Hyperthyroidism in Cats

Hyperthyroidism in Cats


Feline hyperthyroidism is a common endocrine disorder in older cats, and is caused by the overproduction of thyroid hormone. This overproduction of thyroid hormone leads to a multitude of clinical signs, most commonly weight loss, increased appetite, increased activity and/or vocalization, vomiting, diarrhea, increased water consumption, and increased urination. Some cats show all of these signs, but many show just one or two of these signs. Other signs such as decreased activity or reduced food consumption, and problems related to cardiovascular disease occur less commonly. Left untreated, hyperthyroidism can result in heart failure and other complications, and can even lead to death of the cat.

Cats 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', 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. Approximately 2-3% of hyperthyroid cats have a carcinoma (a malignant tumor) that can be variable in presentation involving one lobe or creating multiple nodules.

Following the diagnosis of hyperthyroidism your veterinarian may discuss treatment options such as methimazole (Tapazole®), dietary restriction of iodine (using a prescription diet), surgery, or I-131 treatment.  These treatment options have different pros and cons and should be discussed with a veterinarian. Not all treatment options are suitable for all feline patients.


I-131 is radioactive iodine that is administered as a single injection under the skin. Iodine is the main ‘building block” for creating thyroid hormone, and it concentrates in the thyroid gland. Other tissues do not require iodine and its concentrations are low elsewhere in the body. I-131 concentrates in the thyroid gland where it destroys overactive thyroid tissue. Neighboring thyroid tissue that has been suppressed by the overactive cells are mostly spared, as are other tissues in the body. Side effects of I-131 treatment are uncommon but similar to other treatments for hyperthyroidism. Adverse effects of treating hyperthyroidism include unmasking underlying kidney disease and development of hypothyroidism (decreased production of thyroid hormone).


This will be discussed with you at the initial examination. A “methimazole trial” and a technetium scan might be recommended prior to making a decision regarding I-131. Reasons to pursue I-131 treatment include difficulty in administering oral/topical medication, side effects to oral/topical medication, and having a permanent, one-time, 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. Images of the thyroid gland are then acquired using a gamma camera to provide information on nodule size, the presence of single or multiple nodules, and the location of the abnormal thyroid tissue.

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 occurs in only one thyroid gland, then alternative therapy may be discussed

Cats need to be hospitalized for 24-hours following this scan 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 increases blood flow to the kidney so that it masks (but does not improve) the underlying kidney problem. Therefore, before embarking on a permanent treatment (surgery or radioactive iodine) that might cure the hyperthyroidism but unmask kidney disease, we typically want to evaluate what happens to kidney function when the hyperthyroidism is treated with a reversible treatment such as methimazole. If deterioration in kidney function does not occur, permanent treatment is a typically viable option. If deterioration in kidney function does occur, depending on severity, it might be preferable to avoid permanent treatment such as I-131 or thyroid surgery.

Tapazole® (methimazole) may or may not be discontinued prior to treatment with I-131, and this will be determined by the clinician overseeing the case prior to your appointment. Please follow your veterinarian’s recommendations.


Ideally, laboratory tests including a minimum of kidney values (BUN and creatinine) in conjunction with thyroid testing that shows a euthyroid /close to euthyroid status induced by treatment should be available to evaluate the effect of an iodine deficient diet or methimazole treatment on kidney status prior to I-131. Some animals do not tolerate methimazole treatment and will not eat the iodine deficient diet, and in these animals, evaluation of kidney function with a reversible treatment will not be possible. Bloodwork demonstrating a total T4 (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, with or without additional 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 alternatively, the bloodwork can be performed at UC Davis (see variation in cost estimate). Bloodwork may be repeated at UC Davis at the discretion of the admitting/supervising clinician.


The 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 $2,100 (price subject to change). In addition to the costs of the treatment, there might be other diagnostics necessary (additional blood work, cardiac evaluation, ultrasound and possibly other tests) that can increase this estimate further. Your UC Davis veterinarian will provide more details of necessary diagnostics and costs once your cat is examined at UC Davis.

Please have your veterinarian fax records and pertinent lab work prior to your appointment. They can be faxed to UC Davis Internal Medicine Service (attention Kaitlin Zuwala). Her Fax # is 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 steps in diagnosis and treatment of your cat’s hyperthyroidism, blood will be collected for evaluation, or other tests performed. A technetium scan might be scheduled within 30 days of the anticipated date of I-131 treatment. After evaluating all of the results of these tests, an appointment for I-131 treatment will be scheduled. Appointments for I-131 treatment are usually scheduled on a Monday morning. If the wait time for the treatment appointment is prolonged, the clinician might ask to schedule an additional appointment in the week prior to I-131 treatment to assure the safety of the procedure.


Administration of radioactive material to a pet requires a period of isolation and minimal contact to avoid radiation hazard to family members and care givers. The length of hospitalization is typically 2-3 days with a 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 evaluate your cat, feed it, and provide fresh water twice daily.  If your cat is on any daily medications, please discuss this with the clinician at the initial examination because they may not be able to be administered while your cat is in the treatment facility.

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 your cat home.


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

Important post discharge recommendations:

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

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

Young children or pregnant women should have no contact with I-131 treated cats for 2-weeks after treatment to decrease their risk of radiation exposure. Please discuss radiation risk for this group of individuals with your clinician as additional precautions are recommended.

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


For further information or to arrange referral, please contact Kaitlin Zuwala, the Small Animal Medicine Service Coordinator at 530-752-1393.

Last updated 4/25/2018 (CG; SLM)