Feline Hyperthyroidism | Care | Treatment | Spokane, WA
The Cat's Meow Feline Veterinary Clinic and Hyperthyroid Treatment Center Resting two cats Veterinary clinic
Different breeds of cats

Feline Hyperthyroidism

Care and treatment

Common hormone imbalance

The most common hormone imbalance in cats is Hyperthyroidism.  Most cats are over 8 years of age when they are first diagnosed, although they can be younger.  The average age at diagnosis is 13 years of age.  


Treatment options for Hyperthyroidism include life-long medication, surgery and radioactive iodine, or I-131. Where available, I-131 is usually the treatment of choice, unless concurrent illness gives an overall poor prognosis.


An article in the Journal of the American Veterinary Medical Association (Feb 15, 2006) showed that cats lived more than twice as long when given radioactive iodine therapy then when treated with medical management (methimazole) alone. Success rate of I-131 is over 95% with just one injection and, if a second is required and the thyroid is not cancerous, it is almost 100% effective. I-131 is painless and very well tolerated and side effects are rare.

Treatment Options

There are 3 treatment options available to treat hyperthyroidism. I prefer to think of them as good (medical management), better (Surgical removal of affected thyroid tissue), and best (I-131 treatment). All have their pros and cons, which are described below. It is important to note that an article in the Journal of the American Veterinary Medical Association (Feb 15, 2006) showed that cats lived more than twice as long when given radioactive iodine therapy then when treated with medical management (methimazole) alone.


Medical management (good)

Medical therapy consists of giving methimazole or PTU. Medicine has the advantage of being an inexpensive way to begin treatment of hyperthyroidism. However, besides the downside of having to give medicine to an often uncooperative patient, in some cases it must be given up to 3 times daily and side effects are common-almost 20%. The most common side effects are anorexia, vomiting and lethargy but cats can also experience far more severe side effects including liver failure and blood abnormalities. On methimazole, most cats improve but are never cured and when discontinued or if doses are skipped, clinical signs will recur.


On medication, blood tests should be performed every 6 months to ensure your cat has normal thyroid levels as well as normal liver function tests. In addition, medicine can cost up to $2 per day. Although it would initially appear that medicine is less expensive than other treatment options, if your cat lives even 1 year from the time of diagnosis this ceases to be true. Over time, with giving life-long medication and bi-yearly blood-work, medical management often becomes the most expensive form of treatment. Additional costs may also accrue should your cat experience any of the above side effects and medical attention be required.


That said, medical management may be the best choice in cats with concurrent life threatening diseases. Because we are often dealing with an older population, our patients often have concurrent severe kidney disease, heart disease or cancer and medication may benefit these patients if tolerated.


Surgery (better)

Surgery has long been a very popular treatment option for hyperthyroidism. It includes surgical removal (thyroidectomy) of diseased thyroid tissue. Surgery has the benefits of decreasing thyroid hormone rapidly and is often the treatment of choice when it is imperative to do so, such as severe heart failure caused by or exacerbated by hyperthyroidism.


However, Surgery presents significant risks to the patient. First, and foremost, surgery requires anesthesia of a geriatric patient who typically has compromised renal function and/or heart disease. Also, the parathyroid glands, which help maintain normal calcium and phosphorus levels, are located on both ends of the thyroid glands. During surgery these can be destroyed or disrupted, resulting in a life-threatening change in calcium and phosphorus levels. These will then need to be controlled with either injected or oral calcium and vitamin D. Sometimes life-long supplementation is required. Also, cats with bilateral thyroidectomy often have to have thyroid hormone supplementation.


Due to the risks of anesthesia and hypocalcemia from parathyroid loss, surgery for this disease is generally not recommended if I-131 is available.


I-131 therapy (best)

I-131, also known as radioactive iodine, is the treatment of choice for most cases of hyperthyroidism in cats. The only organ in the body that is affected by I-131 is the thyroid gland. When injected, it gets absorbed by the thyroid gland and "burns out" the hyperactive tissue typically leaving normal tissue intact. It is the safest and most effective therapy available. With the exception of hyperthyroidism due to thyroid cancer (thyroid adenocarcinoma) I-131 has an approximately 95% cure rate after a single injection and a purported 100% cure rate after a second injection. At The Cat's Meow Feline Veterinary Clinic, cats requiring a second injection will be retreated at no charge unless a carcinoma existed. A very small percentage of cats treated with I-131 will become hypothyroid (0.25%) but even fewer require thyroid supplementation.


Side effects of I-131are rare and include mild lethargy and decreased appetite. Occasionally a sore throat and/or voice change can occur and are usually transient. Although significant side effects in cats are rare, if your cat exhibits other symptoms besides mild lethargy/decreased appetite do not assume this is normal. Because we are dealing with a geriatric population, other underlying health concerns may arise and your veterinarian should be consulted immediately.


If you have any questions or concerns regarding treatment, please call the clinic at 509-535-6369.


Please call our clinic at 509-535-6369 to make your appointment.  Your cat will thorough examined and the procedure explained with time for any questions.  Phone consultations are encouraged to peruse bloodwork results as well as determine candidacy status for I131.  Feel free to phone the office at 509-535-6369.




It is imperative that, unless there are extenuating circumstances, such as serious cardiac disease, that methimazole be discontinued one week prior to administering I-131. Cats with significant cardiac disease, which cannot be easily controlled even with cardiac drugs, may need to remain on methimazole until 48 hours prior to I-131.


In addition, avoid giving your cat supplements that contain iodine (including kelp), calcium or magnesium. These medications as well as any holistic or herbal nutritional supplements should be discontinued 1 week prior to I-131 administration. These supplements should be withheld for 1 week following I-131 administration as well. You can continue all prescription medications as directed by your veterinarian.

I-131 and how it works

The dose of I-131 is injected between the shoulder blades and is immediately absorbed into the blood stream. Because the thyroid glands are the only tissue to utilize iodine, I-131 is taken up exclusively by any tissue producing thyroid hormone (T4). Normal thyroid tissue, which is typically inactive when a cat is hyperthyroid, does not produce T4 and therefore, is typically left intact (hence the rare occurrence of hypothyroidism after treatment). The overactive cells, however, need increased amounts of iodine to supply their increased production of thyroid hormone. The hyperactive tissue binds most of the radioactive iodine within the first 24 hours after injection. I-131 kills the hyperactive cells and generally spares normal thyroid tissue. This treatment has the added benefit of sparing the adjacent parathyroid tissue that surgery will often damage/destroy. Once the hyperactive tissue is gone, the normal thyroid tissue is stimulated to start producing T4 at more normal levels. This may take several months before the normal thyroid tissue rebounds and no supplements are required during this time.


Up to 90% of the unbound I-131 is excreted in the urine within the first 2 days. The reason that we keep them hospitalized for 3 days prior to release is so that this radiation is contained in our facility and doesn't go home with your cat.


Most cats will show clinical improvement almost immediately with a decrease in thirst and appetite as well as a return to normal activity levels. Up to 75% of cats will have normal thyroid levels within 1 week, 90% within 1 month, and 95% within 3 months of I-131 treatment. Up to 10% may actually have low thyroid levels but less than 1% will require oral thyroid hormone replacement medication and will depend on whether or not your cat has clinical signs of hypothyroidism.


The remaining 5% of cats which continue to have high thyroid levels can be re-treated 3 months after their initial therapy. If the cat is not showing clinical signs of hyperthyroidism, recent studies suggest that many of these will become euthyroid (normal thyroid) within 6 months of treatment. If required, with the exception of thyroid cancer, we will give that second I-131 injection at no additional cost for the treatment.


Remember to have your veterinarian perform blood tests 1 month and 3 months after therapy to assess thyroid function as well as kidney and liver function.

However, Hyperthyroidism can mask kidney disease.  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 marked deterioration, medication may need to be discontinued or adjusted. In that case, 1-131 likely is not the treatment of choice. If kidney function remains stable on treatment with anti-thyroid medications, then a more permanent therapy (such as radiotherapy) can proceed.  

Success rate of I-131 is over 95% with just one injection and, if a second is required and the thyroid is not cancerous, it is almost 100% effective.  I-131 is painless and very well tolerated and side effects are uncommon.


In order to treat your cat with I-131, we require recent lab work (CBC, blood chemistry, urinalysis) from your referring veterinarian.  If your cat is currently on methimazole treatment, it should be discontinued for 1 or more weeks prior to I-131 administration, if possible.  Any supplements containing iodine, calcium or phosphorous should be discontinued for 1 week prior to treatment and should not be re-started for 1 week following treatment. If your cat is on prescription life-long medication these may be continued during the stay but due to the exposure risk, temporary medications should be finished prior to treatment.  All medications should be brought with you for the hospital stay.


After your cat is treated with I-131, your cat will enter an isolation ward for an average of 3 days.  Due to state regulations, once your cat has entered the isolation ward you will be unable to visit your cat.  You can call daily for updates on your cat at 509-535-6369.  Personal items such as blankets / bedding and toys are not allowed in the isolation ward. It is recommended that you bring 5-7 days' worth of food your cat is used to eating to avoid any change in diet while in the hospital.


Laboratory tests should be performed 1 and 3 months after I-131 administration to assess the thyroid as well as the kidney and liver values.  All follow-up lab work should be performed by your referring veterinarian. If the thyroid still shows hyperactivity after 3 months and an additional injection is required, our clinic will give this injection at no additional cost to you.

Reasons for failure of treatment

Reasons for failure to respond to treatment include thyroid cancer, certain drugs/chemicals (including "natural" or homeopathic medications), a very large or overproducing thyroid mass and prior treatment with methimazole. In addition, a very small number of cats will have a poor uptake of I-131, leaving some of the hyperactive tissue intact.


However, nearly all cats will be cured by a second treatment. As stated before, repeat blood tests should be performed at 1 and 3 months after the treatment to assess thyroid, kidney and liver functions. Relapse can occur 2-5 years after initial treatment but is rare (<0.5%)


Your cat will be housed in an isolation room in our clinic. Most cats stay an average of 3 days and nights in this isolation room where fresh food and water are always available. Cats that require higher doses or excrete the radioactive iodine at a slower rate may require additional hospitalization. Once I-131 is administered your cat cannot be released until her/his radiation level is at or below the level determined by the Washington State Department of Health. State regulations also dictate that no clients be allowed in the isolation room so we are unable to allow you to visit your cat once she/he has been isolated.


If you are unable to pick your cat up at the designated time, you can arrange to keep your cat in the isolation ward for additional days. An additional fee, which will be discussed with you when making the arrangements, will apply.

The Cat's Meow Feline Veterinary Clinic and Hyperthyroid Treatment Center

1017 South Perry Street

Spokane, WA 99202


Fax- 509-535-6377


Mon-Fri 8:00-5:30

(7:30 drop off)

Sat  9:00-12:00

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