19203 Aurora Ave N
Shoreline, WA 98133
(206) 546-1243


5506 Pacific Avenue
Tacoma, WA 98408
(253) 471-9200

Restoring Health & Happiness to Senior Cats Since 1992

Restoring Health & Happiness to Senior Cats Since 1992

Understanding Your Cat's Semi-Aggressive Thyroid Tumor(s)


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 associated with higher thyroid hormone levels have often progressed toward a more aggressive version, a process which usually takes 2-3 years. 


Over the years, elevated thyroid hormones cause heart disease. Unfortunately, controlling the hormone production with methimazole doesn’t 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 and having an echocardiogram before the appointment with us. Minimally, the blood pressure should be checked, and hypertension controlled. If the HR is greater than 250 bpm, the primary veterinarian may consider a B-blocker (atenolol or propranolol) for HR control. I also suggest initiating alprazolam 0.125 mg (½ of a 0.25 mg tab) daily from before travel, during hospitalization, and for the trip home in the car and/or flight.


‘Semi-aggressive’ thyroid tumors may be associated with T4’s in the 12 to 20-ish µg/dl range.  The majority may still be adenomas, but are starting to behave more aggressively and normal hormone levels in those on methimazole may be more difficult to control. Because we must give these cats a more assertive dose of radioiodine, the charge is anywhere from $200-500 more than our base fee ($1100), in addition to the $95 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 the thyroid hormones remain above normal after one treatment of I-131 is that some have thyroid cells cells that have poor radioiodine-uptake ( RIU ) ability. These ‘stubborn’ tumors may require a second treatment. If we don’t need a very high second dose of I-131, we’ll cover the cat’s retreatment. If the second dose is much higher and we think the hospital stay will be much longer, we won’t be able to retreat for no charge. In order to recapture at least some of our cost for the I-131 from the nuclear pharmacy, we charge approximately $550-750, depending upon the exact dose, for a second high-end-dose treatment. These cats would also need to be reevaluated (same exam charge, $95.)


One reason why some cats with large tumors become hypothyroid after the hyperactive tissue is destroyed is ‘diuse atrophy.’ That is, normal thyroid tissue sometimes dies after longterm 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.


I will evaluate each set of labs and PE findings from your rechecks 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’ve any questions or concerns!


Faythe Vaughan



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 was adopted with another senior, all-black kitty we had in care.... read more »

Seattle, WA

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 medication much consideration because she was difficult to pill and knowing... read more »

Lynnwood, WA

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 fortune continues! We highly recommend The Feline Hyperactive Thyroid Center for their... read more »

Tacoma, WA

View All Testimonials »