Hyperthyroidism in Cats

Radioiodine (I-131) Therapy-Commonly Asked Questions

Feline hyperthyroidism is a common endocrine condition in older cats. It is caused by overproduction of thyroid hormone. This overproduction of thyroid hormone leads to the multitude of signs, most commonly weight loss with an increased appetite, increased activity and/or vocalization, vomiting, diarrhea, increased water consumption and increased urination. Some cats show all of these signs and others may show only one of these signs. Other signs such as decreased activity or food consumption, and problems related to cardiovascular disease, occur less commonly.

Cats normally have 2 thyroid lobes, one located on each side of the trachea (windpipe) in the neck. More than 80% of hyperthyroid cats have a condition known as 'hyperplasia' of the thyroids, resulting in excessive function and enlargement of both thyroid lobes. However, both lobes cannot always be felt, even if both are functioning excessively. About 15% of hyperthyroid cats have a benign tumor (an adenoma) and in this situation, only one of the 2 lobes is involved. And lastly 2-3% of hyperthyroid cats have a carcinoma (a malignant tumor) that can be variable in presentation involving one lobe or have multiple nodules.

Following the diagnosis of hyperthyroidism your veterinarian may discuss treatment options such as methimazole (Tapazole®), specialized diet, surgery, and I-131 treatment.  Methimazole blocks the production of thyroid hormone and can be administered orally or topically (transdermal - for absorption through the skin).  Specialized veterinary prescription diets for treatment of hyperthyroidism are iodine deficient.  Iodine is a necessary building block in thyroid hormone synthesis and without it the body cannot make thyroid hormone.  Thyroid surgery is where the surgeon removes all abnormal thyroid tissue, and is a good option for cats with an adenoma when only one thyroid lobe is involved. These treatment options should be discussed with your veterinarian or can be discussed at the initial consultation at UC Davis, because not all treatment options fit for all patients.

I-131 treatment is where radioactive iodine is administered as a single injection under the skin. Since iodine is the main ‘building block” for creating thyroid hormone, this iodine concentrates in the thyroid gland where it will destroy overactive thyroid tissue.  Side effects are not expected but similar to other treatments for hyperthyroidism, unmasking underlying kidney disease, and development of hypothyroidism may occur but are uncommon.

This will be discussed with you at the initial examination and following the technetium scan. Reasons for I-131 treatment include difficulty in administering oral/topical medication, side effects to oral/topical medication, or for a permanent treatment option with minimal adverse effects. 

This is a safe, minimally invasive test that provides information on anatomy, diagnosis, and may alter treatment dose or recommendations.  Technetium, a short acting radioactive substance, is administered through a catheter into a vein, and concentrates in the thyroid tissue then images are acquired using a gamma camera providing information on nodule size, location, and presence of multiple nodules. Although not perfect, this is the most reliable test for determining if a cat has an adenoma (one lobe involved), hyperplasia (2 lobes involved), or carcinoma (>2 lobes). If thyroid carcinoma is a concern or the thyroid uptake is unilateral, alternative therapy may be discussed.  

Cats need to be hospitalized 24-hours following this study to prevent owner exposure to radiation. This will not be able to be performed on the day of the initial examination, but will require a second visit separate from initial examination and I-131 treatment. 

Some cats with hyperthyroidism also have kidney disease (a common condition of older cats), but their thyroid condition improves blood flow to the kidney so that it masks the underlying kidney problem. Therefore, before embarking on a permanent treatment (surgery or radioactive iodine) that might cure the hyperthyroidism but worsen kidney problems, we typically want to see what happens to kidney function when the hyperthyroidism is treated with a reversible treatment such as methimazole. If problems with the kidney do not occur, permanent treatment is an option. If problems with the kidney do occur, we usually avoid surgery or radioactive iodine treatment.

Tapazole® (methimazole) may or may not be discontinued prior to treatment with I-131, this will be recommended by the clinician overseeing the case, prior to your appointment please follow your veterinarians recommendations.

Ideally, lab tests including a minimum of kidney values (BUN and creatinine) and thyroid hormone test (total T4 or TT4) that show a normal or nearly normal T4 induced by methimazole treatment should be available to evaluate the effect of treatment on kidney status. Some animals do not tolerate methimazole treatment and evaluation of kidney function with a reversible treatment will not be possible. Bloodwork demonstrating TT4 at the time of diagnosis, and current TT4 (within the last month) is also necessary. If a cat is showing signs of other illness a current complete blood count, chemistry panel, urinalysis, and possible further diagnostics may be required prior to I-131 therapy. Bloodwork can be performed at a local clinic and brought with your pet for evaluation at UC Davis, or performed at UC Davis (see variation in cost estimate).  Bloodwork may be repeated at UC Davis at the discretion of the admitting/supervising clinician.

Approximate cost, which includes the examination fee, technetium scan, intravenous catheter placement for technetium administration, I-131 treatment, and hospitalization after technetium scan and I-131 therapy is $1800 (price subject to change). If cardiac evaluation or blood work is necessary this could add $200-500 to this estimate.

Please have your veterinarian fax records and pertinent lab work prior to your appointment. They can be faxed to UC Davis Internal Medicine Department (attention Meggan Brownrigg) 530-754-7993. 

During the initial exam the medical records will be reviewed, and a complete history and physical exam performed. At that time we will determine the next step in diagnosis and treatment of your cat’s hyperthyroidism, blood will be collected for evaluation, or other tests performed. A technetium scan will be scheduled within 30 days of expected I-131 treatment. Following the technetium scan, an appointment will be secured for I-131. Appointments for I-131 are usually scheduled on a Monday with treatment to follow on Tuesday.

Length of time in hospital is typically 2-3 days with technetium scan, and 5-8 days following the I-131 treatment. Your cat will then remain in our treatment facility for a minimum of 5 days (Monday through Friday) during which the radiation technician will check in with your cat, feed, give fresh water twice a day.  If your cat is on any daily medication, please discuss this with the clinician at the initial examination since they may not be able to be administered in hospital.

The radiation technician, not the clinician, will authorize release of the cat once radiation levels are determined to be at a level safe to take home.  

Discharge instructions will be given to you when your pet is discharged from the hospital. These instructions will include guidelines on special handling of urine and feces and exposure to people. Below is brief summary of discharge recommendations:

Important post discharge recommendations:

We recommend the use of flushable litter for 2 weeks post treatment. If you have a septic tank please notify your doctor so alternative arrangements can be made.

Please limit HANDS ON contact time to a maximum of 10 minutes per person per day for 2 weeks after trearment. Wash your hands thoroughly after holding the cat, the cats’ litter pan, or food dish. Exposure to other pets is fine.

Young children or pregnant women should have no contact with cats for 2 weeks after treatment to decrease their risk of radiation exposure. Please discuss radiation risk for this group with your clinician as additional precautions are recommended.

Cat must be kept indoor exclusively for 2 weeks after discharge in order to minimize the possibility of radiation exposure to other people.

For further information or to arrange referral, please contact the Internal Medicine Coordinator at 530-752-1393.