in thirst, litter box use, vomiting or behaviour to tell your vet.
Never put a cat on a crash diet or withhold food; cats that stop eating risk fatty liver disease.

When to see a vet

Any unexplained weight loss, especially 10% or more
Weight loss with a bigger appetite, more thirst or more urine
Weight loss with vomiting, diarrhea or eating less
Bad breath, drooling or dropping food

Prevention

Weigh your cat monthly and record it
Senior cats: vet checks every 6 months, with blood tests that include thyroid and kidney values
Feed cats separately in multi-cat homes if one is a food bully
Keep up with dental care and parasite control

Free tools for this

Sources

We check guides against veterinary sources and update them when guidance changes. How we write and review.

Questions.

Is it normal for old cats to lose weight?

Some muscle loss can come with age, but weight loss is never something to put down to old age without a vet check. The common causes are often treatable.

How much weight loss is serious in a cat?

Losing 10% of body weight without trying is significant. For a 5 kg (11 lb) cat, that's just 500 g (about 1 lb).

What tests will my vet do?

Usually a full exam, blood tests (including thyroid levels, kidney values and blood sugar), a urine test and sometimes imaging.

Key facts

Overview

Go to a vet now if you see

Common causes

1. Hyperthyroidism

2. Diabetes

3. Chronic kidney disease

4. Dental disease and mouth pain

5. Inflammatory bowel disease and intestinal lymphoma

6. Parasites

7. Stress, competition or food access