I was talking about one of my cats earlier and it made me think about this, so wondering if there are any others out there who have seen it. My late cat Mikey had lots of issues, including diabetes, but was also hyperthyroid (common in older cats). We opted to try to treat with medication (methimazole). It worked well, and his thyroid was in the normal range once we got the dose right. But, maybe 6 months or so later, he developed a tumor on his hind leg. It might have been an an injection site tumor from his rabies shot, which can happen, or maybe not; vet could not be certain. Anyway, at 13, we agreed that keeping him comfortable for as long as possible was the best answer. Surgery wasn’t financially an option for me at the time, and the vet didn’t feel like it was a good option anyway. When the time came, I let him go. I still miss him.
But, he was an old cat who had issues. It happens. But then I had another senior cat (15) diagnosed with hyperthyroid. Again treated with methimazole as radioactive iodine was out for financial reasons. Again, she did well on the medication with none of the usual side effects, levels came within normal range, and she gained weight and looked overall much better. Until she developed a tumor on her head. Inoperable, but she did OK for 6 months or so even though it looked fugly. When her quality of life declined at 16 1/2, I let her go. She and Mikey were a pair from he start; their ashes are next to each other on the mantle.
But was this a coincidence? I’ve done some research on the drug, and while there are some studies that show it caused cancer in mice, those can be deceiving as they give them so much that it doesn’t reflect reality, and just about anything causes cancer in mice in a lab, it seems. Most of what I’ve read doesn’t mention it as a side effect. So, I’m wondering if maybe the change from hypothyroid to normal is the trigger…Suddenly their metabolism is slower, blood pressure lower, etc.
Anyway, not sure of the answer, but wondering if anyone else had a similar experience? Maybe my two really were just bad luck. Just seems odd to me.
I’d be suspicious, too. It must be awful to lose two kitties in near-identical fashion. My experience was only slightly similar… my kitty had been tested throughout her senior years for thyroid problems and repeatedly she was in normal ranges. She did have kidney disease. When she was over 19 and a half, I took her to the vet because I thought she was finally in kidney failure, after having battled it for more than 6 years. Nope: she was hyperthyroid, and also had an extremely high white blood cell count. (Kidney values were good.) The likely culprit was cancer, most likely stomach or intestinal.
We decided to let her go rather than surgery (she was an extremely poor surgery candidate), so I don’t know for 100% certain that she had cancer.
My point is that perhaps rather than the thyroid meds causing cancer, it’s possible that hyperthyroid cats are simply prone to cancer.
I’m very sorry for your loss. The questioning never ends, does it?
I was thinking the thyroid condition itself might be part of the equation, though for both of mine, it was only after it was regulated by medication and their levels were normal again that the cancer occurred. As both were older cats at 13 and 16, it could just be that the cancer possibility is greater. Plus, my 16-year-old was white, and I know that can predispose them to cancer as well. It just gives me pause that both tumors happened after their metabolism was very high for a period before hyperthyroid diagnosis and then both developed tumors when it was reined back in.
It’s certainly a curious question. It’s possible that the higher metabolism = greater cell turn over = more cellular mutations (normal, but just happening more quickly) = cancer more quickly than otherwise.
I’m sorry for your losses 
My hyperthyroid cat is 15. No cancer at this time but he only recently started medication. He does have a small lesion on his ear we are watching. It could be skin cancer, but the vet thought it “looked” benign . He is pure white with yellow eyes.
My other cat is 11years old and she has breast cancer (normal thyroid). My other cat is 13 and has a lump on her tongue - either a granuloma or cancer. She gave us a scare last year- the vet said either intestinal cancer or ibd, but she made a complete recovery in about a week. I still think it was food poisoning. The vets i saw had no idea what was wrong, sent me in for a $600 ultrasound, and they still couldn’t tell me anything as far as diagnosis. I suspect she has dental disease related to the bacteria from the lump on her tongue, but she seems to be thriving despite that. The lump is non operative at the base of the tongue. She eats really well and stays rotund.
My last dog died at 11 from nasal cancer, and my dog prior to that died from kidney failure.
Since cancer is so common in the elderly, i don’t think it sounds like the medication. Association does not equal causation… Cancer and hyperthyroidism risk increases with age. Any dog or cat over ten years is considered old, in my opinion.
Honestly I suspect you’ve been unlucky. I haven’t personally seen any correlation between cats starting thyroid tx and development of nasty tumours. You’re also correct that mice studies do not always cross over properly into other species for a number of reasons. (There’s good reasons why clinical trials in the species the drug is designed for need to be done before it can be sold, not mice studies only.) It’s more likely to be an older age thing as like in humans, older cats are more prone to many types of cancer unfortunately. Also don’t feel bad about not going with radioactive iodine in a 15 year old cat. Most people won’t at that age for financial and/or medical reasons and it’s often ok to manage with medications in older cats like this as long as the cat can tollerate the meds ok. Take care.
