Hyperthyroidism in Cats

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Treatment
- Outlook

What is hyperthyroidism?

Hyperthyroidism (an over-active thyroid) is one of the most frequently diagnosed conditions in older cats. It is diagnosed by an increased level of thyroid hormone, called T4, in the blood. 

What are the symptoms of Hyperthyroidism?

Symptoms include: 

  • Weight loss, often despite increased appetite 
  • Increased thirst 
  • Vomiting 
  • Diarrhoea 
  • Behaviour changes 

How is Hyperthyroidism treated?

There are several treatment options to manage hyperthyroidism but not all will be suitable for every cat. Approximately 5% of cats have functional thyroid tissue elsewhere in the body (known as ectopic), and not all options account for this. 


Medication for hyperthyroidism works by blocking the production of T4. It must be given daily, as either a tablet or liquid. Tablets must be given whole and not crushed, and neither the liquid nor tablets should be handled by pregnant women, unless they are wearing gloves. Blood tests are used to monitor the response to treatment. Doses often need to be adjusted to each individual cat, sometimes requiring multiple blood tests over a few months. 

Treatment of hyperthyroidism by medication:  

  • can be used safely long-term 
  • requires regular blood tests for life 
  • removes the need for your cat to go under anaesthetic
  • means the cost is spread across the cat's lifetime 
  • can result in adverse reactions, which usually resolve if medication is stopped 

Radioactive Iodine

Radioactive iodine treatment permanently destroys all functional thyroid tissue, including tissue not in the thyroid glands. It is given as an injection under the skin and a single dose is effective in over 90% of cats. Radioactive iodine is very safe for cats and has few to no side effects. Due to the radiation risk to humans, cats must be kept in a special unit for up to 2 weeks after treatment. 

Radioactive iodine treatment:  

  • is a permanent treatment 
  • has minimum, long-term monitoring required 
  • requires cats to stabilise on medication first 
  • has minimal ongoing costs 
  • is only offered at certain centres 
  • is not suitable for patients with complex needs, such as diabetics  


Surgical thyroidectomy involves removing one or both thyroid glands (both are affected in 70% of cats). This can offer a complete cure for some cats. The thyroid glands are very closely attached to the parathyroid glands, damage to which can cause a life threateningly low calcium. Additionally, not all hyperthyroid cats are suitable candidates for anaesthesia. Surgery does not treat ectopic thyroid tissue so if this is present, surgery will not achieve a cure. 


Hills y/d diet has an extremely low iodine content, which is required for T4 production. Cats on dietary management must not ingest any iodine, so should be kept indoors with no access to any other food or treats, including human food. In some areas, offering deionized water may be necessary for dietary treatment to be effective. 

What is the outlook for cats with Hyperthyroidism?

Generally, hyperthyroidism can be very successfully managed. Elevated T4 levels can cause secondary problems such as heart disease and high blood pressure, which don’t always resolve with treatment of the hyperthyroidism.  

Additionally, hyperthyroidism can sometimes mask kidney disease. High blood pressure is generally detrimental, but in the kidneys, it can compensate for them not filtering blood properly. As thyroid treatment reduces blood pressure, waste products that aren't being removed by the kidneys build up and begin to show on blood test. Recent studies suggest this doesn't necessarily mean a worse prognosis, but it may affect suitability for treatment such as radioactive iodine or surgery.  

Some cats may need additional treatment to manage kidney disease, heart disease or high blood pressure

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Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.

Page last reviewed: 7th August 2023
Next review due: 7th August 2025

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