I-131 Treatment for Cat Hyperthyroidism

After her near-death experience last summer we got a diagnosis.  Not a rare disease, just one that slipped under the radar.  Hyperthyroidism.  And not a mild case, either.  I believe the first time she was tested, her thyroid hormone was about five times the normal level.

For about two months we managed it with pills.  First we hid it in her food.  Then when she stopped eating her food entirely we tried to pill her.  When that became such a hassle that we didn’t know how to instruct our cat-sitting neighbors, we got the medicine in a topical form, put it into her ear once a day.  This turned out to be the only sustainable way to medicate this cat.  We did the math, figured it would cost us about $600 a year.

The vet recommended radioactive iodine.  There were a few places that administered it in the Boston area.  She was healthy otherwise, a good candidate for the treatment.  She is 13, and they said, 95% of cats are cured with one treatment, and they go on to live 5 more years on average.  Secretly, I felt like she would go on to outlive those other cats too.  So after some agony, it was decided that we’d drop the $2000 on this cat for the I-131 treatment.

We chose Angell Animal Center in JP since they seemed to have the most consistent results.  I was appalled by how hard it was to find testimonials online.  So, I suppose the only reason we thought that this center gave us the best chance of success was their assurance that since they took images of every cat’s thyroid tumor before administering the medicine, they’re able to better calibrate the dosage.  And maybe one Yelp review.

We dropped her off at the center during the height of the first wave of the pandemic.  Everything was locked down.  There were few cars in the parking lot.  An aide came outside in a mask when we called and picked up our cat at the door in her carrier and we watched her watch us recede slowly before the doors finally slid shut.

They sent us updates every day.  In total she was gone about 5 days.  We had separation anxiety almost as bad as the cat.  It was always a relief to see her little sad face.  

But the treatment didn’t work the first time.  Not for us.  There was no glaring indicator early on that it didn’t work.  Her energy level was about the same.  Her appetite little reduced.  There was no profound personality change or weight gain.  When I drove her to get her blood checked it was a hot day and she panted in the passenger seat.  Her thyroid levels came back only reduced by about 20%.  The vet recommended a second treatment.  She would have expected at least a 50% reduction at this point if the treatment had been successful.  Our own vet said she’d never seen a cat not respond to the I-131 treatment before.

We were assured another approximately 95% of patients is cured in the second round (only 8 out of 4000 cats still hyperthyroid after a second treatment).  We would pay only for the syringe full of I-131 this time around.  After some discussion, weighing her odds and her suffering, we decided to give it one last try.  So in July, we packed her up once more in her carrier, and shuttled her off to be locked up for another 4 days.  

This time a friend lent us a Geiger counter.  So we started tracking her radiation levels every day.  You know how they tell you to keep the cat inside for 2 weeks but to keep what she poops and pees during this time locked up for multiple months?  We suspected that this is because the half-life of her radioactivity is actually much shorter than the half-life of I-131 itself (8 days).  In other words, she eliminates the I-131 from her system faster than it decays on its own.  

The data was a bit hard to collect because she would not hold still.  Turning off the clicking sound helped us get measurements close to her face.

We learned a few things from this data: The cat is quite radioactive the first few days after returning from the vet. The half-life of I-131 elimination from her system appears to be around 3-4 days. The 2 week quarantine seems to be timed to coincide with your dosage rate dropping below that of a typical daytime flight (~3 uSv/hr), assuming the cat is in your lap making maximum contact with your body.

We had quite a scare after this round of therapy. About 10 days after her treatment she because extremely lethargic and began to refuse food and water. We had been warned that there may be a bit of a shock as the body returns to normal thyroid function. But being an older cat, both vets were quite worried about this development.

She slept most of the day and would come out at meal time, sniff her food, lick her lips a few times and return to her nap pod. We tried all kinds of expensive cat food. We bought single-serving catfood in a jar. We tried catnip. We gave her anti-nausea meds as prescribed by the vet, we tried an appetite stimulant, and finally, after almost 2 days of not eating or drinking, it was Costco rotisserie chicken which we went out specifically to acquire (thanks Costco) that finally piqued her interest.

I’ll say that she eats a LOT less now, about 6 more weeks after her second round of treatment, than I’ve ever seen her eat. But she has kept her weight up (8.3 lbs now!) and she sleeps normally and is generally behaving like a significantly less hyper version of her former self.

She’ll go in for her blood test in a week or two. At that point, I’ll update this post with what’s hopefully the last chapter of this saga. Wish us luck!

@jplicksthecat on Instagram 🙂

UPDATE: The kitty’s thyroid levels tested at around 1.0 ug/dL, which is on the low end of the 0.8-4.7 reference range. She is also healthy in every other respect, liver function, kidney function, weight, etc. She is lower energy than she used to be before the treatment: cuddlier, sleeps a lot more, but fundamentally her little personality is still what it is. Looking forward to healthy years to come!