Felimazole side effects

[Binxie]

I have a 17 yr old cat that was put on Felimazole for her hyperthyroidism, 2.5 mg once a day, after 4 days she became very lethargic and would not eat much, so it was stopped for several days and her activity and appetite returned to normal. She does not exhibit the ravenous appetite that some hyperthyroid cats do. The vet advised to try 2.5 every other day, she tolerated that but it did not bring her T3T4 levels to a normal level, so the vet said give daily again, here we are 4 days into daily and once again she is more quiet and appetite down once again, obviously she cannot tolerate this dosage. I have treated many cats in the past with Felimazole and have not run into this reaction, has anyone else seen this side effect? Has anyone used the transdermal med for hyperthyroidism? Vet has suggested the possibility of radioactive iodine treatment as well, on the surface that seems like the perfect treatment but I am wondering what if anything is the drawback to it other than all the precautions that have to be taken after the treatment is completed.

 

[FeebysOwner]

I would guess the dose is too much for her. Back in October 2020, Feeby's T4 was 4.5 - she was losing weight but was not showing any signs of ravenous eating. After a series of discussions with the vet, we started her on a 1.25 mg dose twice a day. It brought her T4 down to 2.6. She has had some increases in her T4 since then, and we have only incrementally increased the Felimazole to where she now takes 1.85 mg twice a day. There was one occasion when the vet wanted to increase it to 2.5 mg AM/1.875 mg PM and it dropped her T4 too low, so we went back to 1.85 mg twice a day. I can't recall her being any more lethargic or losing her appetite, but both are very common if the dose is too high.

I don't know what her T4 level was/is, but I would suggest trying 1.25 mg twice a day. Once a day causes too much of a fluctuation in the T4 level because a single dose does not last as long as 24 hours - and using it every other day could have contributed to why her T3T4 didn't show improvement. Giving a dose every other day is just causing her to ride a roller coaster with the amount of Felimazole in her system. Even with the transdermal, which does tend to be less effective, I believe it is still appropriate for twice a day administration.

There are many members who have gone the I-131 route and are very happy they did. Most of the after-treatment precautions appear to be 'overkill', and many folks only limitedly followed them without repercussions. It would seem the biggest hurdle is dealing with how long your cat would need to remain at the treatment center afterward. Some release the cats in as little as 2-3 days, others more like 3-5 days.

Until you hear directly from others who have gone through the I-131 treatment, maybe this site would be helpful.
Animal Endocrine Clinic | Where science and compassion cure

 

[Binxie]

Thanks for the info and link. Do you have the medication specially compounded for the 1.25 and 1.85 dose, I tried cutting the pill in half and that did not work well because of the hard outer coating on the little tablet.

 

[FeebysOwner]

Thanks for the info and link. Do you have the medication specially compounded for the 1.25 and 1.85 dose, I tried cutting the pill in half and that did not work well because of the hard outer coating on the little tablet.

Nope. I have found a couple of pill cutters that I can cut the 2.5 mg into halves, and then again into quarters as well. Is each piece exactly the same size every time? Nope, but I don't cut them ahead of time, so the ones that are a bit off are used in back to back doses. I am using a pill cutter from Publix and have another from CVS. If I keep them clean (I use a Q-tip to wipe off the blade and to clear any debris in the pill holder section), they seem to cut more accurately.

You could consider using Methimazole instead. I believe it is uncoated and a flatter (not rounded) pill, but I think the smallest dose of it is 5 mg, which would mean splitting it into quarters to get 1.25 mg doses. I won't use it because for me, it would mean cutting some pills into 8ths. However, there are plenty of ways to compound them, if you wanted to.

 

[neely]

Has anyone used the transdermal med for hyperthyroidism? Vet has suggested the possibility of radioactive iodine treatment as well, on the surface that seems like the perfect treatment but I am wondering what if anything is the drawback to it other than all the precautions that have to be taken after the treatment is completed.

We used Methimazole 1.25 mg twice a day in the transdermal gel form because it can have fewer adverse effects than the pill. Some cats exhibit gastrointestinal side effects with the pill. Our cat stayed on the transdermal gel form for approximately nine months then our vet recommended the I-131 radioactive iodine treatment. He went through the appropriate tests and was considered a good candidate because he had no organ involvement. He was between 11 - 12 years old when he had the I-131 and I noticed your cat is 17 years old. I honestly don't know if this makes a difference but it's something you might want to discuss with your vet. I also noticed you're in the Chicagoland area, so am I. Should you decide to do the I-131 please feel free to pm me because it's important to go to a facility that does Scintigraphy prior to the treatment. Wishing you the best of luck, please keep us posted on your progress.