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

Cat Tumor

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
  • Cardiologist/Echocardiogram
  • Anti-anxiety medication

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