Cat losing weight but still eating: the three things a blood test separates

Why a kept appetite is the useful part of the story

Weight loss is not one problem. A cat that eats less and loses weight raises one set of questions, a cat that eats the same or more and still loses weight raises another, and the second list is shorter. When food is going in normally and the cat is still getting lighter, the calories are either being burned too fast, lost in the urine, or not being held on to. That is why the appetite is the first thing a vet asks about, and why your answer changes which tests go on the form.

Weight is also the one figure you can produce yourself. Cats hide loss well under a coat, and a cat that has gone from 4.5 kg to 4.0 kg has lost more than a tenth of itself. Use the same kitchen or baby scales, once a fortnight, and write the numbers down with the date. A sequence of weights is worth more to the appointment than any description.

The three lines of enquiry in a cat over eight

These three also overlap, which is the reason the whole panel is run rather than one test. An overactive thyroid increases blood flow through the kidney and can make kidney values look better than they are, so kidney damage sometimes only becomes visible once the thyroid is under control. Cats with one of these conditions can have a second. And other causes sit behind the three: gut disease that stops nutrients being absorbed, dental pain that changes how a cat eats, parasites, a cancer, or simply an older cat losing muscle. The panel is what sorts the list, not the symptom.

What each marker settles

• Total T4. The direct reading of thyroid output. Above the range it is the strongest single finding on this list. Inside the range it does not fully clear the thyroid in a cat that fits the picture, because a concurrent illness can suppress the value, hence a repeat or a free T4.
• Blood glucose. High and staying high is the diabetes pattern. One high reading in a cat that has just travelled is not, because stress alone moves feline glucose a long way.
• Fructosamine. Glucose sticks to blood proteins over time, so fructosamine reflects the average of roughly the previous two to three weeks. It is the test that separates real diabetes from a stressful morning, and it is the reason a vet may ask for a second small sample rather than acting on the first.
• Creatinine. Waste from muscle, cleared by the kidney. Its weakness in exactly this situation is that a cat which has lost muscle makes less of it, so creatinine can sit inside the range while filtration is already reduced. In a thin cat it understates the problem.
• SDMA. Cleared by the kidney, leaves the range earlier than creatinine, and barely depends on muscle mass. That combination is why it earns its place on a panel for a cat that is losing weight.
• Urea. Moves with filtration but also with dehydration and with a protein rich meal before the sample, so it is read as support rather than as the headline.

Alongside these, expect the vet to want a urine sample, a blood pressure reading and a check of the heart, because all three of the main conditions reach beyond the blood panel. Urine specific gravity in particular has to be measured on a sample taken before any fluids are given.

Two reading habits that apply to every line. Compare a value only with the range printed on your own report, because ranges follow the analyser and the laboratory. And a single panel freezes one moment, so the comparison between two samples taken in similar conditions is often what carries the answer.

Questions to ask your vet

1. Here is my cat's weight over the last few months. How much has been lost in percentage terms?
2. Has total T4 been run, and if it came back inside the range, does the picture still justify repeating it or adding a free T4?
3. Was the glucose reading taken in a stressed cat, and is a fructosamine worth adding to settle it?
4. Do creatinine and SDMA agree, and could lost muscle be making creatinine look better than it is?
5. Was urine specific gravity measured, on a sample taken before any fluids?
6. Has blood pressure been checked, and the heart listened to?
7. Which of these results would you want to repeat, and how soon?

What is T4 in cats?

T4, or thyroxine, is the hormone the thyroid gland releases to set the speed at which the body burns fuel. Total T4 on a blood panel measures how much of it is circulating. When the gland is overactive the value climbs above the laboratory's range, and the cat loses weight because it is running hot, not because it is eating too little. Total T4 is the usual first test in the UK and is reported in nanomoles per litre by most laboratories.

What is a high T4 level in cats?

High means above the range printed on your own report, and that range differs between laboratories and methods, so a number copied from elsewhere is of no use. The more useful point is the opposite case: a minority of cats with an overactive thyroid, often quoted as up to about one in ten, have a total T4 that still sits inside the range, usually early on or when another illness is pulling the value down. That is why a normal T4 in a cat that fits the picture is often repeated, or followed by a free T4.

Why is my cat's glucose high?

In cats, the single commonest reason for one high reading is the appointment itself. The stress of the carrier, the car and the consulting room can push blood glucose a long way up in a healthy cat, which is the leading cause of a false diabetes alarm. Fructosamine is what settles it, because it reflects the average glucose over roughly the previous two to three weeks and is not moved by a stressful morning. Glucose in the urine also weighs in.

My cat is losing weight and drinking lots of water. What does that add?

Increased thirst narrows things, because all three of the usual causes can produce it. It does not sort between them, but it does make the urine sample more valuable: specific gravity read alongside creatinine and SDMA is what separates a kidney that can no longer concentrate from a cat drinking for another reason. Measure what your cat drinks over 24 hours if you can, and mention any litter tray that now needs changing more often.

My cat is losing weight and always hungry. Is that a good sign or a bad one?

It is neither, it is a clue. A kept or increased appetite alongside falling weight means the calories are going somewhere rather than not arriving, which is what points towards an overactive thyroid or diabetes before anything else. A cat that is losing weight and has also gone off its food sends the enquiry in a different direction. Either way the weight itself is the hard data, so weigh your cat at home if you can and bring the figures.