Radioactive Iodine Treatment

What is hyperthyroidism?

Hyperthyroidism (an over-active thyroid) is one of the most diagnosed conditions in older cats. Symptoms include weight loss, often despite increased appetite, increased thirst, vomiting, diarrhoea and behaviour changes. Hyperthyroidism is diagnosed by an increased level of thyroid hormone, called T4, in the blood. Radioactive iodine treatment is a safe, effective and permanent treatment for hyperthyroidism, which aims to eliminate the need to give daily medication. 

How does radioactive iodine treat hyperthyroidism?

Radioactive iodine permanently destroys all functional thyroid tissue, including tissue not in the thyroid glands (occurring in approximately 5% of cats). It is given as an injection under the skin in the back of the neck, from where it travels around the body and is absorbed by any thyroid-hormone producing tissue. The radioactivity destroys the overactive thyroid cells but does not harm any other tissues and is a completely painless process. 

What are the advantages?

  • A single treatment is effective in over 90% of cats 
  • Treatment is permanent, with minimum long-term monitoring required 
  • There are few to no side-effects
  • Well tolerated and considered very safe
  • There is no requirement for general anaesthesia

What are the disadvantages?

  • Patients must be stabilised on medication first 
  • Cats must be hospitalised for at least 7 days following treatment 
  • Radioiodine treatment is not suitable for patients with complex needs such as diabetics 

Why do cats have to stay in hospital after treatment?

It takes an average of 2-4 weeks for cats to clear all the radioactivity from their system. While this is not dangerous to the cat, it can be harmful to people. For the first week after treatment, cats must be housed in a special isolation unit. Although experienced staff will be looking after your cat, handling must be kept to a minimum and you will not be allowed to visit. 

After seven days, the level of radioactivity being emitted by the cat is checked daily. Once the level of activity is below a set level, the patient can be discharged following these rules for a further 2 weeks: 

  • The cat must be kept indoors 
  • The cat must be confined to a room which cannot be the kitchen, bedroom or other high occupancy room 
  • The cat mustn’t be allowed on beds 
  • Any vomit or fur balls must be cleaned immediately, wearing disposable gloves, and the floor scrubbed with soap and water 
  • The litter tray must be cleaned at least daily, and any urine-soaked litter and faecal matter disposed of in the sewage system immediately. Flushable litter should be used. 
  • People must limit contact with the cat and if the cat is handled/stroked, hands must be washed with soap and water 

Are there any requirements for treatment?

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. For this reason, cats should be treated medically first, until their T4 levels are within the normal range. Blood tests, blood pressure, urine and the heart are checked prior to arranging radioiodine treatment. 

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 become apparent on blood tests. Recent studies suggest this doesn't necessarily mean a worse prognosis, but it may mean that a permanent treatment such as radioactive iodine isn’t suitable. 

Most centres will require cats to be vaccinated, microchipped and up to date with flea and worming treatment before being admitted for radioiodine treatment. 

Are there any risks with treatment?

Generally, radioiodine treatment is extremely safe with no side effects. A small number of cats may experience hypothyroidism (underactive thyroid) after treatment. This is often short-term, will resolve and may not cause symptoms. A very small number of cats will require thyroid supplementation long-term. 

Unfortunately, conventional radioiodine treatment is not suitable for the treatment of thyroid carcinomas (cancer). This is very uncommon in cats, but specialist referral is usually required for management. 


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.

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