Mechanism by Which Certain Dietary Substances Interfere with the Peripheral Activity of Thyroxine

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Abstract
A number of dietary substances interfere with the absorption and enterohepatic recycling of thyroxine (T4) in the rat. These substances produce a marked increase in the fecal excretion of orally administered T4 and a proportionately lower serum protein-bound iodine (PBI) concentration. The extent to which the PBI is reduced depends upon the relative potency and dietary concentration of the substance tested, but the reduction in PBI accounts for the corresponding decrease in the peripheral activity of the administered T4. Therefore, it is possible to feed T4 at a dose level (0.5 mg/100 g diet) that will produce a large metabolic rate and liver [alpha]-glycero-phosphate dehydrogenase response in the rat and to almost completely block these physiological responses by feeding an appropriate amount of one of the antithyrotoxic substances. By interfering with the absorption and recycling of T4, these substances prevent the serum PBI from reaching levels that would otherwise produce the usual physiological responses to T4.