What is hyperthyroidism?

If left untreated, hyperthyroidism can result in issues such as heart disease and high blood pressure, which can cause damage to the eyes, kidney and brain.

Typical symptoms include:

• Weight loss
• Increased appetite
• Increased thirst and urination
• Vomiting or diarrhoea
• Increased activity or restlessness 

How is it diagnosed?

Hyperthyroidism is diagnosed with a blood test that looks for increased levels of thyroid hormone.

There are several treatment options for hyperthyroidism, which fall into two categories: lifelong medical management with an oral medication, and curative therapy, involving either radioactive iodine or surgical treatment.

What does medical management involve?

Medical management is recommended for all patients initially to stabilise them, and can either then be stopped in order to proceed with curative therapy, or continued lifelong. This involves anti-thyroid medication given twice daily. We recommend starting with an oral medication, which is available as either a tablet or a syrup:

• The tablet medication can be given with food but cannot be crushed. Liquid treats or cat putty can help with administration.
• The syrup should be given directly into the cat’s mouth without food. 

If you are unable to orally medicate your cat, there is also the option of a gel that is applied directly to the ears. However, this is not absorbed as effectively as with the oral medication, and is used off-licence (i.e. this medication hasn’t been authorised for use in cats). Therefore, we would recommend trialling the oral medication first. 

If you or a family member, are pregnant, it is important that they don’t handle the medication. It is also recommended that women of child-bearing age wear gloves when handling the medication. as the main ingredient is a possible teratogen (can cause damage to foetuses).

What are the possible side effects?

Side effects include:

• Vomiting
• Lethargy
• Itchiness, especially around the face
• Changes to red and white blood cell parameters

Some of these side effects can be transient and should resolve within a week of medication. For further information, please see the product information leaflet that accompanies the medication.

What happens after starting medication? 

We will revisit after 3-4 weeks of consistently taking the medication. We would recommend repeating the thyroid bloods at this visit. We also recommend assessing other parameters such as:

• Kidney parameters, as kidney disease can be obscured by uncontrolled hyperthyroidism, and then detected once hyperthyroidism is controlled
• Red and white blood cells, as we can see changes in these values as a side effect of medication
• Blood pressure, which can be increased by hyperthyroidism

These can be discussed at your revisit appointment. It may take several visits until the hyperthyroidism is stabilised. Once the hyperthyroidism is under control, we need to reassess the patient every 6 months. 

Radioactive iodine is the gold-standard treatment for hyperthyroidism and involves the injection of a substance that will destroy abnormal thyroid cells. It is effective in around 95% of cats and often has minimal side effects, although occasionally cats will develop a low thyroid level after treatment. Sometimes sedation is required for injection. 

This requires referral to a specialist centre and the patient would need to be hospitalised for a set period of time before returning home. There are often additional tests that are required before referral, such as follow-up blood tests and a heart scan. This is only suitable for cats that are not especially fractious, or have other serious health conditions. Costs depend on the individual and the referral centre, but are usually in the region of £2500-4000 (as of January 2026). 

Surgery involves the removal of one or both of the thyroid glands under a general anaesthetic. We have a fixed fee of £1000 for removal of one gland, or £1400 for removal of both glands, which includes pain relief after the procedure and post-operative checks (as of January 2026).

The main risk of surgery, aside from the risks of general anaesthesia, is the possible damage to the parathyroid glands, which are associated with the thyroid gland. These are involved in calcium regulation, and if they are damaged the cat’s calcium control may be compromised, which can cause seizures. We may therefore perform a blood test to assess calcium levels if both glands are removed, which is included in the fixed cost (waiting on confirmation). 

There is also the possibility of thyroid tissue present in abnormal locations, which is not accessible surgically. This may be identified after surgery if the thyroid hormone isn’t falling to normal levels or clinical signs persist. 

In cats that cannot be managed by medication or by curative therapy, we sometimes consider feeding them a special diet with strictly controlled levels of iodine. However, this management is generally less effective than the above treatments and cannot be used at the same time, so we would not suggest this as a first-line management option. We would recommend keeping their diet the same, unless another diet has been recommended by the vet