Cat Losing Weight but Still Eating: 6 Causes

A cat that eats well and still loses weight is not simply getting old. That specific pattern, good appetite plus shrinking body, is one of the most useful diagnostic clues in feline medicine. It narrows the list fast, because it rules out the obvious explanation that the cat is not eating enough.

Owners often notice it late. The fur hides a lot, and a cat that has always been slim can lose half a kilo without looking dramatically different. By the time the ribs feel sharp, the process has usually been running for months.

Measure Before You Guess

Weigh the cat on a baby scale once a week, same time of day, and write it down. A kitchen scale with a box works if the cat tolerates it. Then run your hands along the spine and hips. Fat feels soft and covers the bones. Muscle feels firm, and a cat losing muscle while eating is losing tissue, not just padding.

Body condition scoring charts from your vet put a number on what your hands are feeling. A single weight is a data point. A trend over four weeks is a diagnosis waiting to happen.

One: Hyperthyroidism

This is the classic cause in cats over eight. An enlarged thyroid gland pumps out too much T3 and T4, and the body burns through calories at a rate the food cannot match. The cat eats voraciously, loses weight, and often becomes irritable, vocal at night, or unable to settle.

Cornell's Feline Health Center notes that most cats with hyperthyroidism have elevated T4 levels, but a small percentage have T4 within the normal range while still being hyperthyroid. In those cases, additional testing such as a free T4 or a suppression test is needed. The vet will also palpate the neck for an enlarged gland, which is sometimes visible as a bulge.

Treatment options are medication, radioactive iodine, surgery, or a prescription diet. Radioactive iodine is often preferred because it destroys abnormal thyroid tissue while sparing the parathyroid glands, but it requires a specialist facility and a short hospital stay.

Two: Diabetes Mellitus

A diabetic cat cannot use glucose properly, so the body breaks down fat and muscle for energy. The result is weight loss despite a huge appetite, usually with excessive drinking and urination. Cornell estimates that between one in one hundred and one in five hundred cats will be diagnosed with diabetes in their lifetime.

Diagnosis rests on persistent high blood glucose plus glucose in the urine, often confirmed with fructosamine. Treatment is insulin injected under the skin roughly every twelve hours, with a diet change and a strict routine. Syringe size matters: U-100 and U-40 insulin require different syringes, and mixing them up is a genuine hazard. In 2023 an oral medication was approved for feline diabetes, which gives some owners an alternative, though it does not suit every cat.

Three: Gastrointestinal Disease

Inflammatory bowel disease and small-cell lymphoma both cause weight loss with a normal or increased appetite, because the gut cannot absorb what passes through it. Vomiting, softer stool, and a dull coat usually accompany the weight change. Ultrasound and biopsy are often needed to tell the two apart, and the distinction matters because the treatments differ.

Four: Parasites

Roundworms, tapeworms, and giardia steal nutrition directly. Indoor cats are not exempt, because flea eggs and shoes travel. A fecal test is cheap, and the treatment is simple. Any cat with weight loss and a ravenous appetite should have one before more expensive tests.

Five: Dental Pain

A cat with a painful tooth or resorptive lesion may still eat, but it eats differently: dropping food, chewing on one side, swallowing kibble whole, or preferring soft food. It also eats less efficiently. A full oral examination under anaesthesia finds what a conscious look cannot.

Six: Chronic Kidney Disease

Kidney disease usually reduces appetite rather than increasing it, but it belongs on the list because it causes muscle loss and nausea that owners read as fussiness. Bloodwork and a urine test, including urine specific gravity, are how it is found. Early detection buys years of management.

What the Vet Will Run

A sensible first pass is a complete blood count, a chemistry panel, a T4, a urinalysis, and a fecal test. That combination finds thyroid disease, diabetes, kidney disease, and most parasites in one visit. If those come back clean and the weight is still falling, the next step is usually abdominal ultrasound.

Bring the weight log. A vet who can see a steady decline over six weeks will move faster than one hearing that the cat seems thinner lately.

What Not to Do

Do not free-feed to compensate. A hyperthyroid or diabetic cat will eat more and still lose weight, and you will have added a second problem. Do not assume it is ageing. Do not start a senior diet without bloodwork, because senior diets often restrict protein and that is exactly wrong for a cat losing muscle.

Go Now If

• Weight is dropping fast, more than a few percent in a month
• Drinking and urination have increased sharply
• The cat is vomiting, has diarrhea, or has stopped grooming
• Breathing is laboured, or the cat hides and stops interacting
• The abdomen looks bloated while the spine looks sharp

Weight loss with a good appetite is a signal that something is running in the background. In most cases it is treatable, and in almost every case early treatment is cheaper and kinder than the late version.