Iodine-131 Therapy for Feline Hyperthyroidism

Next to diabetes, hyperthyroidism is one of the most common diseases that cats experience.

Hyperthyroidism occurs when there is an abundance of abnormal but generally non-cancerous cells being generated. Those cells secrete thyroid hormones which cause the cat’s thyroid levels to become unbalanced. In approximately 2% of cats, hyperthyroidism is caused by a cancerous tumor.

Yes, hyperthyroidism in cats can cause stress to many of the body’s organs and may eventually be deadly if not treated. This chronic stress may lead to the following:

1. Heart: hyperthyroidism may cause a thickening of the heart wall that can lead to eventual heart failure, abnormal heartbeats, and possibly the formation of blood clots. Much of these heart changes may be reversible with appropriate therapy.
2. Kidney disease: it is reported that up to 30-35% of hyperthyroid cats also have pre-existing kidney disease. Hyperthyroidism can mask kidney disease by increasing the kidney blood flow and glomerular filtration rate (GFR), thus artificially driving down the kidney enzymes. This masking of the kidney disease may prevent proper medical management of the kidney disease to be instituted. Furthermore, poorly controlled or untreated hyperthyroidism can also lead to progression of the kidney disease by contributing to renal hypertension, proteinuria, glomerulosclerosis, renal tubule dysfunction, and sodium retention.
3. Breathing abnormalities: breathing issues may develop as a result of the development of heart disease or due to respiratory muscle weakness, increased airway resistance, and diminished lung compliance.
4. Neuromuscular abnormalities: may include muscle weakness, ventral neck flexion, inability to jump, gait abnormalities, tremors, collapse, and seizures (uncommon).
5. Eye changes: hyperthyroidism may result in retinal hemorrhages and retinal detachment that can lead to blindness.

1. Methimazole orally or transdermal application: this is one of the most common therapies, and while it can be effective, it is a life-long therapy with frequent monitoring and dose adjustments being needed. Adverse reactions as a result of methimazole therapy may occur in approximately 20% of treated cats and typically develop within the first 3 months of therapy. The most common side effects include a reduced appetite, vomiting, and lethargy. A few cats may experience more severe abnormalities, such as significantly low white cell counts, low platelet counts, allergic reactions, facial and neck excoriations, and jaundice. Also, methimazole is considered a non-antineoplastic hazardous drug and gloves should be used when handling the medication. Potential adverse effects to people may include lowering of the thyroid level, risk of birth defects, and the appearance of the drug in breast milk.
2. Diet: Y/D (a limited-iodine food) may be considered, but it must be understood that no additional food/treats are fed to your cat, or else this therapy will not be effective.
3. Surgery: surgery may be effective for unilateral disease, but there are other glands and nerves in this region. Surgery may lead to decreases in calcium that can lead to muscle twitching and seizures, neurologic abnormalities such as Horner's syndrome, laryngeal paralysis, and persistent hyperthyroidism if there is ectopic tissue present in addition to the thyroid nodule.
4. Radioactive iodine therapy: this therapy is generally considered the safest and most effective method of treatment for feline hyperthyroidism.

This therapy can be effective in approximately 94% of cats. In the other 6%, approximately 2.0% of cats become hypothyroid and need to be supplemented with thyroid medication; 2% of cats remain hyperthyroid (possibly due to the presence of a malignant thyroid tumor) and need to be treated a second time; and approximately 2% will develop hyperthyroidism in the future.

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.

A pertechnetate scan will be performed prior to the treatment day to aid in determining the I131 dose. Short sedation will be required for this part of the procedure.

On a separate day, the radioactive iodine (iodine 131) is administered by a single injection into the vein. That is it!

The dose of the radioactive iodine may be determined by the following parameters:

1. The original pre-treatment thyroid level.
2. How long your cat has been treated medically.
3. Features noted with a pertechnetate scan. There is a much lower cost to radioiodine therapy if this scan is skipped, however, there may be a loss of information in doing so that may influence our dosing. 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, if there is ectopic disease, and may reveal changes that may suggest the presence of a malignancy, all of which may influence the final dose determination.

Yes, your cat will be veterinary hospitalized when they receive treatment for hyperthyroidism until the radiation levels have reduced adequately to allow your cat to return home (usually in three to four days). Treatment is not invasive and most cats tolerate the brief separation from home without significant stress.

Tier 1 Veterinary Medical Center offers premier, luxury veterinary hospitalization for pets. Your cat will be treated with extraordinary care and continued monitoring while they stay at our veterinary hospital. Our staff provides around the clock monitoring so that you can rest easy knowing that your pet is being cared for by the most dedicated, professional and educated veterinary team in the State of Alaska.

When your cat is discharged, there will still be a period of time, usually about 2 weeks, where restricted human and other pet contact is necessary because your cat is still emitting low levels of radiation (do not allow your cat to sleep with your or with the other pets, do not allow your cat to sit on your lap, and especially limit all contact with children and pregnant women). Also, during this time, flushable litter is recommended, even though the amount of radiation in the urine and feces is almost zero when your cat is discharged.

After this period, life returns back to normal and all there is left to do is return for some follow-up lab work over the next few months.

As mentioned above, this is a very safe therapy for hyperthyroidism that is curative in approximately 94% of cats. As 30-35% of cats with hyperthyroidism also have concurrent kidney disease, there was a past concern that the treatment of hyperthyroid cats may not only unmask the kidney disease but may also worsen the kidney disease. Recent studies have shown that for most cats, this is not true. What we now suspect includes:

1. Persistent hyperthyroidism may mask kidney disease and thus, prevent appropriate management of the kidney disease. When the hyperthyroid state resolves with treatment, the true state of your cat's kidney disease is revealed, but does not generally worsen; so if the kidney values do increase, they generally increase to the level that they would have been if hyperthyroidism was not present. Proper management of the kidney disease is then pursued.
2. Persistent hyperthyroidism may be damaging to the kidneys and may promote the progression of kidney disease.
3. In the rare event we do cause hypothyroidism (2% of cases), the kidneys may eventually become compromised, however, thyroid supplementation would be beneficial in correcting this event if it should occur.

After treatment, we recommend to have blood work and a thyroid level measured in 30 days and then 90 days post treatment. This is to ensure that the thyroid level is controlled (94% chance), that the thyroid level is not low, which may require supplementation (2% chance), and that the kidneys are still happy.

Then, you are done! No more medications to treat the hyperthyroidism and no more extensive blood monitoring!

Make an appointment for your cat to receive Iodine 131 treatment at Tier 1 Veterinary Medical Center by calling our Veterinary Hospital at 907-745-TIER.

When you schedule your treatments with our Specialty Department, we will walk you through the process of how the radioiodine will be administered and answer your questions in detail so that you feel comfortable making any informed decision. We will also discuss the cost in depth with you so that you know what to expect. If you have pet insurance, our Veterinary Hospital is happy to work with you and your insurance company to file a claim.

Call our Veterinary Hospital at 907-745-TIER, option 1 if you would like additional information.