AbstractPurified rat hemoglobin catalyzes the oxidative degradation of iodothyronines to form iodide and an iodine‐containing intermediate that reacts with protein. Hemoglobin also catalyzes peroxidation of linoleic acid. These observations are consistent with the reported intrinsic peroxidase activity of hemoglobin and other heme‐proteins. However, incubations containing both linoleic acid and an iodothyronine produced a surprising result: deiodination was stimulated rather than competitively inhibited. In contrast, linoleic‐acid peroxidation was inhibited by iodothyronines. Thus, low levels of iodothyronines (2.6×10−7M) are effective inhibitors of linoleic‐acid peroxidation. Thyroxine and reverse T3 were found to be more effective in this antioxidant activity than vitamin E, glutathione, ascorbic acid and DTT. Since linoleic‐acid peroxidation proceeds by a propagating free‐radical mechanism, we have concluded that iodothyronines can effectively terminate the free‐radical chain reaction to become oxidatively deiodinated. Consistent with this antioxidant mechanism, reverse T3 is effective in preserving red cell membranes as measured by the inhibition of erythrocyte hemolysis.