12 May 2025
Deborah James
Job Title
Image: Kristina Blokhin / Adobe stock
While hyperthyroidism can be seen in both canine and feline species, it is very rare for it to occur in dogs and, therefore, we will be looking at hyperthyroidism in our cats.
Hyperthyroidism is the most diagnosed hormonal condition in cats and is most seen in our older felines, with 95% of cases being above the age of 10 years.
In 97% of cases, a benign growth on one or both thyroid glands that produces thyroxine at an increased rate is the main cause. However, the remaining 3% are caused by a malignant growth that has the potential to spread to other areas of the body.
This increase in thyroxine causes the body’s metabolism to work harder and so often we see hyperthyroid cats presenting with a voracious appetite, but losing weight at the same time.
In addition to this, more than 70% of the hyperthyroid cases we see in practice have ectopic thyroid tissue – thyroid tissue that is present elsewhere in the body (Jones, 2022).
In this article, I will discuss the diagnostics, treatment options and financial considerations when treating our feline hyperthyroid patients and highlight the nurse’s role in each of these stages.
Normally, by this stage, a veterinary nurse would likely have seen a patient and recommended a consultation with a veterinary surgeon because of the presenting clinical signs (weight loss and increased appetite). Other signs we may see on clinical exam are vomiting and poor skin and coat condition, in addition to tachycardia and other cardiac changes such as murmurs or gallop rhythms developing.
Blood tests checking our patient’s biochemistry and haematology levels are likely to show an increase in alkaline phosphatase (ALP) and alanine aminotransferase (ALT) alongside increases in our red blood cell count, neutrophils and lymphocytes (Jones, 2022). Total T4 (thyroxine) should also be tested and is likely to be elevated, but this should be the lone parameter you are testing for.
This is because some medications, such as NSAIDs, phenobarbital, furosemide and steroids, alter the level of T4, as well as non-thyroid disease processes such as renal disease, hepatic disease and cardiac disease (Jones, 2022). Therefore, it is so important to check all parameters and not just our T4 levels when diagnosing hyperthyroid patients.
Blood tests are not the only way to diagnose and should be used as an aid alongside other diagnostics. Blood pressure monitoring will indicate if the patient has hypertension (a key signalment of hyperthyroidism) alongside an echocardiogram (ECG) if a new heart murmur is present. In specialist referral centres, a test known as nuclear scintigraphy, where the patient receives an injection of technetium-99 (a radionuclide) and then is imaged using gamma to detect enlarged thyroid glands and/or ectopic thyroid tissue (Jones, 2024).
Four main treatment options need to be considered when treating a hyperthyroid cat: medical management, nutritional management, surgical approach and radioactive iodine. Each treatment option has its own associated advantages and disadvantages, which must be considered when creating a treatment plan.
Medications such as methimazole (felimazole or thyronorm) and carbimazole (vidalta) are commonly seen in dispensaries in practice and inhibit the production of T4. Methimazole products require twice-daily administration whereas our carbimazole products only require once-a-day administration as it is converted to methimazole in the body and is a sustained release product (Jones, 2022).
Overall, these medications are well tolerated, with the biggest issue being owner compliance and whether the cat will be tolerable of being medicated. They can be used to stabilise a patient prior to surgery or radioactive iodine therapy or can be used as a lifelong treatment option.
Response rate is greater than 95% while on the treatment, and is reversible if the kidney function declines. However, they relapse 100% when off treatment and regular visits to a vet are required to monitor effectiveness for the rest of its life (Carney et al, 2016).
In total, 25% of the patients on medical treatment develop reactions, which vary from facial itching to vomiting, liver failure, bleeding episodes and abnormal blood cell levels. While it has a good response rate, the treatment does not solve the primary issue and the tumour will continue to grow and is at risk of becoming malignant (Carney et al, 2016).
These are a more recent innovation and have been designed to replace the medical management therapies. They work by restricting the dietary iodine, which is needed to synthesise the thyroid hormones – therefore, preventing an excessive amount being produced (Jones, 2022).
This method of management could be ideal for older cats whose owners cannot afford treatment or that cannot be given tablets, or have shown to have side effects to the medications (Feline Hyperthyroid Treatment Centre, 2025). This treatment option is also safe for cats with renal insufficiency (Carney et al, 2016).
This approach is not for every patient. This diet cannot be fed to cats who do not have hyperthyroidism, so is not ideal if you have an owner with multiple cats or other cats on a different prescription diet and may not be practical (Peterson, 2020). In addition to this, if the treated cat goes outside and visits other households or hunts, then this will diminish the effectiveness of the diet, even if the owner is using pill pockets or treats (Feline Hyperthyroid Treatment Centre, 2025).
The final thing to bear in mind is that patients will 100% relapse if taken off the diet. As this is the only food they can eat for the rest of their lives, it can become expensive over a patient’s lifetime – particularly if for a young cat (Carney et al, 2016).
So, how useful are the iodine restricted diets? Well one study suggests that the diets were effective in reducing and maintaining serum T4 levels for cats with spontaneous hyperthyroidism for more than a year, but not all the clinical signs showed improvement (Hui et al, 2015).
Another study found that iodine-restricted diets were able to control moderate to severe hyperthyroidism in six out of eight cats by the four-week mark, while patients with higher T4 levels took significantly longer to normalise (longer than four weeks) if at all (Loftus et al, 2019).
Finally, a study carried out by RCVS Knowledge (2021) concluded that these diets reduced the serum T4 levels in 100% of the cats studied. However, this did not guarantee a return to euthyroid status (normal thyroid gland function), nor a resolution of clinical signs, so it is certainly a topic requiring further investigating.
Surgery is one of the two curative approaches (curing it within one to two days) and has a 90% cure rate if both thyroid glands are removed and a 35% to 60% cure rate if only one is removed; relapse rates are 5% if bilaterally removed and below 30% if unilaterally removed (Carney et al, 2016).
This surgery requires no specialist equipment and generally most surgeons can perform this permanent procedure. This is a highly recommended procedure for young, otherwise healthy cats.
However, surgery comes with its own risks. Many of our hyperthyroid cats are compromised cardio-vascularly and anaesthetising a cat with this comorbidity comes with a high risk. As mentioned previously, many patients will require stabilisation using medication for at least a two-week period and will require hospitalisation post-surgery.
The parathyroid gland sits closely to the thyroid gland, and a risk exists that damage may occur during the surgery can cause calcium crisis (transiently or permanently) and will require treatment for this – either short term or longer term (Carney et al, 2016).
Something to make the owners aware of is a cat’s meow or purr may be altered because of the surgery, and if the surgeon fails to remove all the abnormal thyroid tissue, then revision surgery will be required (Carney et al, 2016) and in patients with ectopic thyroid tissue, hyperthyroidism will still reoccur even in those with bilateral thyroidectomy’s (BVNA, 2022).
Radioactive iodine is a newer treatment option and is often considered “gold standard” for treating patients with hyperthyroidism (De Voogt et al, 2023).
It involves injecting a radioactive iodine isotope subcutaneously, which destroys the hyperfunctioning tissue within the thyroid gland and spares the hypo-functioning and parathyroid tissue (Jones, 2024).
It has a curative rate of 95% and survival times have been reported to exceed that of cats who are medically treated by three to four years on average (Warland, 2018). Generally, no further treatment is required afterwards and has very few side effects (Warland, 2018). For young, healthy cats (other than hyperthyroidism), radioactive iodine therapy is highly recommended.
However, these patients will require hospitalisation for one to three weeks as our patients are radioactive after the injection (the isotope has a half-life of 8.1 days). They should have minimal contact, and clinical waste (for example, cat litter) should be stored in a concrete storage until the radiation has decayed (Jones, 2022).
During this time, owners cannot visit the facility. In addition, few facilities are able to offer this in the UK, with just 16 practices holding the licence and facilities to conduct this treatment and aftercare (Carney et al, 2016). This treatment has an expensive upfront cost; on average in the UK it is priced around £3,400 for the treatment and hospitalisation, but this will vary from practice to practice (The Hyperthyroid Cat Centre, 2025).
Following discharge, for the following one to three weeks, owners should follow strict procedures at home, including minimising close contact with their cat, preventing pregnant women as well as children from handling the cat or its waste, and disposing of the cat’s waste down the toilet (Jones, 2022).
It has been noted that depending on the dose administered, up to 75% of cats become hypothyroid post-treatment, and require treatment with up to 30% remaining hypothyroid for three months after radioactive iodine therapy (Carney et al, 2016).
As aforementioned, the medical and nutritional management options are both termed “palliative” as neither are curative.
Both choices benefit the owner by giving them time to decide what option is best or can be used as a way of stabilising the cat prior to further treatment.
However, it is vitally important that owners are aware that neither of these options will address the underlying cause of the disease and the tumour will remain, and over time will grow. This disease progression varies, as every cat is different and owners must understand that a benign tumour always has the potential to become malignant (Peterson, 2020).
Understandably, one of the owners’ concerns will be regarding the cost of treatment regardless of insurance status or if a specific treatment has been recommended.
Curative treatments, such as surgical thyroidectomy or radioactive iodine, cost the most initially. Palliative treatments of medical and nutritional management have an initial lower cost, but ongoing it can exceed that of the curative options, as treatments continue for months or even years (Peterson, 2020).
Dietary management and medical management options can become cumulatively more expensive over time and surgery could be a one-off high cost or could involve multiple high costs, dependent on complications, whereas radioactive iodine has initially high upfront cost but generally proves to be the most cost-effective option over time.
Currently, a surgical thyroidectomy or radioactive iodine treatments are the only curative options available.
Each individual cat must have its own tailored treatment plan to consider the best treatment.
The short-term and long-term effects must be considered in addition to the quality of life of the cat and therefore it is imperative the owner plays a major part during the decision-making process.
A study has been conducted that has shown that radioactive iodine therapy is more effective in normalising T4 levels when compared to thyroidectomies (unilateral and bilateral).
However, each option will be down to the owner and the veterinary team to make a decision that suits the owner and the cat.
The best option will depend on owners’ finances/insurance, age of the cat, co-morbidities, such as kidney disease or heart disease, owner compliance and ability to medicate and household status, for example, multi-cat household.
While the aforementioned options are down to the veterinary surgeon and the owner to decide, veterinary nurses play a role in the ongoing management of these cases. Small steps like having posters in the waiting room can enhance owner awareness and can guide owners to relevant websites.
Introducing geriatric or old cat clinics in your practice provides an opportunity to detect trends, such as losing weight or increasing heart rates or hypertension, which can then be used to encourage owners to book a consultation and seek treatment earlier (Bodey, 2015).
During this consultation process, the veterinary nurse can allocate time that the veterinary surgeon may not have to re-explain all the treatment options to the owners, rather than leaving them to Google it themselves – something 95% of owners will do (Bodey, 2015).
Veterinary nurses play a role in informing owners and helping them detect hyperthyroidism early, facilitate communication and understanding between the owners and the wider veterinary team and help during the longer-term management of these patients.
Alongside owner support and education, veterinary nurses provide practical support by monitoring anaesthetics during surgical procedures and in the recovery period, too.
