This is definitely one of the more serious topics we’ve addressed on VeryRealVet, but it’s also a topic I passionately want to address.
As a first opinion small animal vet, I see on a weekly if not daily basis the effects that feline hyperthyroidism has on pet owners, on the bond between cat and owner, and of course, on the quality of life of the cats themselves.
And – I’ll be brutally honest here, as always – in general, we as pet owners and individuals within the veterinary profession could be doing a much better job.
As we will discuss, in around 98% of cases feline hyperthyroidism is very treatable, and with treatment does not tend to significantly shorten the patient’s lifespan.
However, it is important to also realize that if left untreated, the condition will progress and result in suffering and eventual death for the cat.
- Signs Of Hyperthyroidism In Cats
- What Is Feline Hyperthyroidism?
- Thyroid Adenocarcinoma (2% Of Cases)
- Benign Growth Causing Hyperthyroidism (98% Of Cases)
- Life Expectancy Following A Diagnosis Of Hyperthyroidism
- Treatment With Anti-Thyroid Medications
- Prescription Diets For Hyperthyroidism
- Radioactive Iodine Treatment (I-131 Therapy)
- When To Euthanize A Cat With Hyperthyroidism
Let’s start by taking a look at some of the signs of feline hyperthyroidism.
Signs Of Hyperthyroidism In Cats
Hyperthyroidism is a common disease of older cats. The average age at which a cat develops the condition is 12 years. Around 5% of cases are diagnosed in cats aged 10 years or less.
The following signs may indicate that your cat has developed hyperthyroidism.
Please note that several other diseases can have similar signs, including kidney disease, liver disease and diabetes. It is never appropriate to make a diagnosis of hyperthyroidism based solely on the signs below; your vet will need to carry out a blood test to determine what is going on for your pet.
- Weight loss, usually with normal or increased appetite
- Increased thirst and urination
- Greasy, unkempt or matted coat
- Vomiting and/or diarrhea
- Being very active and/or easily stressed out
Your vet may also noticed the following signs during their clinical examination:
- A higher than usual heart rate
- The presence of an abnormal heart sound known as a “gallop”
- High blood pressure
- Mild dehydration
- A lower than normal body condition score
What Is Feline Hyperthyroidism?
In around 98% of cases, hyperthyroidism in cats is caused by a benign (non-cancerous) overgrowth of thyroid gland tissue. And in around 2% of cases, the condition is caused by a malignant, cancerous tumor (an adenocarcinoma) in the same gland.
The two thyroid glands as situated within the neck, close to the windpipe, one on each side, and – when enlarged – can oftentimes be felt by an experienced vet as they run their fingers along your cat’s neck. We call this noticeable enlargement of the thyroid glands “goitre”.
The cells of the thyroid gland produce thyroid hormones. So in hyperthyroidism, because of the new, additional thyroid cells that have popped up within the enlarged gland, excessive amounts of thyroid hormone are produced.
Thyroid hormones have many vital roles in terms of regulating your pet’s metabolism. But, when produced in excess, they make the body work too hard.
If left untreated, over time this leads to negative consequences. As a single example, high levels of thyroid hormone can make the heart work too hard (it beats faster and harder). Over time this may lead to hypertrophy (thickening) of the heart wall muscle and in some cases, eventually heart failure.
Treatment of hyperthyroidism will be touched upon in greater detail below, but in short, it aims to suppress the production of thyroid hormone to within normal levels.
The key takeaway from the above, is that in 98% of cases feline hyperthyroidism is a treatable disease. Unfortunately though, as mentioned in around 2% of cases, the thyroid gland has become malignant and the outlook is much worse.
Thyroid Adenocarcinoma (2% Of Cases)
When a cancerous thyroid gland adenocarcinoma is to blame for the hyperthyroidism, unfortunately treatment of the thyroid disease is more complicated and has a significantly worse outlook. In these cases it is likely that euthanasia will need to be considered much sooner after the diagnosis of hyperthyroidism.
Like any cancer, before they can accurately discuss treatment and expectations your vet will want to establish:
- The size of the tumor and extent to which it has already invaded the surrounding tissues.
- Whether any metastases are present elsewhere in the body. Metastases are new tumors that grow elsewhere in the body due to spread of cancerous cells from the original tumor to a new location, via the bloodstream or lymphatic system .
- To what extent thyroid hormone production is being affected by the tumor.
Surgical removal of malignant thyroid tumors is unfortunately sometimes not a viable option as these tumors can be invasive and can be closely intertwined with local blood vessels, nerves and other vital parts of your cat’s anatomy.
For this reason, radiotherapy and/or chemotherapy may be suggested as alternative treatment options.
If surgery is possible, and the tumor is relatively small (<4cm) at time of removal, you might expect your cat to live for 1-3 years following the procedure. They are likely to need daily medication throughout this time.
If surgery is not possible, you should prepare for a shorter survival time, possibly around 1-2 years with oncological treatment.
Without any treatment, if your cat has developed a thyroid adenocarcinoma you are likely going to need to make the difficult decision to euthanize your pet in their own best interests, within 12 months.
Benign Growth Causing Hyperthyroidism (98% Of Cases)
Obviously it’s never fun when your fur baby is diagnosed with a health problem, however please be reassured that benign hyperthyroid cases usually respond very well to careful medical management!
Many of these cats can live for five years or more if their hyperthyroidism is treated appropriately, which is really impressive given that the average age of diagnosis is 12 years old, and the average feline lifespan generally is around 15 years.
So, realistically, a diagnosis of hyperthyroidism won’t necessarily shorten your cat’s overall lifespan, so long as it is well managed.
The key thing as an owner is to stay motivated and stay on top of attending regular vet visits. Make sure you show up with your cat for routine blood tests etc.
If (like so many cats) your kitty becomes super stressed at the vets, request a calming medication such as gabapentin (link to other article) that can be given prior to each visit to make the experience a pleasant one for your pet.
Of course, it’s also super important to make sure you’re in the hands of a really good vet who is ideally familiar with your cat and their medical history. Continuity can be key, so try to see the same vet each time you visit the clinic.
Life Expectancy Following A Diagnosis Of Hyperthyroidism
The information below relates to the 98% of feline hyperthyroidism cases where a benign overgrowth of thyroid tissue is to blame.
Unfortunately as we already mentioned, the outlook in the 2% of cases where a thyroid adenocarcinoma is to blame, is significantly worse.
So, maybe your cat recently received a diagnosis of hyperthyroidism and you’re understandably wondering about life expectancy.
From my experience, I would say typically that for well-managed cats with motivated and dedicated owners, the disease does not tend to shorten their natural lifespan by much, if at all.
Whereas for the less well managed cats (ie, those whose owners commit to giving daily medication but skip on their regular vet checkups and blood tests) a life expectancy of around 2 years post-diagnosis is more normal.
Obviously the above is anecdotal, and I want to make sure I’m provided a better answer than that.
So, what does the research say? The study below looked at 167 cases of feline hyperthyroidism and found that the average life expectancy for cats treated with the medication methimazole, or radioactive iodine treatment, or both, was 5.3 years following diagnosis. That’s pretty impressive!
To break that down a little further though, be aware that cats treated with methimazole alone lived on average for only 2 years after diagnosis; those treated with radioactive iodine lived for 4 years; and patients treated with both, lived a fabulous 6.5 years following diagnosis.
So clearly, we have a winner there; radioactive iodine is the treatment you should be requesting from your veterinarian. So long as your vet deems that your cat is a good candidate for radioactive iodine treatment, this gold standard combination will give your kitty the best chances of living as long as possible, and with the best possible quality of life.
Treatment With Anti-Thyroid Medications
The two main ones we use are thiamazole (also called methimazole – same drug, different name) and carbimazole.
Thiamazole and carbimazole both work by suppressing thyroid hormone production.
After starting treatment, it is ideal that your cat receives a blood test at 3, 6, 10 and 20 weeks into treatment, and then every 3 months ongoing. This is so your vet can ensure that their thyroid hormone level stays within a safe, ideal range; not too high, not too low.
In the real world where people have busy lives and finances are often limited, it is often possible to get to a point where bloods can be run every 6 months, and in most cases this still tends to work pretty well in terms of keeping an eye on things.
The bloods at 3 and 6 weeks are vitally important though, not just to check the thyroid hormone, but because the medications themselves can affect the liver, and in some instances suppress red and white blood cell levels. In a minority of cases, this can be a reason to need to stop the medication.
We also need to keep an eye on your pet’s kidney function once they have started thyroid medication. Whilst the medication itself does not harm the kidneys, sometimes an overactive thyroid can “mask” (hide) kidney disease, that was there all along but which only becomes apparent once the thyroid disease is treated.
Kidney disease is super common in older cats (in fact, as many as 1 in 3 will suffer from it), so it’s something we need to make sure we identify and address alongside the thyroid disease so that we can keep your cat healthy and happy for as many years as possible.
Thyroid medications are available as liquids or tablets that your cat will need to take on a daily or twice daily basis. For cats who simply will not take oral meds (and as a cat owner, believe I’m aware of the struggle!), a transdermal gel also exists.
This gel is applied to the skin of your cat’s ear, and they absorb the medication through their skin. It’s not recommended as a first-line treatment because it’s more difficult to achieve an accurate dose this way, however if your cat won’t take oral meds it’s vastly, vastly better than nothing!
Be aware that if you do opt for the transdermal gel, gloves are mandatory, because you will absorb the medication through your skin too, and could end up with an underactive thyroid as a result!
Other side effects to keep an eye out for if your kitty is started on thyroid medication including loss of appetite, vomiting and diarrhea, which can often be addressed by your vet adding anti-sickness medication in the short-term.
Usually these side effects subside on their own and treatment can be continued, but in a few instance they may be severe enough to require treatment to be stopped.
Prescription Diets For Hyperthyroidism
Following a prescription diet is a natural way to treat hyperthyroidism. These work on the premise that the body requires iodine (from the diet) in order to produce thyroid hormones.
No iodine = no thyroid hormone production!
But of course, you need a tiny amount of iodine to live; so, prescription diets for hyperthyroidism don’t contain zero iodine; just a very, very, very tiny amount. Enough to cater for vital functions (including production of some thyroid hormone), but without providing enough iodine to allow excessive thyroid hormone production.
Prescription diets for feline hyperthyroidism can be effective, so long as you are really strict with sticking to them. If your cat eats anything other than their prescription diet, this method won’t be effective; and I really do mean anything, ever.
No stealing the other cat’s food. No dreamies, munchies, smooshies, creamies or tuna on the sly, ever again. No scavenging from the kitchen table. And no hunting.
Realistically, the prescription diet method only works for indoor cats, because outdoor cats will find additional food sources – I guarantee it!
Whether it’s bin scraps, mice, or an overly generous neighbor. So if your cat likes to roam, I really recommend sticking to one of the other treatment methods.
Prescription diet for hyperthyroid cats are available as wet (canned or pouches) and dry (kibble) options these days. I would encourage you to invest in a smallish amount of a particular brand first, in case your cat does not like that brand. That way, you can always switch to a different brand of iodine-restricted food if you need to.
In general, it takes 2-3 weeks for your cat to achieve a normal thyroid hormone level on an iodine restricted diet. A few cats will take longer than this. Your vet should run bloods regularly to check that a normal level of thyroid hormone has been achieved, and is being maintained.
Radioactive Iodine Treatment (I-131 Therapy)
My personal favorite method for treating hyperthyroidism in cats, radioactive iodine treatment actually cures the vast majority of cats who receive it!
This treatment method has been proven by studies to be the gold standard in terms of producing the best long-term outlook.
Radioactive iodine treatment sounds scarily scientific, but really it’s quite simple.
Your cat’s thyroid gland is constantly hungry for iodine. In fact, it sucks up nearly all of the iodine they take into their body via their diet.
The thyroid gland uses this iodine to make thyroid hormone. So, this treatment method involves giving radioactive iodine by injection, and this radioactive iodine is then sucked up by the greedy thyroid gland.
Of course, being radioactive, it destroys the majority of the thyroid gland tissue; which is actually a good thing if you have too much thyroid tissue and are producing excessively high levels of thyroid hormone.
What’s more, because the “overactive” abnormal thyroid tissue is more greedy than the normal, original thyroid tissue, this normal thyroid tissue survives and is able to produce a normal level of thyroid hormone going forward for the rest of your cat’s life!
For obvious reasons, radioactive iodine can only be handled and given at specialist, licensed treatment centers, and because of the radioactivity of the substance involved your cat will need to spend a few days as an in-patient at one of these centers.
When they return home, you will need to follow some special instructions regarding litter trays and handling your cat for 2 weeks following treatment because during this time your kitty is still a little bit radioactive!
But please don’t be alarmed – the instructions really are very simple; I personally know individuals in their 80’s who have managed to follow the guidelines with no issues. And it’s well, well worth it to cure your cat of hyperthyroidism once and for all.
The only drawback to radioactive iodine treatment is that it is the most expensive treatment option in terms of upfront cost, and this often puts people off.
However, I’ve done the math (so you don’t have to) and in fact, if your cat lives for 2 – 2.5 years following their diagnosis of hyperthyroidism (which they likely will, with treatment), it is actually cheaper in the long run to choose radioactive iodine treatment when compared to daily anti-thyroid medications for the rest of their life!
So, if the initial upfront cost is affordable, this is 100% the best option so long as your cat passes a few health check criteria. A small minority of cats will not be suitable for radioactive iodine treatment, usually in the case of advanced kidney disease.
The best place to start – as always – is a discussion with your vet about suitability for your cat, on an individual basis.
Thyroidectomy refers to the surgical removal of the thyroid gland(s) and to be honest has been largely replaced by radioactive iodine treatment in the realms of modern veterinary medicine, for a couple of reasons.
For one, radioactive iodine treatment is safer and less invasive; it doesn’t require a general anesthetic, or cutting repeatedly into your cat’s neck (scarily close to the windpipe and major blood vessels).
Another reason is that when the thyroid gland is removed, even the most skilled surgeon is at risk of accidentally removing the parathyroid gland, the tissue of which is closely cuddled up to the thyroid, and practically identical to the eye of the beholder.
The parathyroid gland has an important job to do; he’s responsible for your kitty’s ability to regulate their blood calcium levels. So, as you can imagine, if he’s inadvertently extracted, this is not a good thing.
For these reasons we don’t really tend to remove thyroid glands anymore these days, unless the cat is one of the 2% of hyperthyroid cases who is suffering from a malignant thyroid gland tumour (see ‘Thyroid Adenocarcinoma’ section).
When To Euthanize A Cat With Hyperthyroidism
My experience as both a vet and a pet owner have taught me the following. The right time to euthanize any pet is when suffering outweighs joy, in their life.
Unfortunately as a vet I have noticed that very frequently pet owners are not fully aware, or indeed are in denial of, the fact that their pet is suffering.
I regularly meet arthritis pets who are struggling to walk, but whose owners will tell me they “are not painful”; I would passionately argue otherwise, given that we know arthritis is a painful condition, but when the tail is still wagging and meals still being finished, some individuals may not immediately recognise the signs that their fur baby is in pain.
In terms specifically of a cat with hyperthyroidism, it is appropriate to euthanize when treatment options have been exhausted, or tried without success, or when your cat has other coexisting diseases or factors negatively affecting their quality of life.
We also have to consider – because we live in the real world – that euthanasia is probably the best option for your cat if you are very limited in terms of funds and/or practically limited and unable to keep up with medication and vet visits. This is because in this instance, the disease will be allowed to progress untreated, resulting in suffering for your cat.
It is always, always better to euthanize an animal before they are suffering, then to wait until they are actively suffering on a daily basis. As far as the cat is concerned, better to die a day or two too soon, than too late.
Sadly I frequently see owners allowing their cat to outlive a good or even acceptable quality of life, always from a place of love, but sadly never in the best interest of the cat.
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