Understanding Your Cat's Aggressive Thyroid Tumor(s)

Cat Tumor

All cats with hyperthyroidism have a tumor on one or both thyroid glands.

Of all thyroid tumors, about 97% are benign adenomas. However, very large thyroid tumors with extremely high (greater than 20 µg/dl) thyroid hormone levels have often progressed toward a more aggressive state, which usually takes more than 3 years. Up to 20% of large, old tumors have evolved into adenocarcinomas. Control of thyroid hormones with methimazole becomes much more difficult.

Over the years, high levels of 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 for thyroid treatment with I-131. I encourage you to engage a veterinary cardiologist through your regular veterinarian, who will perform an echocardiogram, before the appointment with us. A cardiologist may help stabilize the heart during this difficult time. Minimally, the blood pressure should be checked, and controlled if high, usually with amlodipine. If your cat’s heart rate is much faster than one would expect for a frightened cat in a veterinary office, your veterinarian may consider a B-blocker (atenolol or propranolol) to slow it down to a safer rate. I also suggest a medication for anxiety, such as alprazolam or gabapentin, for travel and hospitalization. Your veterinarian may dispense these.

 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, more likely 5-14 days. Prepare to be flexible; we cannot legally release your cat from our facility until the radiation discharge rate drops to the required level.

Potential outcomes are less predictable in cats with longer-term hyperthyroidism. In this ‘aggressive’ category, about 70 % are cured after 1 dose of I-131 with no other thyroid therapy needed. With that said, about 30% 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 sometimes remain too high after one treatment of I-131 is that some hyperthyroid cats have thyroid cells that have a poor ability to absorb (uptake) the radiation. These ‘stubborn’ tumors may require a second treatment if the thyroid hormones don’t steadily decrease to normal over weeks (to months) after I-131. Because the dose needed for large tumors is much higher than usual, the hospital stay will be much longer; we will not 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 $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 have low thyroid hormones (hypothyroidism) after the tumor is destroyed is that the normal thyroid cells have died off after years of chronic suppression. In other words, if the normal cells are laid off long enough, because the tumor has taken over production, they won’t go back to work. This is called ‘disuse atrophy’. The longer your cat has had thyroid disease, the greater the chance of hypothyroidism after I-131. Another possibility is that the amount of radiation necessary to destroy the abnormal tissue destroys some normal tissue as well. If the thyroid hormone remains low after sufficient time for residual tissue to reactivate, l-thyroxine should be supplemented indefinitely.

If your cat has had no adverse reactions to methimazole, we may advise restarting it a few days after I-131, and continuing it until 1 week prior to the first recheck, which is 1 month after I-131. Controlling the hormone production while the tumor is atrophying (withering) can help decrease weight loss and reduce the risk of heart failure.

Naturally, if your cat has had side effects to the methimazole, do not resume it after I-131.

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 with your primary veterinarian, as well as conscientious management of any other illnesses, such as kidney or intestinal diseases, will be crucial to your cat’s recovery in the weeks to months following discharge from our hospital.

Please feel free to contact us 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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