Radiation Therapy for Hyperthyroidism in Cats

At our North Nowra Veterinary Hospital we have the expertise to treat cats diagnosed with hyperthyroidism using radioactive iodine. We are the only veterinary hospital on the NSW South Coast that performs this procedure. Cats are treated in a lead shielded cat cage manufactured by Gammasonics. The cage is made with a lead glass window so that cats can be monitored at all times whilst in for treatment. This purpose-built cage removes any chance of radiation exposure to operators.

What is hyperthyroidism?

Hyperthyroidism is a common disease found in cats caused by the excess production of thyroid hormones (known as T3 and T4) from an enlarged thyroid gland. It is most often diagnosed in cats over the age of 10 years old.

Thyroid hormones are responsible for controlling the body’s metabolic rate, and impact heart rate, temperature and speed of digestion.

What causes it?

Hyperthyroidism is caused by an enlargement of the thyroid gland (goitre). In most cases (over 97%), this enlargement is due to a benign (non-cancerous) process called adenomatous hyperplasia. In rare cases, the cause of thyroid enlargement is due to a malignant cancer called thyroid adenocarcinoma.

Clinical Signs

Clinical signs of hyperthyroidism in cats often become more severe as the disease progresses. Common signs include:

• Weight loss
• Increased appetite
• Unkempt, rough or greasy coat
• Behavioural changes such as increased activity, vocalisation or more aggressive
• Gastrointestinal upset, including vomiting or diarrhoea

Clinical signs of concurrent diseases including cardiac and kidney disease may also be present.

Diagnosis

After a physical examination is performed by the veterinarian, if hyperthyroidism is suspected the vet will conduct a blood test. The blood test will provide information about the cat’s general health, and include specific tests of thyroid hormone. Elevations in thyroid hormone in conjunction with appropriate clinical signs will diagnose hyperthyroidism.

Treatment and Management

Daily Oral Medication

Management of hyperthyroidism in cats can be accomplished with the oral thyroid hormone suppression drugs carbimazole and methimazole. This medication suppresses the production of T3 and T4 hormones assisting to decrease clinical signs. It is important to note that this approach does not eliminate the underlying cause of hyperthyroidism. It simply suppresses thyroid hormone production from the diseased and enlarged thyroid gland. The thyroid gland remains enlarged and often continues growing and changing. This means that cats whose hyperthyroid state is controlled on a certain dose may later relapse, and need further blood tests and dose adjustments over time.

Daily Transdermal Medication

For patients where the oral administration of carbimazole or methimazole is a challenge (many cats are difficult to tablet!), methimazole is available in a transdermal formulation, meaning it can be applied to and is absorbed through the skin of the inner surface of the cat’s ear (pinna). While this treatment may be easier to implement than oral administration, it introduces additional variables around application and absorption of the drug. Like oral medication, while it is able to effectively manage the hyperthyroid state for periods of time, because the progression of the underlying thyroid pathology continues, relapses and the need for further blood tests and dose adjustments over time are common.

Radioactive Iodine Therapy

Radioactive iodine therapy (Iodine 131) is considered the gold-standard treatment for hyperthyroidism in cats (and humans!). With this approach, radioactive iodine is administered to the hyperthyroid cat as a simple injection under the skin, or given orally. Once administered, it is quickly absorbed by the hyperactive thyroid gland (the thyroid gland naturally uses dietary iodine to manufacture thyroid hormone). The radioactive iodine targets the abnormal thyroid tissue, destroying it. In doing so, the disease is almost always cured with a single treatment (success rate approx. 97%), and the thyroid gland pathology is arrested and cannot progress further (progression over time is a downside with oral and transdermal treatments). It is a safe procedure and has minimal side effects. Treated cats need to spend 5-7 days in hospital after receiving this treatment, while their radiation output declines to safe levels, before they can go home.

Prognosis

Prognosis for cats with hyperthyroidism is generally excellent with appropriate therapy. The use of oral and transdermal medications do often present challenges with keeping the hyperthyroid state of some cats controlled on an ongoing basis, which can often lead clinical relapses, and the need to repeat blood tests and adjust doses. Because radioactive iodine is curative in 97% of cases and arrests the progression of thyroid gland pathology permanently, it is generally recommended as the best and most effective treatment in the vast majority of cases.