These compounds inhibit 5'deiodinase (5'DID-1 and 5'DID-2) enzymes, which catalyse T4-T3 conversion in the thyroid cell, liver, kidney, skeletal muscle, heart, brain, pituitary.

This accounts for the dramatic improvement in both subjective and objective symptoms of hyperthyroidism, particularly when they are used as an adjunctive therapy with thioamides (propylthiouracil, carbimazole).

They can be used in the treatment of patients with severe thyrotoxicosis (thyroid storm) and significant morbidity (e.g., myocardial infarction, or stroke) for rapid control of elevated plasma triiodothyronine concentrations.

The use of iopanoic acid for treatment of thyrotoxicosis has been discontinued in the United States.

Iopanoic acid underwent monodeiodination in the presence of type 1 deiodinase in a microsomal mouse liver preparation.