Thursday, February 24, 2011

Q & A: Hyperthyroid Cat Not Controlled on High-Dose Methimazole

Tommie is a 14-year old MC DSH cat that presented 6 months ago for vomiting and weight loss. Hyperthyroidism was diagnosed based on a high serum T4 value (11.5 μg/dl; reference range, 0.8-4.0 μg/dl). Results of a CBC, serum chemistry panel, and urinalysis were normal.

I started him on 2.5mg methimazole twice daily. On recheck his serum T4 values was still high at 10.3 μg/dl so I increased the dosage to 5 mg twice daily. After 4 months on this dose, his serum T4 has decreased but remains slightly high at 6.5 μg/dl

I am afraid of increasing his dose any higher than 10 mg per day. All of the other cats I've treated have responded to this or even lower daily doses of methimazole. Any thoughts about the dosage in this cat?

My Response:

The dose of methimazole needed to control hyperthyroid cats is based on what it takes to lower the serum T4 concentration into the reference range, so there is not an upper limit to what methimazole dosage can be used. I've gone as high as 30 mg per day in some cats, although the incidence of side effects tends to be higher as you raise the dose. So in this patient, I would raise the dose to around 7.5 mg twice a day (total dose, 15 mg/day).

A few things that must be considered in this and all hyperthyroid cats that are treated chronically with methimazole is the following:
  • Methimazole and other antithyroid drugs work by being taken up by the thyroid tumor where the drugs inhibit thyroid hormone production and secretion. With time, the thyroid tumor will grow larger and larger. Therefore, most hyperthyroid cats will require higher dosages to inhibit thyroid hormone secretion as their goiter becomes larger and the thyroid volume increases.
  • In some of these cats, especially those treated for many months to years, the thyroid adenoma can transform into thyroid carcinoma while on these antithyroid drugs.
So, getting back to your cat, are you planning on using the methimazole long-term? At the moment, his daily dose is still quite low, but you are already having trouble regulating him. You might want to discuss the use of surgical thyroidectomy or radioiodine with the owner if you haven't already. Both of these are definitive treatment that are better choices for long-term prognosis, at least in my opinion.


Barbara Chappuis said...

Hello Dr. Peterson,
Thank you for your very informative blog. My cat has had very poorly controlled hyperthyroidism for nearly 3 years, now and we have had to continue to escalate his methimazole dose over this time period. He did well, initially with 5 mg of transdermal methimazole, twice daily, but after about a year, he started to lose weight, again, so we were essentially back to square one and up to a level of 12, again. We upped the dose to 7.5, 2x daily to no avail, so now have him on 10 mg oral tablets 2x daily. Very little improvement after 40 days. He went from 6.3 lb to 6.7 lbs. Would you suggest increasing him to 25 or 30 mg in divided doses daily? I'm feeding him high quality canned food and supplementing with cooked, ground turkey to give him a bit more protein. He seems to tolerate the oral Methimazole fairly well.

Dr. Mark E. Peterson said...

There is no upper limit of methimazole dose, although I've never used a dose higher than 30 mg per day.

Are you monitoring serum T4 and renal levels. You can't use body weight alone to determine the methimazole dose - that could be very dangerous.

Ellie Braun said...

My 15 year old cat, Preshus is on methimazole10mg f2f, prednizone5nd x2 and anti vomiting meds. From 15lbs in one year she is down to 6.2lbs. She was also diagnosed with adenocarcinoma in the intestines and hyperthyroidism. As soon as I started the thyroid medication her condition seems to get worse. Before the thyroid meds she was on the predisone, anti diareah/vomited meds and her diet to science hill medicated food.I feel in my gut to stop the thyroid meds cuz it's making her worse. Maybe I'm in denial but right after starting this medication she has suddenly became lethargic, not happy and extremely calmed down. Is this medication helping her at all out making her condition worse?
Thank you.

Dr. Mark E. Peterson said...

The methimazole is associated with side effects in 10-20% of cats. Talk to your vet.