Veterinary Internal Medicine Nursing

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34 | How to understand hyperthyroidism in 4 easy steps

Laura Jones

We’ve all seen those older cats who come in with the body condition score of a hat-rack and an appetite that rivals ours at the end of a busy shift.

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Yep, that’s right - they’re hyperthyroid.

Hyperthyroidism is the most common endocrinopathy seen in cats, so we know we will be treating and nursing these patients very commonly. But to do that effectively, we need to understand what hyperthyroidism is and how it affects our patients.

So, let’s waste no time and get stuck into what happens when our patients get hyperthyroidism.

What IS hyperthyroidism, anyway?

If we think back to episode 26 and our introduction to the endocrine system, our patients have two thyroid glands located on either side of the ventral neck.

Our thyroid glands secrete thyroid hormones, T3 and T4. They form these hormones from dietary iodine, among other substances.

These thyroid hormones are released from the thyroid glands under the influence of the pituitary gland. The pituitary gland detects when more thyroid hormone is needed and releases thyroid-stimulating hormone, TSH, which prompts the gland to release T3 and T4.

That’s great - but what do these hormones do?

Thyroid hormones control our body’s metabolism. Our metabolism is the rate at which food is converted into energy, the rate at which food is converted into the building blocks of things like proteins and enzymes, and the elimination of metabolic waste.

They’re also responsible for growth and development and play an important role in controlling cardiac, musculoskeletal, and digestive function and maintaining healthy bones and brain development.

So, when our patients produce too little or too much thyroid hormone, we see changes in their metabolism as a result.

Hyperthyroidism is the excessive secretion of these thyroid hormones.

It’s seen most commonly in cats (dogs can also get it if they have a thyroid tumour, but that’s very rare).

In most cats, it’s due to a benign tumour (adenoma) affecting one or both of the thyroid glands. This tumour releases thyroid hormones, bypassing the body’s negative feedback loop, so the patient’s metabolism speeds up.

In some cases, this can be caused by a malignant tumour (adenocarcinoma) - though thankfully, this is very rare.

What signs do we see in cats with hyperthyroidism?

As our patients’ metabolism speeds up, they’ll begin burning through energy at a faster and faster rate, meaning they’ll lose fat (and then muscle) stores and become polyphagic as they try to eat more and more food to keep up with the speed at which they’re burning through food.

However, as they continue to produce more thyroid hormones, they’ll never be able to eat enough to meet the body’s demand for energy, meaning they continue to lose weight.

On top of that, we see an increase in the sympathetic nervous system tone - our ‘stress system’. The combination of high metabolic rate and SNS tone means that we see several other changes in these patients, including:

  • Vomiting

  • Diarrhoea

  • Constipation

  • Irritability or hyperactive behaviour

  • Tachycardia

  • Hypertension

  • PUPD (due to increased renal blood flow)

On clinical examination, these patients also often have poor skin and coat quality and may have cardiac changes, such as a murmur and/or gallop rhythm on auscultation.

So those are the signs we see, but how do we diagnose these patients?

Hyperthyroidism is diagnosed based on bloodwork, but we perform a few other tests with these patients. These include blood pressure measurement, urine analysis, and potentially echocardiography as well.

Let’s start with blood pressure measurement.

Systemic hypertension is a real risk in our hyperthyroid patients because they are tachycardic and have increased sympathetic drive. Where systemic hypertension is present, so too is the risk of target organ damage - with the CNS, kidneys and eyes commonly affected.

So, blood pressure should be measured regularly in these patients, beginning at the time of diagnosis and repeating at revisit appointments.

And then there’s bloods…

Biochemistry, haematology and thyroid hormone levels are vital diagnostics in these patients.

Biochemistry is used to determine baseline renal function (treating hyperthyroidism often unmasks renal disease) and look for evidence of other diseases. Remember that our hyperthyroid cats are seniors, so they are at risk of concurrent diseases. Pay close attention to their BUN, creatinine, phosphate, and electrolytes, for example. We can also see increased liver values in hyperthyroid cats, so look at their ALT and ALP.

Haematology may be normal or show increases in RBCs, neutrophils and lymphocytes.

But the real diagnosis comes from measuring thyroid hormone levels. We measure T4—or thyroxine—in two different ways. We can measure the total T4 (which measures ‘free’ and bound T4) or just the freely circulating or unbound T4 (free T4).

To begin with, we’ll measure the total T4. This is a better test for initial diagnosis, and we can then add a free T4 test on top if needed.

In a hyperthyroid cat, total T4 levels are usually elevated—but they might not always be. This is because other things in the body can affect metabolism. For example, if a patient is sick, their T4 levels will drop since the body slows the metabolism while they recover or deal with their illness.

This can cause a falsely normal total T4 level in a sick, hyperthyroid patient. In cases where the clinical exam findings and history fit with a diagnosis of hyperthyroidism but the blood results don’t, we’ll add a free T4 test. This is a more specific test, so it’s better at ruling in hyperthyroidism in these grey-zone cases.

What other diagnostics do we perform?

Other diagnostic tests in hyperthyroid patients include cardiac ultrasound and/or ECG monitoring in patients with heart murmurs or abnormal rhythms on auscultation and thyroid scintigraphy.

Thyroid scintigraphy is rarely performed but involves imaging a patient with a gamma camera after an injection of radionuclide. We compare the density of the thyroid glands in the image to the salivary glands to determine if the thyroid glands have taken up a normal level of radionuclide.

So you’ve got your patient diagnosed - how do we treat them?

There are three options for treating hyperthyroidism - medical management, surgical management or radioactive iodine treatment.

Let’s start with medical management

Medical management uses thyroid inhibitor medications. These prevent the synthesis of thyroid hormones, normalising T4 levels.

The main medications used are methimazole (aka thiamazole) or carbimazole. Methimazole is a twice-daily medication, whereas carbimazole is converted to methimazole in the body, so it only requires daily administration.

These can be given lifelong or administered before other treatments (such as radioactive iodine or surgery) to stabilise the patient and allow assessment of renal function before anaesthesia or I-131 treatment.

Then there’s surgery…

Surgery is performed less frequently than medical management or radioactive iodine treatment. It involves the removal of one or both thyroid glands (depending on whether both glands are affected).

A thyroidectomy isn’t always an option for every cat - many have ectopic thyroid tissue in other locations (e.g. the mediastinum), so this means they can still develop hyperthyroidism after surgery.

There are a few risks to be aware of in surgical patients. Firstly, if iatrogenic damage to or removal of the parathyroid glands occurs, hypocalcaemia can result. Calcium levels should be closely monitored in recovery, and patients should be observed for signs of hypocalcaemia.

Other risks include damage to the nerves in that area - including a risk of laryngeal nerve dysfunction or facial nerve damage. These patients can (albeit rarely) develop Horner’s syndrome or laryngeal paralysis after surgery; this may be temporary or permanent.

What about radioactive iodine?

Radioactive iodine therapy is a newer treatment for hyperthyroidism. It’s simple, safe and effective and involves injecting a radioactive iodine isotope (I-131). The I-131 destroys hyperfunctioning tissue in the thyroid gland while sparing hypofunctioning and parathyroid tissue.

Success rates are reported at 95% with a single injection, with a follow-up injection needed in a few cases. Rarely can patients become hypothyroid after I-131 treatment; in these cases, treatment with levothyroxine will be needed (just like our hypothyroid dogs, which we’ll be chatting about in the next episode!).

After injection, our patients are radioactive. The half-life of I-131 is 8.1 days, and during this time, it’s important that we follow strict health and safety guidelines when managing these patients.

They are usually hospitalised for 10-14 days, depending on dose, location, and local rules (some clinics hospitalise for shorter periods, so always check with your local radioactive iodine unit to find out what they do). Throughout hospitalisation, we must minimise contact with these patients and their waste since it contains traces of radioactive iodine.

And what about iodine-restricted diets?

Management with an iodine-restricted diet is another technique used to treat hyperthyroidism in some cases.

These diets replace medical therapies since dietary iodine is needed to synthesise thyroid hormones. 

These diets are unsuitable for non-hyperthyroid cats, so they cannot be free-fed in multicat households. They also need to be the pet’s sole food source, so they’re not ideal for cats who visit other homes, hunt or have access to other foods. Even treats, pill pockets, or a small amount of table food can reduce the effectiveness of the diet since these all contain iodine.

When using these diets, the considerations are exactly the same as for patients under medical management. Rechecks, including bloodwork, should continue at least every six months, like when managing a hyperthyroid cat on medication. 

Patients transitioning from medications to diet should be weaned off medical management over several weeks while gradually introducing the new diet.

So there you have it - my guide to understanding and treating feline hyperthyroidism! To do this, we first must understand how our thyroid glands work and what happens when they begin overproducing thyroid hormones. This explains the signs we see - especially weight loss and polyphagia, alongside those sympathetic nervous system tone changes. Once we’ve diagnosed them, we’ll think about treatment options, including medication, surgery, and radioactive iodine, depending on the individual patient and what you and their family think will be best.

Did you enjoy this episode? If so, I’d love to hear what you thought - screenshot it and tag me on Instagram (@vetinternalmedicinenursing) so I can give you a shout-out and share it with a colleague who’d find it helpful!

Thanks for learning with me this week, and I’ll see you next time!

References and Further Reading