Iopanoic acid is of minimal benefit in the treatment of severe hyperthyroidism: conclusions from a case study.

Iopanoic acid (1 g/d) was used together with propylthiouracil (1200 mg/d) in the treatment of a patient with very severe hyperthyroidism and associated cardiac failure. Although serum total T3 decreased by 75% within 48 h and reached normal after 72 h, free T3 levels did not fall to normal. Total and free T4 remained markedly elevated and features of hyperthyroidism persisted. Estimations of theoretical in vivo occupancy of nuclear thyroid hormone receptors, based on serum free T4 and free T3, suggest that the marked decrease in total T3 would not result in a corresponding decrease in thyroid hormone action. Hence, estimates of potential benefit from oral cholecystographic contrast agents, based only on measurements of total T3, may be unduly optimistic. When temporary agranulocytosis developed in this patient, the prior use of iopanoic acid, by markedly reducing thyroidal iodine uptake, restricted the therapeutic options. Caution should, therefore, be exercised in the use of iodine-containing contrast media as adjunctive antithyroid agents.