Understanding Your Cat's Semi-Aggressive Thyroid Tumor(s)
All cats with hyperthyroidism have a growth (tumor) on one or both thyroid glands.
Of all thyroid tumors, about 97% are benign adenomas. However, relatively large thyroid tumors producing A LOT of thyroid hormones have often progressed toward a more aggressive state. It usually takes 2-3 years to reach this level of hyperthyroidism.
‘Semi-aggressive’ thyroid tumors may be associated with T4s in the 12 to 20-ish µg/dl range. The majority may still be adenomas, but are starting to behave more aggressively. Control of thyroid hormones with methimazole becomes more difficult and expensive to monitor, requiring repeated veterinary visits and sets of lab work. It’s best to simply eliminate the tumors with I-131 and be done with it.
Over the years, elevated thyroid hormones cause heart disease. Unfortunately, controlling the hormone production with methimazole does not fully protect the heart.
Decompensation (congestive heart failure) is more likely when they go off hormonal control via methimazole, and when they travel and are hospitalized. It’s never a bad idea to engage a cardiologist, who may perform an echocardiogram, before the appointment with us. Minimally, the blood pressure should be checked, and hypertension controlled. If the heart rate (HR) is greater than 250 bpm (tachycardia), the primary veterinarian may consider a B-blocker (atenolol or propranolol) for HR control. I also suggest initiating anti-anxiety medications, such as alprazolam 0.125 mg (½ of a 0.25 mg tab) or gabapentin (50-100 mg), before travel, during hospitalization, and for the trip home in the car and/or flight.
Because we must give these cats a more assertive dose of radioiodine, the charge is anywhere from $200-300 more than our base fee ($1465), in addition to the $115 exam.
Cats with higher doses of radiation are required by WA State law to remain in the hospital longer than the average 2-3 days. These cats usually stay 3-6 days.
Potential outcomes are less predictable in cats with longer-term hyperthyroidism. In this ‘semi-aggressive’ category, about 80% are cured after 1 dose of I-131 with no other thyroid therapy needed. With that said, about 20% of these could either require a second dose of I-131 or, on the other hand, may need to receive thyroxine supplements for life after I-131 destroys the abnormal tissue.
The reason why thyroid hormones remain above normal in some cats after one treatment of I-131 is that their thyroid cells may have poor radioiodine-uptake (RIU) ability. These ‘stubborn’ tumors may require a second treatment. If we do not need a very high second dose of I-131, we will cover the cat’s re-treatment. If the second dose is much higher and we think the hospital stay will be much longer; we will not be able to re-treat for no charge. In order to recapture at least some of our cost for the I-131 from the nuclear pharmacy, we charge approximately $1000, depending upon the exact dose, for a second high-end-dose treatment. These cats would also need to be reevaluated (same exam charge, $115).
One reason why some cats with large tumors become hypothyroid (low thyroid hormone production), requiring a natural thyroid hormone supplement after the hyperactive tissue is destroyed is ‘disuse atrophy.’ That is, normal thyroid tissue sometimes dies after long-term suppression. Another possibility is that the amount of radiation necessary to destroy the abnormal tissue destroys some normal tissue as well, or the normal residual tissue is incapable of reactivating even if present. If the thyroid hormone remains low after sufficient time for residual tissue to reactivate, l-thyroxine should be supplemented indefinitely.
Our doctor will evaluate each set of labs and physical exam findings from your rechecks in 1 and 3 months post-I-131, and fine-tune the direction. This is a ‘work in progress'.
These cats should be fed canned or raw diets that are, ideally, plant-free. Adequate amounts of meat-based protein are essential for recovery of both overall muscle mass, and cardiac muscle.
Despite the more complex nature of treatment for more significant thyroid disease, a cure with I-131 gives them the best medical chance, by far. More often than not, thyrotoxic hearts repair with time, and these cats generally rebuild muscle mass and recover if they don’t have too much else going on.
Ongoing communication between the primary veterinarian and their client, as well as conscientious management of any other illnesses will be crucial to their success.
Please feel free to contact me if you have any questions or concerns!
Faythe Vaughan, DVM
Christine Wilford, DVM
Pre-appointment Check List:
- I have read and understand this information (required)
The following are suggested, but not required:
- Systolic Blood Pressure (SBP), correct hypertension
- Anti-anxiety medication
My family has had Lila since she was a kitten. She is turning 15 in October, I was only 10 when we got her. I moved away as an adult for a few years... read moreLila
Great place. They were so good to my kitty who was really scared. He seems to be completely cured. read more
I wanted to drop you and everyone at FHTC a note to let you know that Byron (the PAWS™ kitty FHTC treated pro bono earlier this year) was adopted last month! Even better, he... read moreByron
You took care of our Nina Kitty and it was like she found the Fountain of Youth! At age twelve, she had been slowly losing weight and getting a bit more "yowly" but we... read moreNina
Monty is 12 years old. Dr. Vaughan had been monitoring Monty’s thyroid over the years and identified that he was borderline for hyperthyroidism. After discussing his overall appearance, behavior and appetite, we decided that... read moreMonty
We want to thank you for your extraordinary efforts in looking after our Samson's well being. You took the initiative of a problem we were not aware was even present. We are very, very... read moreSamson
Port Townsend, WA
Princess started losing weight and after checking with the vet she was confirmed as being Hyperthyroid. After Approximately 13 months of of a paste-like med applied to the ears, switching from left to right... read more
Ripley (kitty) and I just wanted to thank Dr Vaughan and the whole staff for your great service and care. You were all great to work with, and communicated well with us! Ripley left... read moreKali & Ripley
Will & Grace found me in 1999 when they were four weeks old. They were born near in a parking lot off Highway 99. They are twins and do everything together. This year they celebrated their... read moreWill, Grace, and Hunter
As a member of an animal rescue organization on Vashon Island (VIPP), as well as being a chronic cat owner, I've had many opportunities to visit the Feline Hyperthyroid Treatment Center. Over the past... read morePiro Kramar. Vashon Island Pet Protectors
Vashon Island, WA
Hello! Just wanted to let you know Rio Blanco is doing great! As soon as we got home he started emptying the bowls of food! I haven't even opened the anti-nausea or the... read morePatrick and Rio
Pox's Story Does it make sense to treat hyperthyroid in a fourteen-year-old, mostly outdoor cat? That’s what we were asking ourselves in the summer of 2013. Pox let me know his opinion by following me... read morePox
My cat was diagnosed as hyperthyroid when she was 10. We had her treated as soon as she was diagnosed and I was glad it was found early. I didn’t really give the thyroid... read moreSavannah
They are very personable, professional, and knowledgeable about their very specialized practice. If you have a cat with thyroid issues, I highly recommend this clinic. read moreBecky
Thanks to Dr Faythe Vaughan, and her kind staff in Shoreline and Tacoma for giving us at least 5 extra, good quality, years with our buddy, Rocky! He was a rescue cat to begin with, and his good... read moreRocky
Dr. Vaughan; Kathy; Miranda; and anyone who was involved in treating and caring for Tigger. Just wanted you to see the life Tigger is leading because of your excellent care on this upcoming anniversary... read morePepper
Three months ago my 13 year old kitty Miss Fanny spent three days in the care of Dr. Vaughan and the staff. Before treatment she had been losing weight, getting more lethargic and her... read moreMiss Fanny