Feeding & Nutrition

She was back at the bowl for the third time before eight in the morning, shouting at the cupboard door between mouthfuls. Ruby was seventeen, eating more in a week than she used to get through in a fortnight, and lighter every time I picked her up. Her coat had slipped to that unkempt, greasy look cats get when grooming stops feeling worth the effort. The blood panel confirmed what the bowl had been saying for a month. Her thyroid was running the whole cat at double speed.

Hyperthyroidism is the most common hormonal disorder of older cats, and its signature is exactly that contradiction: an enormous appetite sitting on top of steady weight loss. The gland pours out excess hormone, metabolic rate climbs, and the cat burns fuel faster than she can shovel it in. Owners often mention the appetite with relief at first, because an old cat eating with enthusiasm feels like good news. It isn't. A senior cat losing weight while eating well belongs at the vet, not in front of a bigger bag of food.

The weight coming off is not simply fat. Cats in this state break down lean muscle to keep pace with demand, which is why an untreated hyperthyroid cat can feel bony along the spine and over the shoulder blades while still licking the bowl clean. Cats are obligate carnivores with a high baseline protein requirement and little ability to spare their own tissue, so muscle goes quickly and returns slowly. On a cat approaching twenty, rebuilding it is genuinely hard work.

Which makes the feeding priority before the disease is controlled an unusual one: feed generously. Energy needs are elevated, sometimes dramatically, and this is not the moment for portion control or a weight-management formula. A calorie-dense, highly digestible food with plenty of good quality protein, split across several meals a day, gives the cat something to burn other than herself. Cats who are also vomiting or passing soft stools, both common here, cope better with small frequent servings.

What happens next depends on which treatment route you choose, and there are four. Daily anti-thyroid medication, usually methimazole, is the flexible one: cheap to start, reversible, easy to adjust, but it is a tablet or a smear of transdermal gel twice a day for life, with regular bloodwork. Radioactive iodine resolves the problem in most cats in a single treatment and is what many specialists would choose for their own animal, at the cost of a stay in an isolation ward and a bigger bill. Surgery is done less often than it once was. And then there is the diet.

An iodine-restricted therapeutic diet works on a straightforward principle. The thyroid cannot manufacture hormone without iodine, so restricting the raw material brings hormone output down. It suits certain cats very well: the ones who will not tolerate being medicated twice a day, the households where nobody is home at the right hours, the cats with other conditions that make travel or anaesthesia a poor idea. Like any therapeutic diet it is prescribed by a vet and monitored with blood tests, because the only proof it is working is a thyroid level that actually falls.

What it demands in return is total compliance, and that is where it comes apart in real houses. The diet works only if it is the sole thing the cat eats. No treats. No dental chews, no lickable tubes, no saucer of milk, no shred of chicken off the roast. No stealing from the other cat's bowl, which in a multi-cat home means genuinely separating meals rather than half watching the kitchen. And no hunting, because one mouse carries enough iodine to undo a week. It is realistically a diet for indoor cats in disciplined households, and if you cannot promise that, pick another route rather than fail slowly and blame the food.

Be clear about what the diet is doing, too. It manages the hormone level and does nothing about the thyroid nodule itself, which carries on growing quietly in the neck. Daily medication has the same limitation; radioiodine is the only routine option that deals with the tissue rather than its output. Long-term outcome data comparing diet-only management with the alternatives is thinner than anyone would like.

Then there are the kidneys, and this is the part owners are rarely warned about. An overactive thyroid drives blood flow through the kidneys harder than normal, which flatters the numbers on a routine panel and can conceal kidney disease that is already present. Bring the thyroid under control and that artificial boost vanishes, sometimes exposing values that were hidden all along. This is not an argument for leaving the thyroid alone, because uncontrolled disease damages heart and kidneys both. It is the reason many vets start with medication: a tablet can be stopped and the dose walked back, while radioiodine cannot be undone. Where both conditions exist, the ideal plans pull against each other on protein and phosphorus, and untangling that needs your vet with the bloodwork in front of her, occasionally with a board-certified veterinary nutritionist involved. No compromise recipe circulating online will do it safely.

A few things to steer clear of. Kelp, seaweed and the thyroid support supplements sold online are iodine-rich, and giving one to a hyperthyroid cat is pouring petrol on the fire. Home-cooked low-iodine diets are close to impossible to get right, because iodine content varies between ingredients and is almost never listed.

The most useful thing you can do at home is dull. Weigh the cat on a kitchen scale on the same day each week and write the number down. Appetite lies in this disease, cheerfully, right to the end, so your hands on her spine and a line of weekly numbers tell you more than an empty bowl does.

Ruby went onto medication first, then to radioiodine three months later once her kidney values had held steady. She put a pound and a half back on, her coat came right, and she stopped yelling at the cupboard. She still turns up at the bowl three times before breakfast. That part, apparently, is permanent.