Alterations in thyroid function after cholecystographic contrast agents

The radiographic contrast agents commonly used for oral cholecystography have diverse effects on thyroid hormones in man. They may: (i) decrease serum triiodothyronine (T3) and increase reverse T3 in both hyperthyroid and euthyroid subjects, an effect attributable to inhibition of phenolic (outer) ring deiodination of iodothyronines; (ii) acutely increase serum thyroxine (T4) by displacing it from the liver, so inviting an erroneous diagnosis of hyperthyroidism; (iii) precipitate hyperthyroidism in apparently euthyroid subjects who have autonomous thyroid tissue. In comparison with the cholecystographic agents which are taken up by the liver, the renally-excreted contrast agents used for angiography or intravenous urography are much less potent in producing these effects. The paradox of T4 excess with normal T3 may arise after cholecystography, either by an acute T4 increase in a euthyroid subject, or by normalization of T3 in hyperthyroidism, thus creating a diagnostic dilemma. The recent trend towards use of oral cholecystographic agents in the urgent management of hyperthyroidism, because of their effect on serum T3, needs to be regarded with caution in view of the risk that hyperthyroidism may eventually be worsened if synthesis of T4 is not effectively blocked. For this reason, contrast media should probably be used in the treatment of hyperthyroidism only in conjunction with conventional antithyroid drugs. A history of exposure to contrast media should be sought in any acute or unexpected exacerbation of hyperthyroidism, or when T4 excess is found without an increase in T3.