Internal Medicine & Oncology Service

I-131 Radioactive Iodine Treatment for Feline Hyperthyroidism

WAVES HOSPITAL

Want to know more about hyperthyroidism and I-131?

Please see the list of questions we are commonly asked regarding I-131 treatment.

What is hyperthyroidism?

Hyperthyroidism describes an increased hormone production by the thyroid gland. This is a very common disease in middle-aged to old cats. In over 95% of cats, hyperthyroidism is caused by the development of benign tumour (adenoma) or hyperplastic nodules of the thyroid gland. In rare cases, hyperthyroidism is caused by a malignant tumour.

Thyroid hormones have a wide range of functions in the body and an excess of these lead to various clinical signs. The most common clinical signs include weight loss, increased appetite, vomiting, diarrhoea, drinking and urinating more, and nervousness or hyperactivity. Thyroid hormone affects the heart, causing fast heart rate, heart murmurs, abnormal heart beats, and high blood pressure.

What are the treatment options for feline hyperthyroidism?

There are several therapies for the management of hyperthyroidism:

Anti-thyroid medication (methimazole and carbimazole)
Surgery to remove the thyroid gland(s)
Diet
Radioactive iodine therapy.

How does I-131 work?

Radioiodine treatment is considered the gold standard treatment for hyperthyroidism.

The goal of radioiodine treatment is to destroy the abnormal thyroid tissue that is producing the excess of thyroid hormones. The thyroid gland naturally takes up iodine from the blood to produce thyroid hormones. Treatment with radioiodine ‘utilizes’ this natural iodine uptake mechanism. I-131 is given as a capsule by mouth and once the radioiodine is absorbed it is actively taken up into the thyroid gland, preferentially by the abnormal thyroid tissue. The accumulated I-131 then causes destruction of the abnormal thyroid cells resulting in a decrease in thyroid hormone production.

The dose is calculated for each cat individually based on severity of clinical signs and elevation of thyroid hormones. Unlike medication or a low-iodine diet, administration of radioiodine can provide a permanent cure for feline hyperthyroidism. It is also almost always a one-off treatment; less than 5% of the cats needing a second dose. Most cats will have normal hormone levels within 1 or 2 weeks of I-131 treatment although it can take up to 6 months to take full effect.

What is involved in radioiodine treatment at WAVES?

The first step is to confirm diagnosis of hyperthyroidism and to ensure there are no diseases in your cat that would complicate or prohibit radioiodine treatment. Your cat will be booked in for a consultation with one of our internal medicine veterinarians. During this consultation, the clinician will review your cat’s history and laboratory results and we will perform a physical examination. Unless recently performed by a reference laboratory, blood/ urine tests will be required. Further diagnostic testing may be required if concerns for concurrent disease arise during this consultation and examination, and this will be discussed with you. The medicine clinician will also discuss alternative treatment options for hyperthyroidism in your cat so that you can make an informed decision whether or not to pursue radioiodine treatment. Once we have established that your cat is suitable for treatment with radioiodine and you have agreed to this, we will book an admission time for the treatment.

Why choose WAVES for my cat’s radioiodine treatment?

The assessment and treatment of your cat will be overseen by one of WAVES Internal Medicine Specialists, in a state-of-the-art radioiodine treatment facility. WAVES is the only Radioactive iodine unit for cats in Perth that is run by specialists in internal medicine.