Radioactive Iodine

(Feline Hyperthyroidism)

Feline Hyperthyroidism usually results in marked weight loss in affected cats, and the associated loss of muscle mass may reduce serum creatinine (SCr) concentration and complicate the diagnosis of underlying renal disease. Thyrotoxicosis increases glomerular filtration rate (GFR) and renal plasma flow (RPF), which also may contribute to reduced SCr concentrations and interfere with recognition of underlying renal disease. Treatment of hyperthyroidism may reduce GFR and RPF and allow emergence of azotemia.


Symptoms of Feline Hyperthyroidism may include:

  • Weight loss despite excellent appetite.

  • Changes in behavior - anxiety or nervousness

  • Excessive appetite or decreased appetite

  • Increased water intake

  • Hyperactivity or lethargy

  • Excessive shedding, hair loss (alopecia), poor coat condition

  • Diarrhea or vomiting

  • Increased urination

  • Cardiac symptoms - rapid heart rate, arrhythmia

Planar thyroid scintigraphy of a normal cat.

Hyperthyroid cat with a unilateral thyroid adenoma.

Feline Hyperthyroidism is a disorder that occurs when the thyroid produces and excess of thyroid hormone. Hyperthyroidism occurs in middle aged and older cats. Both sexes and all breeds are equally at risk.

In the normal cat, the lobes of the thyroid gland are butterfly-shaped and located in the neck region cannot be felt with one's fingers. In the hyperthyroid cat at least one lobe is usually prominent and may be detected by your veterinarian during a physical exam. Seventy percent of cats have both lobes affected.

What is thyroid hormone?

Active thyroid hormone (nicknamed "T3," short for "triiodothyronine") sets the body's metabolic rate, sort of like a volume dial. One might say T3 determines how hard or how fast each cell works to do its job. Every cell of the body is affected by T3.

The thyroid glands (there's one gland on each side of the windpipe) do not produce T3. Instead, they produce an inactive form called "T4." Tissues of the body absorb T4 andconvert it to T3.

Doctors will readily make references to T3 and T4. It is a good idea to know what they are referring to. While T3 is the active hormone, it turns out that more meaningful information is gained by measuring T4. Your veterinarian will probably mention monitoring your cat's T4 level.

What causes hyperthyroidism?

Hyperthyroidism is caused by a benign growth in the thyroid gland which are over-producing T4. It is important to realize that these tumors are almost always benign and represent a form of goiter rather than a form of cancer. Only 3-5% of hyperthyroid cats have a cancerous thyroid growth.

Why is it so important to treat hyperthyroidism in cats?

Hyperthyroid cats frequently experience reduced quality of life through weight loss, muscle deterioration, chronic vomiting or chronic diarrhea. Not all cats experience these signs at the time of diagnosis but there are less visible reasons to treat: heart disease and high blood pressure. These problems can result in heart failure, sudden blindness, or sudden death and, if secondary to hyperthyroidism, can be prevented with timely treatment.

The guarantee of services is valid only if the Total T4 value is checked post treatment at 2 specific time points:

  • First check at ~14 weeks post treatment.

  • Second check at ~10-12 months post treatment.

Please forward us the results so that we can monitor values.

Frequently Asked Questions

How long does my cat need to stay at your facility?

Due to our licensing regulations we cannot release your cat for a minimum of 4 days post treatment. However, this is dependent on dose and expected clearance. All animals must reach a specific radiation background (or below) before they are allowed to leave our facility (1mR/hr or 10uSv/hr). We will give you an idea of what we expect for your cat.

What should I do with the Radioactive Litter Waste?

You must use flushable, clumping litter for two weeks post treatment. Most of these litter brands recommend letting clumps sit 20 minutes prior to flushing so please read the Manufacturer Instructions. You must flush the toilet twice (human requirement as well). This is allowed by the government.

We have two other cats and a dog at home. Will we have to keep them completely separate after our cat returns home?

The restrictions are not for animal to animal contact so your cat can socialize with your other animals at all times.

We have a multi-cat household. How do you manage the litter situation after the treatment?

You will need to flush all litter clumps as you will not know which cat produced them. If you have multiple cats and wish to reduce the amount of litter to flush, you will need to separate them into different rooms for the 14 days.

What are the risks of the I-131 treatment?

There are no risks associated with I-131 itself. I-131 only targets thyroid tissue therefore it does not affect the other organs i.e. liver/kidneys etc. Hyperthyroidism masks true renal function so the concern that we have for some cats is once you remove the hyperthyroid state the kidney function could deteriorate. However, there are tests that can be done help determine their kidney function.

Are we able to visit them during the day at all?

Due to our location and the nature of the treatment our site is considered a "closed" facility. However, we do send emails daily to provide you with updates on how your cat is doing. This helps keep the communication open if you have any questions while they are with us.

What can I bring for my cat?

Food, toys, food dishes are all welcome. We also recommend some old towels or T-shirts with home scents - would even prefer if you slept in it. We think it really helps to have home/personal scents around them.

Is there a consultation prior to admission or is it a case of assessing whether our cat is a candidate for treatment based on the tests results that our veterinarian sends you?

No consultation required for any of our cases. We work with your home vet via email/phone and or fax. We obtain necessary information from them and will recommend other tests if needed that can all be done through your home vet.

Do you provide treatment on the day that our cat is dropped off? How early can we pick up our cat?

Yes, treatments are done on the appointment day. Due to our licensing regulations they must stay for a minimum of 4 days after treatment. However, this is dependent on dose and expected clearance. Most cats are discharged between 4-7 days after treatment.

In most cases, why do you wait 10-12 months post treatment to recheck our cats T4?

The T4 takes several months to normalize. Due to some complexities of certain cases we expect different results at the ~14 week mark. However, our main goal at the first check is to determine at ~14 weeks that the T4 is coming down. The second check at 10-12 months post treatment is necessary because the thyroid gland will have been working for about 3 months at its "new normal". This value is needed to determine that treatment has been successful. Each case is treated as an individual and at times we may alter these dates.

What happens if our cat becomes hyperthyroid again? Is there any form of guarantee?

Yes, we do guarantee our services. One of the reasons that we guarantee our services is that we treat conservatively so that we can maintain some healthy tissue. Our belief is that the cats (and humans) were given a thyroid gland for a reason so we like to retain some of that tissue.

If a second treatment is required, how early would this occur?

For those cats that have been given our “step-dose” treatment we typically provide the 2nd dose 4-5 months after the initial dose. For all cases: A second iodine dose cannot occur until after 12 weeks from the first dose.

My cat will be on medication during their time with you. Can they still be given their medication?

All staff are veterinary technicians with many years experience. You must bring all medications and instructions for you cat. We will follow the administration schedule provided.

Where are you located?

Please have a look at this section on our website. Our building is surrounded by St Joseph's Hospital. You will drive down a laneway between two building and stop just before the lane turns right. Please call us on our clinic cell phone when you arrive and we will direct you where to go from there.

Contact Info

Thames Valley Vet Services

268 Grosvenor St.

London, ON N6A 4V2

Phone: (519) 646-6100 x 64679#

🖷 Fax: (519) 646-6205