Antithyroid Drugs and Radioiodine Therapy: A Grain of (Iodized) Salt

Most thyroid specialists prefer to use radioiodine to treat the typical adult patient with hyperthyroidism [1]. Radioiodine therapy is simple, cost-effective, and, except for iatrogenic hypothyroidism, free from long-term side effects. Nevertheless, controversies about its use continue [2]. One major area of dispute is the wisdom and necessity of antithyroid drug pretreatment in the weeks before radioiodine therapy. This practice has become commonplace, especially in elderly patients or those with cardiac disorders, in whom the risk of exacerbating the underlying thyroid problem would be particularly hazardous.

The use of adjunctive drug therapy began early in the radioiodine era [3], when exacerbations of hyperthyroidism were noted shortly after radioiodine treatment. This phenomenon was attributed to radiation damage and subsequent follicular disruption, with leakage of preformed thyroid hormone into the circulation. Because antithyroid drugs block the synthesis but not the release of thyroid hormone from the thyroid gland, it was thought that their use before radioiodine therapy would gradually deplete the thyroid of its hormonal stores, thereby making radioiodine therapy less risky. Modern experimental data support this theory: Larsen [4] noted that normal thyroids contained a mean thyroxine (T₄)_content of 254 µg/g compared with a mean of 295 µg/g in patients with hyperthyroidism treated only with propranolol before thyroidectomy [4]. In contrast, drug pretreatment resulted in a marked reduction in glandular T (₄) content (mean, 115 µg/g).

Despite the apparent logic behind the use of antithyroid drugs in this circumstance, some experts believe that exacerbations after radioiodine therapy have rarely occurred and that the worsening of hyperthyroidism that has been observed in some patients is not caused by the isotope but rather by a “rebound” of hyperthyroidism that is caused by the discontinuation of the drug therapy before administration of radioiodine [5]. Other experts …