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?
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.