Why Is My Cat Losing Weight? Causes and When to See a Vet
Weight loss in cats is never something to dismiss, particularly when it happens despite a normal or increased appetite. Many serious conditions present with weight loss as an early sign, and earlier detection consistently leads to better outcomes. Here's what to look for and when to act.
Key takeaways
- Hyperthyroidism is the most common cause of weight loss with good appetite in cats over ten — a simple blood test confirms the diagnosis.
- Even small weight changes are significant in cats — regular weigh-ins and hands-on body condition assessment help detect early changes.
- A full blood and urine workup is the essential first step in investigating unexplained cat weight loss; early diagnosis consistently leads to better outcomes.
Why Weight Loss Matters in Cats
Cats' small size means that losing even a few hundred grams represents a significant percentage of their total body weight — a 400g loss in a 4kg cat is a 10% weight reduction, which would be the equivalent of a 7kg weight loss in an average adult human. Even subtle weight changes are clinically significant.
Because cats' coats can mask body condition changes, weight loss often goes unnoticed until it's quite advanced. Regular hands-on assessment — feeling the ribs, spine and hips — is more reliable than visual assessment through a coat. Your vet will use a body condition score (BCS) scale at each visit.
Owners often first notice weight loss when they can suddenly feel ribs or spine more prominently, when the cat's waist becomes more pronounced, or when muscle mass visibly reduces — sometimes most apparent around the head and temples.
Hyperthyroidism: A Common Cause in Older Cats
Hyperthyroidism — overproduction of thyroid hormone — is one of the most common medical conditions in cats over ten years of age in the UK. The excess hormone accelerates metabolism, causing weight loss despite a ravenous appetite, increased thirst, increased activity or restlessness, vomiting and sometimes behavioural changes.
Diagnosis is straightforward — a blood test measuring thyroid hormone (T4) levels is definitive in most cases. Treatment options include daily medication (methimazole/carbimazole tablets), radioactive iodine therapy (curative, performed at specialist centres), surgical thyroidectomy, or dietary management with iodine-restricted food (Hill's y/d).
Medication (carbimazole tablets) is the most common initial treatment in the UK and is effective in controlling the condition, though requires lifelong administration and regular blood monitoring.
Diabetes Mellitus
Diabetes mellitus in cats causes weight loss because the body is unable to use glucose for energy and instead breaks down fat and muscle. Classic signs include weight loss despite increased appetite, greatly increased thirst and urination, and a characteristic 'plantigrade' stance (walking on hocks rather than tiptoes) in advanced cases.
Obese cats, neutered male cats and older cats are at higher risk. The good news is that some cats, with appropriate management, achieve diabetic remission — particularly if diagnosed early and treated with appropriate insulin and a high-protein, low-carbohydrate diet.
Diabetes management in cats involves twice-daily insulin injections (owners are trained by their vet to administer these at home), regular blood glucose monitoring and dietary management. The ongoing cost of insulin, syringes and monitoring should be factored into long-term planning.
Chronic Kidney Disease
Chronic kidney disease (CKD) is extremely common in older cats and causes progressive weight loss, reduced appetite, increased thirst, vomiting and lethargy as the kidneys' ability to filter waste products declines. It's one of the leading causes of death in cats over ten.
CKD is diagnosed via blood and urine tests that assess kidney function markers (creatinine, SDMA, urea) and urine specific gravity. The condition is staged using the IRIS (International Renal Interest Society) system, which guides treatment decisions.
Management involves prescription kidney diets, encouraging fluid intake (wet food, water fountains), phosphate binders, blood pressure medication if hypertensive, and in later stages subcutaneous fluid therapy. Early diagnosis via regular senior blood screening improves management options and quality of life.
Other Causes of Weight Loss in Cats
Inflammatory bowel disease (IBD) causes poor nutrient absorption and can present as weight loss despite normal or increased appetite, alongside vomiting and diarrhoea. Intestinal lymphoma — the most common cancer of the feline gastrointestinal tract — can be difficult to distinguish from IBD without biopsy.
Dental disease is a frequent and underappreciated cause of weight loss — a cat with a painful mouth may eat less, lose weight and appear generally unwell. Other causes include liver disease, heart disease, parasitism, certain infections (FIV, FeLV) and other cancers.
The diagnostic approach to unexplained weight loss in cats typically involves a full physical examination, blood and urine tests, and often imaging (ultrasound). A standard vet consultation costs £40–£65 in the UK; a full diagnostic workup may cost significantly more, underscoring the value of appropriate pet insurance.
Find a Vet Near You
Unexplained weight loss in a cat should always be investigated. Use CompareMyVet at app.comparemyvet.uk to find and compare vet practices near you and understand local costs, so you can act promptly when your cat needs help.
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Common questions
Any unintentional weight loss warrants investigation. A loss of more than 5–10% of body weight over a short period (weeks to a few months) is clinically significant. In practice, if you can feel your cat's ribs or spine more prominently than before, mention it to your vet.
Yes — stressed cats often eat less. However, significant weight loss is rarely explained by stress alone. A vet examination is needed to rule out medical causes before attributing weight loss to a psychological or environmental factor.
Most vets recommend annual blood and urine screening for cats over seven, and six-monthly screening for cats over ten. This enables early detection of conditions like CKD, hyperthyroidism and diabetes, when management options are most effective.
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