Ipodate and iopanoic acid
Because 10% to 20% of cats treated with methimazole develop gastrointestinal side effects and carbimazole is not available
in the United States, alternative medications are being studied. One study involved the use of ipodate, which is an iodine-containing
contrast agent that inhibits the peripheral conversion of T4 to T3. This effect is similar to the effects of propranolol on thyroid function. In the study, 12 hyperthyroid cats were initially
given oral ipodate at a dosage of 100 mg/day. The cats' clinical signs, body weight, heart rate, and serum T3 and T4 concentrations were evaluated two, four, six, 10, and 14 weeks after initiating ipodate.5 In addition, complete blood counts and serum chemistry profiles were performed at each recheck to detect any adverse effects.
If a cat did not exhibit a good clinical response, the dosage of ipodate was increased to 150 mg/day and then to 200 mg/day
at two-week intervals. Eight cats responded to the ipodate treatment, and four did not. Cats that responded exhibited mean
body weight increases and mean heart rate and serum T3 concentration decreases during the study. Cats that did not respond exhibited mean body weight decreases and no significant
changes in mean heart rate and serum T3 concentrations. Serum T4 concentrations remained high in all cats. None of the cats showed adverse clinical signs or hematologic abnormalities due
to ipodate treatment. The study demonstrated that ipodate may be a feasible alternative to methimazole, particularly in cats
that cannot tolerate methimazole and that are not candidates for surgery or radioiodine therapy. Cats with severe hyperthyroidism
are less likely to respond to ipodate than are cats with mild or moderate disease. Also, in cats in which serum T3 concentrations do not return to the reference range, clinical signs are unlikely to improve adequately.
Since publication of the study, ipodate is no longer available in the United States. However, a similar product called iopanoic
acid is available through compounding pharmacies. Like ipodate, iopanoic acid inhibits the peripheral conversion of T4 to T3. No published studies on the efficacy of iopanoic acid are available, but in my experience the dose, efficacy, and side effects
appear to be similar to those reported with ipodate .
We do not know whether treatment with these iodine-containing contrast agents results in only temporary abatement of hyperthyroidism,
as occurs in people, because no long-term studies have been done in cats. Animals that have been treated with ipodate or iopanoic
acid probably should have the medication discontinued before beginning radioiodine therapy since both medications affect thyroid