Your senior cat is losing weight. It's not just age.

"He's just getting old" is the most costly sentence in feline medicine. Cats do lose lean mass with age, gradually — but a cat that is visibly thinner than last year has a diagnosis waiting, and in most cases it's one of four, all of which are far more manageable when caught at the start.

The problem is detection. Cats lose weight slowly, they're covered in fur, and you see yours every day, which makes you the worst possible judge of gradual change.

The 5% rule

A 5% loss of body weight is clinically significant. In a 5 kg cat, that's 250 g — about the weight of an apple. You will not see it. You will not feel it. It will not show in a photograph.

This is why weight is the one number worth measuring rather than estimating. Monthly, on a kitchen or baby scale, same scale every time. Weigh the carrier empty, then with the cat in it, and subtract — most cats tolerate that far better than standing on a scale.

A loss of 10% over a few months, unexplained, is a work-up. Not a wait-and-see.

Track it somewhere that draws the line for you. PawSolids charts weight per pet and flags the trend rather than the single reading, which is the whole point: any individual weigh-in is noise, and the slope is the signal. In a multi-cat household it also keeps each cat's curve separate, which is the only way to notice that one of three cats has quietly dropped 400 g.

The four causes behind most cases

1. Hyperthyroidism. The classic picture: an older cat that is losing weight while eating ravenously. Often with increased thirst, restlessness, night-time yowling, a poor coat, and sometimes vomiting or soft stool. It's common in cats over ten, diagnosed with a simple blood test, and treatable several ways including a definitive cure with radioiodine. Untreated it damages the heart and kidneys.

2. Chronic kidney disease. The most common serious disease of older cats. Increased thirst and urination first, then weight loss, reduced appetite, and nausea. Cannot be reversed, but earlier intervention — diet, phosphate control, hydration, blood pressure management — measurably extends both lifespan and quality of life. This is the condition where early detection buys the most.

3. Diabetes mellitus. Weight loss with a strong appetite, plus marked thirst and urination. Overweight cats are at highest risk. Caught early and managed well, a meaningful proportion of feline diabetics achieve remission — that window closes with time.

4. Chronic enteropathy and intestinal lymphoma. Weight loss with chronic vomiting, soft stool, or a gradually reduced appetite. The IBD-to-small-cell-lymphoma spectrum is common in older cats, and small-cell lymphoma in particular responds well to treatment, often for years.

Others worth naming: dental disease (a cat with a painful mouth eats less and eats differently), hyperthyroidism masking kidney disease underneath it, pancreatitis, and cancer elsewhere.

Note the pattern in the first three: weight loss despite a normal or increased appetite. That combination is never normal ageing. Ageing doesn't make a cat eat more.

Feel, don't just look

Body condition scoring takes fifteen seconds and catches what the eye misses:

• Ribs. You should feel them easily under a thin layer of fat, like the back of your hand. Sharp and prominent with no covering is too thin.
• Spine and hips. Palpable but not knife-edged.
• Waist. Viewed from above there should be a visible narrowing behind the ribs.
• Belly. From the side, the abdomen should tuck up rather than hang.

Also assess muscle separately, over the shoulder blades and along the spine. Older cats lose muscle mass (sarcopenia) even at a stable weight — a cat can hold 4.5 kg while trading muscle for fat, and that trade matters clinically. Wasting over the shoulder blades is a genuine finding, not a normal part of getting old.

What to log for the appointment

Bring numbers, and your vet can skip straight to the tests that matter:

• Weight, monthly, at least six months of history if you have it.
• Appetite — up, down, or unchanged. Crucial, because it splits the differential immediately.
• Water intake. Increased thirst points hard toward kidney disease, diabetes, or hyperthyroidism. Even rough daily estimates help — see the 50 ml/kg/day baseline.
• Litter box output. More urine clumps, larger clumps, or a change in stool.
• Vomiting frequency, counted rather than estimated.
• Behaviour. Sleeping more, hiding, yowling at night, grooming less, jumping less.

A cat that has lost 400 g over four months with increased thirst and a big appetite is, on the history alone, a hyperthyroid suspect — and that's a same-day blood test rather than a month of watching. The data doesn't replace the diagnosis; it points at it faster.

Book an appointment if: your cat has lost 5% or more of its body weight, is eating more but getting thinner, has become noticeably thirstier, or has stopped grooming. And treat any cat that stops eating entirely for 24 hours as urgent — cats develop hepatic lipidosis quickly, and it's a serious complication on top of whatever caused the loss of appetite.

The routine that catches it

Cats over seven should have a check-up with bloodwork twice a year, not annually. In between: weigh monthly, run a hand along the ribs and spine weekly, and note appetite and thirst when they change.

That's perhaps two minutes a month, and it's the difference between finding kidney disease at stage 2 and finding it at stage 4.

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PawSolids is a health journal and pattern detector, not a diagnostic tool. Weight loss in an older cat needs bloodwork — the log is what tells you when to ask for it.