Imaging with Nuclear Medicine and Treatment by Radiotherapy

This therapy is generally considered the safest and most effective method of treatment for feline hyperthyroidism.

Your cat will be given an injection of radioactive iodine (iodine 131) and kept in the facility until the radiation levels have reduced adequately to allow the cat to return home (usually three to four days). Treatment is not invasive, and most cats tolerate brief separation from home without significant stress.

After release, there will be a period of time, usually about a week and a half, where the cat will need confinement, flushable cat litter, and restricted human contact because he is still emitting low levels of radiation. After this period, the cat is back to normal without restrictions, and all that is left to do is return for some follow-up lab work over the next few months. The radioactive iodine injection is given under the skin, similar to a vaccine, and the cat is basically boarding afterward until the radiation levels drop.

Nuclear medicine scan of a hyperthyroid cat. The thyroid tumor is shown in red. It is benign but over-secreting hormone and causing all the problems. Scan courtesy of Advanced Veterinary Medical Imaging

This treatment works because iodine (radioactive or otherwise) attaches to thyroid tissue. Radioactive iodine emits high-speed electrons that damage the thyroid tissue from within, effectively destroying the thyroid tumor. The electrons only penetrate a fraction of an inch, which means only the thyroid tissue is affected, and the rest of the cat's body is not. Human beings with hyperthyroidism are similarly irradiated and are not confined in any way afterward, but because, in this situation, the patient is a cat, the Nuclear Regulatory Commission insists on confinement as described above. The potential for side effects with this therapy is very, very low, and it is low stress for the cat.

The Pertechnetate Scan
In most facilities (but not all), therapy begins with a nuclear medicine scan called a pertechnetate scan. There is a much lower cost to radioiodine therapy if the scan is skipped; however, if you opt for a facility that skips the scan, you should be aware of the information that is lost in doing so in order to make an informed decision.

The pertechnetate scan shows the location and size of the cat’s thyroid glands and confirms hyperthyroidism. The scan also indicates whether one or both glands are involved and provides information needed to calculate the therapeutic dose of iodine 131 that will be used in treatment. An additional benefit of the scan is that it can identify the three to five percent of cats who have a malignant tumor and detect areas of tumor spread. If you skip the scan, you are likely to miss malignant disease, but more importantly, dosing of the iodine 131 will not be based on your cat's individual thyroid mass; it will instead be based on your cat's weight. This leads to the potential for underdosing some cats (necessitating another treatment) and overdosing some cats (making them hypothyroid and necessitating treatment or, worse, tipping borderline kidney function into kidney failure). Scanning can be beneficial, but it does increase the cost of the treatment. We recommend working with your veterinarian to decide the benefit to you and your cat.

If the scan identifies a bizarre distribution of thyroid tissue that is typical of malignancy (cancer), a different iodine 131 protocol is used.

Advantages of this Method of Treatment

• Treatment is a one-time event (only two to four percent of cats require a second treatment), and no ongoing therapy is required.
• The disease is not simply managed but is actually cured!
• No anesthesia is required as treatment amounts to an injection followed by three to 7 days of boarding, which is very non-stressful, especially considering the usual patient is an older cat with potential heart disease.
• If a cat is one of the unlucky three to five percent for which the thyroid tumor is malignant, the initial pertechnetate scan is likely to detect this.

Disadvantages of this Method of Treatment

• Owner and pet are separated during the quarantine.
• Typically, facilities require the cat to be confined indoors or have limited contact with owners for a period of time after discharge. Children and pregnant women can have no contact with the cat for a week or two after therapy. If this is too inconvenient to work out at home, the cat may be boarded at the radiofacility until this period has passed, but there will be an additional expense for that.
• Facilities with the capability of performing radiotherapy may not be conveniently located.
• This is a relatively expensive therapy, and the facility's cost does not include the required pre- or post-treatment diagnostics with your regular veterinarian.
• Special flushable cat litter is required for one to two weeks after therapy.
• Some follow-up blood testing is generally recommended after treatment (typically one and three months after therapy).
• There is a chance (less than five percent) that the cat will become HYPOthyroid after treatment, requiring daily oral thyroid hormone supplementation.
• Radiotherapy may not be a good idea for a cat with poor kidney function.

If kidney function is not thoroughly investigated prior to this therapy, latent kidney failure may be unmasked irreversibly by this therapy. This can be avoided simply by screening potential candidates for kidney failure prior to recommending radiotherapy. Those who have possible kidney insufficiency should be treated with medication to bring the thyroid levels under control. If kidney function begins to show deterioration on this therapy, medication is discontinued, and the need for treating thyroid disease must be reevaluated. If kidney function remains stable on treatment with anti-thyroid medications, then a more permanent therapy (such as radiotherapy) can proceed.

Radiotherapy Facilities

General practice clinics do not provide this service. Your veterinarian will refer you to a facility.