Pharmacodynamic effect of iopanoic acid on free T(3) and T(4) levels in amiodarone-induced thyrotoxicosis.

We describe the effects of iopanoic acid on daily levels of free triiodothyronine (FT(3)) and free thyroxine (FT(4)) in a patient with progressive type II amiodarone-induced thyrotoxicosis (AIT) who was undergoing thyroidectomy. The patient was a 59-year-old man who was undergoing amiodarone therapy while awaiting cardiac transplantation; the use of beta blockers and corticosteroids to control the AIT was contraindicated in this patient. Prior to thyroidectomy, the patient was started on iopanoic acid at 1.0 g twice a day; in response to gastrointestinal side effects, the dosage was subsequently reduced to 0.5 g twice a day. The patient responded to iopanoic acid with a rapid decrease in his FT(3) level and slight increase in his FT(4) level. This control of thyrotoxicosis allowed for an uneventful thyroidectomy, which was later followed by successful cardiac transplantation. Based on our findings in this single case, we believe that iopanoic can be used to rapidly lower FT(3) levels and to treat symptoms of thyrotoxicosis in a preoperative setting. We also discuss the different pharmacodynamic effects that iopanoic acid has on FT(3) and FT(4) levels.