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Hyperthyroidism in Cats

This is the most common hormone imbalance in cats. It is when the thyroid glands (on either side of the trachea in the neck) produce too much inactive thyroid hormone (or T4). T4 is absorbed by tissues of the body and converted to the active form, T3.


There are many clinical signs associated with this syndrome, but the most common sign we see is weight loss despite the cat having a huge appetite. Other signs are: increased thirst, hyperactivity/restlessness, ongoing intermittent vomiting/diarrhoea, inappropriate urination.

Affected cats are usually (but not all the time) older. The average age at diagnosis is 13 years old.


We will take a blood sample to assess liver function, kidney function among other body systems. We usually also check the red and white blood cell levels.  If we are suspicious of hyperthyroidism we will also request a T4 level. If the T4 result is very high, it confirms hyperthyroidism. Some cats have borderline results, we usually like to retest these in 4-6 weeks.

What causes hyperthyroidism?

It is caused by a growth in the thyroid gland which over-produces T4. It is important to know that most of these growths are BENIGN. Less than 3-5% of hyperthyroid cats have a cancerous thyroid growth.


If left untreated, a hyperthyroid cat will experience reduced quality of left due to weight loss, muscle degeneration, chronic vomiting or chronic diarrhoea. These are the visible signs. These cats also develop heart disease and high blood pressure. These can result in heart failure, sudden blindness, or sudden death.

Here at New Plymouth Vet Group we offer a variety of treatment options for managing hyperthyroidism.

Radioactive Iodine Therapy:

 This is the “gold standard” treatment as it is curative in over 95% of cases, and doesn’t require ongoing medication. We order the radioactive iodine from Australia, and only do treatments when we have at least 5 patients to treat, so there may be a waiting time.Prior to confirmation of treatment, we require a treatment trial (e.g vidalta) with repeat blood work then showing normal thyroid levels, and kidney function analysis to confirm normal renal function.

During the procedure, your cat will be given an injection of Iodine 131 under the skin (similar to when they have a vaccination). The injection is based on your cat’s weight and elevation of their thyroid hormone. The iodine attaches to the thyroid tissue and effectively destroys the tumour causing the issue. There is no penetration to any tissue other than the thyroid gland.

They will be required to stay in the hospital after the procedure for 7 days. This allows the amount of radiation they emit to drop to a safe level to allow them to return home. While in the clinic you will be unable to visit but our staff will provide regular updates on their well-being. We are trained to handle the cats while they are “radioactive”.

After the initial 7 days, they are able to return home but special precautions should still be adhered to for a further 7 days. These involve avoiding prolonged contact with them, not allowing them to sleep on your bed, no face-to-face contact and ensuring litter is disposed of in the rubbish bin (please wear gloves when handling litter or faeces). In some instances, this is not practical (i.e. young children or pregnant household members) in which case they are welcome to remain at the clinic for a second week. This can be arranged at the time of booking the procedure. There will be a daily hospitalisation charge for this.

The cost of this treatment, which includes the first week stay in hospital, is $1000


This is a once-a-day tablet that “mops up” the excess thyroid hormone produced by the diseased thyroid gland. It should be given at the same time each day, particularly in relation to feeding. Wash hands after handling the medication. Pregnant women should wear gloves while handling this medication, and cleaning up any vomit, faeces or urine from a treated cat. There are two strengths of this tablet: 15mg are used on cats with very high levels of T4 initially, and 10mg tablets are used for lower grade conditions. The dose may change depending on subsequent T4 levels. This is a lifelong medication.

The cost of the 10mg tablets are $54.75/month approx

The cost of the 15mg tablets are $59.25/month approx


This is a gel that is rubbed onto the tip of the cats’ inner ear once a day. You MUST wear gloves while doing this. The dose rate is 0.1ml applied topically every day. The treatment comes in packs containing 30 doses (10 doses per syringe). This treatment is suitable for cats that cannot be tableted. It is a lifelong medication.

The cost of this treatment is $109.70/month.

Restricted Iodine Food:

This is a food made by Hills Science, and contains absolutely no iodine. This prevents the overactive thyroid gland producing any thyroid hormone. This is a simple and cost-effective treatment, as long as the cat can be persuaded to eat the food. As most hyperthyroid cats are ravenously hungry, they will often eat anything, so this can be a good option. It obviously works best if its effect isn’t diluted by other foods, so it needs to be fed as a SOLE diet. This includes cutting out treats. For this reason, it doesn’t work well for a cat who regularly visits the neighbours for a feed, or hunts to supplement its diet. It comes in canned and biscuit forms. 

Ongoing monitoring:

Cats with marginal kidney function can develop renal disease after receiving radioactive iodine therapy, or while being treated with vidalta or methimazole. This is because when the thyroid gland is over active, it “hides” the full extent of renal compromise. This is why all cats must have recent blood tests prior to the procedure to mitigate the possibility of this occurring but there are still a small percentage of cats this can occur with.

With any of the above treatment options, we would like to see your cat for a follow up blood test approximately 28 days after initiation of treatment. We will be measuring the T4 level, as well as monitoring kidney function for the cats that are treated with radioactive iodine, vidalta or methimazole cream. We will also want to weigh your cat as we usually see great gains in this department. For cats on medication long term (i.e. either vidalta or methimazole) we require 6-monthly appointments for blood sampling and health checks.

Updated 27.06.2024 – *Please note pricing is subject to change.

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