Hyperthyroidism & Radioactive Iodine Treatment

What is Hyperthyroidism?

Hyperthyroidism, by definition, is the excess production and secretion of thyroid hormone (T3 and T4) by the thyroid gland. It is now considered to be the most common endocrine disorder in middle to older aged cats (average age 13 yrs). In cats, this is usually caused by a benign thyroid tumor (adenoma or adenomatous hyperplasia) either on one or both of the thyroid glands. Rarely (1-3% of cats), hyperthyroidism is caused by a malignant thyroid tumor (carcinoma).

How do I know if my cat is affected?

The most common symptoms observed in hyperthyroid cats include weight loss despite an increased appetite, increased thirst and urination, restlessness or increased anxiety, vomiting, diarrhea, and changes in skin or hair coat. There are blood tests to determine the amount of circulating thyroid hormone in your cat.

Why is it important to treat?

Thyroid hormone is essential for fueling every cell in the body, however, too much of a good thing can be detrimental on every body system. Eventually, the body can not keep up with the increased metabolic demands which can result in multiple organ failure when the body can no longer compensate.

What are the current treatment options?

The goal of treatment is to reduce the amount of circulating thyroid hormone in the body. There are oral medications that block the synthesis of active hormone in the body, but this is not curative and requires medicating the cat daily for life. Only 85-87% of cats respond to oral anti-thyroid medications and up to 15% can have side effects including vomiting, diarrhea, bone marrow changes, and skin lesions. Surgery can be done to remove the thyroid gland(s), but healthy tissue is also removed and the parathyroid glands are at risk for being removed due to their close proximity to the thyroid glands. Aside from being an increased anesthetic risk (older, highly stressed, and often very skinny cats with rapid heart rates and often compromised kidneys), there are many nerves in that region that have the potential for damage. Often, both glands need to be removed (70-90%) and then these animals need to have thyroid hormone supplementation. Radioactive Iodine (I131) is the safest and most effective treatment modality currently available with a success rate of 96-98%.

What is I131 and its advantages?

Radioactive Iodine is preferentially taken up by the diseased thyroid gland (not by healthy thyroid tissue) and permanently destroys the hyperactive tissue. There are no side effects on the rest of the body and a reduction in thyroid hormone (and consequently, symptoms of the disease) occurs almost immediately. It is given as a single injection under the skin and unless your cat requires sedation for a regular trip to the veterinarian, there is no sedation required. The success rate is excellent (96-98%) with a single injection.

What are the risks of treatment?

A very small percentage of cats may develop hypothyroidism after treatment (too little circulating thyroid hormone), but these cats rarely require treatment (thyroid hormone supplementation). This is a risk seen with all the treatment options available and is dependant on how long the cat has had the disease and how long it takes for the normal thyroid tissue to start functioning again. Older cats have an increased risk of kidney disease despite concurrent hyperthyroidism. In fact, hyperthyroidism may temporarily mask underlying kidney disease and after these animals have been treated, the decreased kidney function may become more apparent. It is important to keep in mind that this is not a direct result of the treatment itself. Your veterinarian may recommend a trial of oral anti-thyroid medication (since it is reversible) to determine the risk of your cat with treatment. Only on rare occasions would this preclude treatment because excess thyroid hormone can directly impair the kidneys and eventually cause kidney disease or failure.

Are there pre-treatment requirements?

Prior to treatment, we require a baseline CBC, super-chemistry panel, urinalysis including urine specific gravity, and a T4 that has been submitted to a lab (not done in-house). All lab work must be current as of 60 days prior to treatment, except the urinalysis should be current from 30 days. Additional thyroid hormone blood work may be done by your regular veterinarian to aid in the diagnosis. If your cat has a heart condition either primary or secondary to the thyroid disease, your regular veterinarian may recommend a cardiac work-up (ECG, echocardiogram, thoracic radiographs). We require that your animal is stable enough to board in a hospital with minimum medical care (cats in the nuclear ward will require minimal handling to reduce exposure of personnel to radiation). We will be able to administer oral medication or administer subcutaneous fluids twice daily if necessary.

What can I expect?

• You will receive a packet of information in the mail regarding specific details of the treatment itself as well as requirements for treatment at our facility.
• The morning you drop off your pet, you will meet with the Dr. and staff that will be caring for your cat to address any questions or concerns and to review the treatment procedures and discharge instructions.
• The duration of stay will usually be 3 days. Families with small children or pregnant household members or cats released with a higher radioactivity level will require stricter guidelines upon discharge or additional hospitalization. You will receive specific discharge instructions in your packet.
• Due to strict state and federal regulations, only licensed and trained personnel will be allowed to enter the nuclear medicine ward. Therefore, you will not be allowed to visit your cat during the treatment stay. You will be contacted every day with an update on your cat.
• If your cat is easily stressed or fractious in nature, one dose of light sedation may be necessary to administer the radioactive material to ensure your cat receives the entire injection appropriately.
• A deposit will be required prior to ordering of the treatment as each dose will be ordered specific to your cats needs.
• Follow up bloodwork will be done by your regular veterinarian 30 and 90 days after treatment.