Therapeutic 131I dose in hyperthyroidism: role of pretreatment with thionamide

Radioiodine therapy has become a cornerstone of treatment of hyperthyroidism. However, the timing of its administration varies between 1) the time of initial diagnosis with concurrent therapy with beta adrenergic blocking drugs or 2) following induction of euthyroidism with thioamide, Propylthiouracil or Methimazole. This study assessed 24-HR 131I uptake values and the thyroid scan in 24 subjects with hyperthyroidism at the time of diagnosis and again after attaining the euthyroid state with Propylthiouracil or Methimazole. Propylthiouracil of Methimazole was withdrawn seven days prior to the second 24-HR 131I uptake and scan. In all subjects, as a group, 24-HR 131I uptake increased following antithyroid therapy as compared to the time of initial of diagnosis [76 + 5% Vs. 54 + 4%; p < 0.01]. The thyroid gland size decreased in nine of twenty-four subjects, but remained unchanged in the remaining subjects. Since 24-HR 131I uptake and the gland size are the major factors influencing the therapeutic radioiodine dosage, it is possible that initial therapy with thioamide drugs may reduce the therapeutic dose of 131I in subjects with hyperthyroidism belonging to both groups, i.e., Graves' disease and Multinodular toxic goiter by inducing a rise in 24-HR 131I uptake. Furthermore, the shrinkage of thyroid glands may further decrease the radioiodine dosage in patients with Graves' disease.