Current Vet

In this episode of Current Vet, Dr Lottie talks about feline hyperthyroidism, the diagnostic tests available, and the pros & cons of different treatment options

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Timeline:
00:00 Intro
00:42 Case
02:03 Aetiology & Pathogenesis
05:53 Clinical Signs
07:40 Diagnosis
15:41 Treatment
21:31 Key Points
23:17 Outro

Recommended Reading
  • AAFP Guidelines for the Management of Feline Hyperthyroidism
Resources
  • Carney H.C. et al. (2016). 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. Journal of Feline Medicine and Surgery, 18(5): 400–416. https://doi.org/10.1177/1098612X16643252
  • Wakeling J, Everard A, Brodbelt D, Elliott J, Syme H. (2009). Risk factors for feline hyperthyroidism in the UK. Journal of Small Animal Practice, 50(8): 406–414. https://doi.org/10.1111/j.1748-5827.2009.00756.x
  • Mooney, C. T. (2002). Pathogenesis of Feline Hyperthyroidism. Journal of Feline Medicine and Surgery, 4(3), 167–169. https://doi.org/10.1053/jfms.2002.0177
  • Nelson, R.W. & Guillermo Couto, C. Small Animal Internal Medicine, 6th ed., pp. 788–800.
  • Tilley L.P. & Smith F.W.K. Jr. Five Minute Consult — Canine and Feline, pp. 709–710


Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host
Dr. Lottie Wilkinson
Creator and host of the Current Vet podcast

What is Current Vet?

The podcast that makes veterinary medicine simple.

In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

Whether you’re cramming for exams or looking for a soundtrack for your dog walk, Current Vet will make veterinary medicine simple

Intro

Hello, hello, and welcome back to Current Vet if you're new here. This is the podcast that makes veterinary medicine simple. I'm your host, Dr. Lottie, and today is our final episode in our miniseries on cat conditions. So today we're gonna be talking about feline hyperthyroid. Every vet in small animal practice will encounter this in their career.

So we should be really confident in understanding the condition, knowing how to diagnose and treat it, and being able to communicate all of this with our owners. So let's get straight into today's case.

Case

It is mid-morning in consults, and your next patient is Cleocatra, a 16-year-old ragdoll, who's been a regular at the clinic for years. Her owner, Sarah says she's been acting a bit odd lately. She's eating constantly, always asking for food, but somehow getting thinner by the week. She's meowing through the night and seems restless and a bit wired.

When Cleocatra jumps out of her carrier, she starts pacing around on the exam table. You notice she's quite bony along her spine, and her coat is a little bit dull and ragged. On auscultation, her heart is racing and you can feel a strong rapid inguinal pulse.

You palpate all over her body feeling for any lumps or bumps, and find a distinct thyroid slip in her neck. Her temperature's normal, but she's breathing heavily in the consult room. You ask about her eating and drinking recently, and Sarah mentions that Cleocatra has been drinking a bit more water than usual and occasionally vomiting after eating.

She's still bright and interactive, but clearly not her usual relaxed self. Given her age, the weight loss, the ravenous appetite and tachycardia feline hypothyroidism should be your key differential.

Aetiology & Pathogenesis

So now that we've met Cleocatra, let's talk about what feline hypothyroidism actually is and how it causes disease. Hyperthyroidism simply means excessive production of thyroid hormones by the thyroid glands. The key thyroid hormones are thyroxine, which is T4 and Triiodothyronine, which is T3.

These hormones regulate metabolism in almost every cell in the body. So when there's too much of them, basically everything just speeds up. So heart rates, gastrointestinal, motility, protein and fat. metabolism, everything is just quicker. Incas, this condition is almost always caused by a benign hyperplasia of one or both of the thyroid lobes.

In around 70% of cases, both glands are affected while 20 to 30% of cases are unilateral, only a tiny percentage around one to 2% of feline cases are due to thyroid carcinoma. So malignancy is rare. Of course, this episode is about feline hypothyroidism as the condition is very rare in dogs, but in the cases that do occur for dogs, malignant carcinomas are actually more common than benign hyperplasia.

So in healthy animals, the hypothalamic pituitary thyroid axis keeps the hormone levels in check. The hypothalamus secretes troopin releasing hormone or TRH, which tells the pituitary to release thyroid stimulating hormone, or TSH, which in turn stimulates the thyroid to produce T four and T three. So hypothalamus releases TRH, which tells the pituitary to release TSH, which tells the thyroid to produce thyroid hormones.

These hormones then provide negative feedback telling the pituitary and hypothalamus to slow down production of TRH and TSH. So the hormone levels are always in a good balance. In hyperthyroidism, the thyroid tissue becomes autonomous, which means it acts on its own regardless of feedback into the tissues, and it produces T four and T three independently of what the pituitary and hypothalamus are doing.

But what is causing this hyperplastic change in the thyroid in the first place? The exact cause isn't really known, but a few factors are thought to play a role. So iodine levels in commercial diets. Both deficiencies and excesses have both been implicated. Feeding food from a tin as many tins aligned with compounds that can be genic.

Feeding a diet of 50% or more, wept food has been linked to hypothyroidism. Feeding diets high in fish old age is a huge risk factor, and nearly all cases occur in cats over 10 years old. And there's also likely a genetic predisposition with some pure breeds like Siamese and Burmese appearing to be less susceptible than domestic short hair.

The result in all these cases is there is excess thyroid hormones which increase the metabolic rate.

They stimulate beta adrenergic receptors, which explains the tachycardia and hyperactivity. They enhance protein catabolism, which is protein breakdown, which contributes to muscle wasting despite their massive appetite.

Clinical Signs

The clinical picture of feline hypothyroidism is classic once you know what you're looking for. So Cleocatra has that weight loss despite a great appetite, high heart rate. She's restless and she's vocal, which is pretty typical hyperthyroid presentation. The most common clinical signs include weight loss despite polyphagia.

Hyperactivity, restlessness and vocalization tachycardia, often above 220 beats per minute. And sometimes you can hear Gallup rhythms or systolic murmurs. A palpable thyroid slip, which is when there's a smooth movable nodule in the neck, in the region of the thyroid gland, which is of course the over large thyroid gland,

a poor coat, quality, vomiting and diarrhea, polyuria and polydipsia, and this can either be from increased renal blood flow or concurrent chronic kidney disease. As disease progresses, cats develop muscle wasting, especially over the spine and the hind limbs, and secondary hypertension, which can lead to retinal detachment or blindness, if not detected early.

It is also worth mentioning that while most hypothyroid cats are hyperactive, a small subset present as apathetic or lethargic, hyperthyroid. and They appear lethargic, inept, and may even look like they have end stage renal disease or neoplasia until you check their T four levels.

So how are we gonna make sure that Cleocatra does have hyperthyroidism and not something else going on?

Diagnosis

When a cat like Cleocatra walks into your consult room, the clinical picture alone might make hypothyroidism seem obvious, but we have to confirm the diagnosis and also importantly, assess if there are any underlying or concurrent conditions, especially chronic kidney disease before we start any treatment.

So first we're gonna start with a minimum database. Which is gonna include hematology, biochemistry, and urinalysis. On hematology, it's typically quite normal, but you might see a mild erythrocytosis and occasionally stress leuko changes. So a neutrophilia, lymphopenia,

cytosis, and eos. Biochemistry typically shows elevated liver enzymes, so a LT and a ST, and they're very common due to the increased metabolic demands on the liver, not necessarily due to concurrent liver disease. Azotemia can appear due to concurrent chronic kidney disease or due to dehydration, and we typically see hyperphosphatemia.

Urinalysis can show a low to normal specific gravity, but interpretation is really difficult and results are deceptively normal. Because hyperthyroidism increases the glomerular filtration rate through the kidneys, which means that until thyroid levels are corrected, an accurate assessment of renal function is not really possible.

Next we're gonna check our serum total thyroxine, which is total T four. In most cases, T four is clearly elevated, which confirms the diagnosis. However, about 10 to 15% of cats, maybe if they have early or mild disease, can have values in the upper reference range, but not markedly high, especially if they're stressed, hospitalized, or have concurrent illness, which suppresses T four, also known as Euthyroid sick syndrome.

So if the T four level is borderline, you have a few options. We could follow up with a free T four test. So free T four is a more sensitive test as it's looking at the unbound biologically active fraction of T four, if free. T four is high and total T four. So that test we did before is borderline that supports a diagnosis of hyperthyroidism, but.

We should also be aware that free T four can rise in non thyroidal illness, so it should never be used as diagnostic on its own. We can also measure endogenous TSH in hyperthyroid cats. TSH is usually undetectable because the high thyroid hormone levels are suppressing pituitary production. So a high or a normal TSH level effectively rules out hypothyroidism, but a low TSH on its own is not enough to confirm the condition.

So if we did all of the total, T four, free, T four and TSH tests, we would have a much clearer diagnostic picture. If results are still a bit hazy, we can do two different dynamic tests, but these aren't commonly used in practice. So first we could do a T three suppression test.

And this is where owners give exogenous T three hormone for two to three days. In a normal cats, this hormone is going to suppress T four production. But in hyperthyroid cats, as we said before, the thyroid gland acts on its own and T four, it's gonna continue to be produced. So T four levels remain high.

It is pretty accurate test, but it's time consuming and owner dependent. So not typically first line use. The second dynamic test is the TRH stimulation test, and this measures T four levels or TSH response to administered TRH. But again, it's not really used for the same reasons.

A final diagnostic test to assess hyperthyroidism is thyroid scintigraphy, and this can be used to localize and identify thyroid tissue and it's very useful if you're planning radio, iodine therapy or a surgical approach. It's also useful if you suspect there might be any ectopic thyroid tissue or if T four levels don't match the severity of the clinical signs.

It essentially works by using Technium 99 M Ate, which. Is a bit of a mouthful, but it concentrates in active thyroid tissue. So then when you do scintigraphy, the images show really clearly where the thyroid is, its size, its symmetry, and if there's any ectopic tissue somewhere else in the body that you're not expecting it to be B.

But this test requires specialized equipment, radioactive handling. So it's mostly available in referral already. Specialist centers.

We could also do neck ultra sonography might be useful, but it has a lower specificity because many older cats can actually have incidental thyroid nodules that aren't functional and not causing hyperthyroidism. So if we're doing ultrasound and we find an overly large thyroid gland, we might suspect that that's hyperthyroidism when actually it's a benign inactive nodule.

Once we have a clear diagnosis of hyperthyroidism, we have to make sure that there aren't any secondary or underlying conditions. Because excess thyroid hormone stimulates these beta adrenergic receptors, cats often develop oxic cardiomyopathy and systemic hypertension. We have to be performing blood pressure measurements on every suspected or confirmed hyperthyroid cat.

We can also consider thoracic radiographs or echocardiography if you hear a murmur, a gallop rhythm, or tachy arrhythmias. Fortunately though, these changes often reverse once we've started treatment and the thyroid levels normalize, so they typically aren't anything to worry about long term.

And the last thing that you have to be really aware of is the renal masking effect. So thyroid hormones increase cardiac output, which leads to more renal perfusion. It's causes hypertension. So the glomerular filtration rate through the kidneys is going to increase, which can mask underlying chronic kidney disease.
Once the hyperthyroidism has been addressed and glomerular filtration rate falls to its true level, we can actually unmask this kidney disease that wasn't apparent before. So that's why before committing to irreversible treatments like radioiodine or surgery, we have to stabilize thyroid hormone levels medically first.

Reassess renal parameters once the cat is euthyroid and make sure that they don't have any underlying conditions for Cleocatra. Her total T four came back as 165 nanomoles per liter, which is well above the reference range. Her a LT is mildly elevated and her urinalysis shows a USG of 1.025. So that is a straightforward diagnostic confirmation, but how are we going to treat her?

Treatment

So for hyperthyroidism, we have a couple of different treatment options to go with, but we need to choose one that fits, Cleocatra’s overall health, the owner circumstances, and what your clinic can realistically provide. We then need to make sure we monitor carefully, watch for renal, unmasking, or any other comorbidities as well.

So there are three main ways we can approach hypothyroid cases, and those are medical management, dietary control, and curative options, which include radio iodine or surgery. So medical therapy is going to be with antithyroid drugs. The most commonly used drug is methyl masal, which works by inhibiting thyroid peroxidase enzyme, which is essential for thyroid hormone synthesis.

The advantages are it is immediately available. There's no need for referral, and the treatment is reversible because we can stop these drugs at any time. The disadvantages are, we're going to need to do this. For the rest of the cat's life, it requires monitoring with liver parameters, blood counts, renal function, tests, and T four level tests to make sure that we are dosing at the right level and it can cause some quite nasty side effects.

We get the pretty standard vomiting, lethargy, and anorexia, but it can also cause changes to bone marrow, can cause liver disease and facial pruritus and excoriation. So if any of those are seen, the medication has to be stopped and reevaluated immediately.

Essentially these medications are gonna be the best first step, and as we said before, we should be stabilizing these patients with medication first, even if we wanna go down other routes later on. The second thing we need to do is an iodine restricted diet with special cat foods formulated to limit iodine so some cats can be maintained on this diet. diet alone, or it can be given alongside other treatment options. The advantages are, it's not a drug,

so we're not medicating these cats for the rest of their lives and it's good for cats where tableting can be problematic. The disadvantages are it may not achieve full control in every cat, and we can't be feeding other foods or scraps or treats, so that can be quite sad for both the cat and the owners

the long-term outcomes are less robustly documented compared to other options because it is so. Dependent on owner compliance and what they do in their own home.

So essentially, if clear catcher has mild hyperthyroidism or we source some side effects with our drugs, this could be an option. But if she's losing weight still or has cardiac or renal concerns, you'd likely lean towards a more definitive control method.

So our third treatment option is going to be curative therapy. The first thing we could do with this is radio iodine therapy with iodine 1, 3 1, and this is considered the gold standard and it's effective in around 95% of cases, is basically an injection of IDM 1 31, which is injected into the body

and it's concentrated by the thyroid gland. The iodine then emits beta and gamma radiation, which destroys the thyroid tissue. While this actually sounds a little bit dodgy, it only destroys tissue in around a two millimeter radius, so it's very localized and doesn't damage any other organs. As we said, it's extremely effective and curative in a lot of cases.

There is a risk that some cats can become hypothyroid after this procedure, in which case they would require hormone supplementation for the rest of their lives. While most cases only require one injection, the treatment must be done in a facility because the iodine is radioactive.

So we need to be able to isolate the cat safely, dispose of their urine, their feces for the duration of the treatment, as the cat's waste is technically radioactive and potentially dangerous. The second curative therapy option is surgical thyroidectomy, and this is the surgical removal of the thyroid glands.
It's less commonly used now, but still an option, especially if radio iodine is not available. It requires obviously general anesthesia, a skilled surgeon available to perform the procedure and , you need to know that there's a risk of hypocalcemia if the parathyroid glands are accidentally removed alongside the thyroid nodules.

As well as that, there's gonna be the classic risk of hypothyroidism if too much of the thyroid glands are taken through the surgery. Okay, so the advantages to these are is a potential cure and there aren't really any lifelong drugs or monitoring required. The disadvantages are, it's gonna be more costly referral or specialist centers are gonna be needed.

There's an anesthetic risk, especially if we've got an older cat and post-treatment monitoring for hypothyroidism and hypocalcemia. If we're going for the surgical root.

For anyone wanting to read more about the management of feline hypothyroidism, the American Association of Feline Practitioners now called the Feline Veterinary Medical Association has their 2016 guidelines for the manager.. Management of hypothyroidism, which is an amazing reference that summarizes the diagnostics, outlines treatment options, and chats through the pros and cons of all of them.

Key Points

Okay, so before we wrap up, let's go back through the absolute essentials about freeline hypothyroidism that every vet needs to know. Feline hypothyroidism is caused by excessive production of thyroid hormones, and it's one of the most common conditions seen in older cats. The classic presentation is weight loss despite a good appetite, tachycardia, hyperactivity, polyphagia, and a palpable thyroid slip.

But don't forget the possible apathetic, hypothyroid cat, which can be quiet, incompetent, but still hypothyroid. For diagnosis, we should always start with a minimum database to assess general health and then do a total T four test. If the results are borderline, we can do the free T four and TSH options.

Always check renal parameters and blood pressure. In all cats suspected to have hyperthyroidism. Our treatment options include medical control with methyl masal, radio, iodine, thyroidectomy, and iodine restricted diets.

Two to four weeks after starting therapy, always recheck TT four, creatinine, SDMA and blood pressure, and then recheck these parameters every three to six months. Once the patient is stable, be really vigilant in watching for renal unmasking and iatrogenic hypothyroidism.

The prognosis for these cases is excellent if diagnosed early and managed appropriately, and cats can live several more healthy years with a great quality of life. So that's a really positive note to end on.

Outro

And that is feline Hyperthyroidism. Thank you so much for tuning into this episode. If you found it helpful, follow or subscribe or like or love the podcast wherever you listen to your podcasts, and share it with a colleague or friend who needs to hit this topic too.

You will find all the links to the resources used in the show notes. And thank you again to those people who requested our March Cat topics. I really hope you enjoyed them. For more vet tips and case quizzes, find us on Instagram and TikTok at Veterinary Vista and let us know if you have any topics that you would like to hear.

But otherwise, we'll see you next time.