Hyperthyroidism, (the term for an overactive thyroid gland, producing excessive amounts of thyroid hormone) is the most common endocrine (hormonal) disease of cats, usually affecting middle-aged to older cats. It is more often caused by a benign (non-cancerous) overgrowth of one the thyroid glands in the neck, causing an overproduction of thyroid hormones. Rarely, it is caused by a malignant (cancerous) thyroid carcinoma.

Signs of hyperthyroidism

Signs you may see at homeClinical signs picked up on a clinical exam by your veterinary surgeon
Weight loss despite an increased appetitePoor body condition
Vomiting +/- diarrhoeaHeart murmur
Increased thirstTachycardia (increased heart rate, sometimes with an abnormal rhythm)
Dull, unkempt hair coatPalpable goitre (enlarged thyroid nodule in the neck)
Increased activity, restlessness, or behavioural changes e.g., aggressionSigns of hypertension (high blood pressure) such as sight loss 

How will your cat be diagnosed?

Your vet will likely perform a clinical exam and order a blood test to check thyroid levels and your cats’ general health. Your vet may also assess a urine sample; as kidney function is closely related to thyroid function and so they often need to be assessed together to make sure your cat feels well. 

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What should you think about whilst choosing a diet for your hyperthyroid cat?

When considering your cats’ dietary requirements, it is easy to get caught up researching the best food for their condition. Don’t worry! The research has been done and the scientific studies are out there, so you don’t have to reinvent the wheel. Modern clinical studies mean that we should view hyperthyroidism as a disease that affects the whole body. The effect of too much circulating thyroid hormone, caused by an overactive thyroid, is that the cats’ metabolism goes into overdrive. As a result of altered metabolism and its effects on other organs within the body, your cat can develop potential complications with organs such as the kidneys as well as being predisposed to other disorders like diabetes. 

Do you choose a diet for thyroid health, kidney health or diabetes?! 

Discussing your cats test results with your veterinarian is a must here; as it’s important to look at cats as individuals, what are their preferences alongside what the clinical picture suggests. In reality, unless using diet as the sole control for your cats’ illness, (see infographic for treatment options) the most important factor is palatability and finding a diet that is a compromise of considered factors, to first, do no harm. 

Does your cat like their food?

Older cats, especially those with an overactive thyroid often have comorbidities (related problems) leading to a reduced appetite. Hyperthyroidism causes weight loss and muscle loss, so diet is an important consideration. It is notable that at diagnosis, your hyperthyroid cat will often be hyperactive and polyphagic (overeating) due to the condition itself. Once the condition is treated, your cat may appear quieter, resting more, and appearing to be a pickier eater. In other words, activity levels and appetite have returned to a more “normal” level for their life-stage / age. 

What should you avoid when feeding your cat?

Foods high in iodine will contribute to the thyroid producing excess levels of hormone and therefore exacerbate the disease. It is important to consider diet composition if feeding a homemade diet or shop bought non-prescription diet. If you’re unsure, ask your veterinarian for advice. 

About Iodine restricted diets

Research shows that iodine intake is necessary for the production of thyroid hormone. And that strict iodine restriction prevents the thyroid being able to overproduce that hormone. It has been proven that cats with hyperthyroidism consuming an iodine restricted diet see thyroid levels reducing within 4 weeks of starting the diet. Within twelve weeks of consuming an iodine restricted diet, thyroid levels are seen to normalised, without compromising other areas of health. 

Iodine restricted diets are available in the form of prescription diets such as hills Y/D. There are wet and dry forms of this diet to suit any pets’ preference. These are designed to be fed as a sole diet, so no other food at all to be fed alongside. That includes treats, mice, birds and even spiders or other edible crunchies your pet finds!

What if you didn’t choose diet as a sole treatment option, then what should you feed your cat?

Cats not being treated with a prescription diet as a treatment option, need to consider other factors like kidney disease that often comes hand in hand with hyperthyroidism. Your vet may recommend a different prescription diet depending on test results or a clinical examination. If your cats’ kidneys need support, you may be advised to try a protein-controlled diet with a higher quality protein source that is high in energy and has a controlled phosphorus and low sodium content. Diets like Hills K/D or Royal Canin Renal diet may be suitable. 

Sometimes, it’s appropriate to choose a high-quality non-prescription diet. But again, depending on your cats’ test results, the composition of which should be discussed with your veterinary team. 

Is your cat a picky eater?

Cats can be difficult at the best of times. And at a whim, deciding that the imported salmon you re-mortgaged the house for, that was the only food they would eat last week, is now an offence! Sometimes you need to consider rotating flavours of the same diet; before thinking about switching brands altogether, as this can cause tummy upsets. 

Try not to mix flavours daily. Instead stick with one flavour until your cat decides that’s not for them and then move on. This way if your cat feels nauseous at any point in their treatment, which isn’t uncommon, they won’t be put off all of their food. 

If in doubt, there is a whole nursing team at your veterinary surgery equipped to answer questions about nutrition in your cat with hyperthyroidism. The nurses have a myriad of tricks up their sleeves and there isn’t a scenario they’ve not come across before, so don’t be afraid to call! 

If you’re worried about your cats eating, what should you do?

It is common for owners to present their cat in a consultation post diagnosis and treatment of hyperthyroidism for “not eating” or “reduced appetite”. Consider, is your cat now eating throughout the day instead of in one ravenous, unsatisfied, hormone driven frenzy, that you have become used to? The important focus is body condition score (BCS) which focuses on a healthy weight for your cats’ size. If their BCS is in range, and your cats’ weight is remaining stable over time, their disease is likely well controlled. If they are enjoying their food, playing, and sleeping without vomiting or diarrhoea – I’m a happy vet, and you should be assured you can relax, you have a happy cat. You and your vet will continue to monitor to ensure this stays the case!

Depending on which treatment option you chose to control your cats’ overactive thyroid, you may have regular check-ups and testing such as blood tests, urine tests and weigh-ins to assess your cats’ stability and health. These are designed to make sure the dose of medication is correct and your cat is healthy without experiencing complications of treatment or the disease itself. If you notice a change in your cats’ condition, such as inappetence, vomiting, diarrhoea or significant weight loss, alert your veterinarian. Together you can assess if changes need to be made to the treatment protocol to keep your cat happy and healthy.