Warfarin toxicity and exacerbation of thyrotoxicosis induced by radioactive iodine therapy for Graves' disease

Described here is a patient who developed a severe exacerbation of his thyrotoxicosis and warfarin toxicity shortly after radioactive iodine (RAI) therapy for Graves' disease. Although not often seen, these are serious complications of RAI therapy. Patients who could be harmed by exacerbation of thyrotoxicosis, such as those who are severely thyrotoxic, elderly, or with cardiovascular disease, would likely benefit from a course of antithyroid drug therapy that renders them clinically and biochemically euthyroid before RAI therapy as well as from the preventive measure of treatment with a beta-blocker. Our patient also illustrates the interaction between thyroid hormone levels and warfarin requirements. Hyperthyroid patients are more sensitive to warfarin and, therefore, require a smaller dose of warfarin than when they are euthyroid. Close monitoring is necessary in patients receiving warfarin who experience changes in thyroid function to prevent the potentially lethal occurrence of a thromboembolic or hemorrhagic event because of inappropriate warfarin dosage.