Changes in thyroid volume in response to radioactive iodine for Graves hyperthyroidism correlated with activity of thyroid-stimulating antibody and treatment outcome

This study investigated 1) the relationship between thyroid volume and thyroid function in radioactive iodine (RAT) treatment for Graves' disease, and 2) the activity of thyroid-related Ig in serum on the responsiveness of thyroid tissue to RAI. The changes in thyroid volume per megabecquerel (MBq) of I-131 retained in thyroid tissue was calculated by ultrasonography as a quantitative indicator of the effect of RAI on thyroid volume. Of the 52 patients treated with I-131 (3.7 MBq retained/g thyroid tissue), 26 patients showed thyrotoxicosis, 20 patients became euthyroid, and 6 patients developed hypothyroidism 6 months after therapy. The change in thyroid volume per MBq I-131 was lower (P < 0.01) in the hyperthyroid patients than in the euthyroid or hypothyroid patients. The activity of thyroid-stimulating antibody in serum immediately before the therapy was greater (P < 0.01) in the hyperthyroid patients than in the euthyroid patients and was greater (P < 0.05) in the euthyroid patients than in the hypothyroid patients; it was inversely correlated with the changes in thyroid volume per MBq I-131 (r = -0.667; P < 0.01). Accurate measurement of changes in thyroid volume during the course of RAT treatment provides evidence of the responsiveness of Graves' disease thyroid tissue to RAT, which is related to the outcome of thyroid function. Thyroid-stimulating antibody determination may be useful in deciding the appropriate dose of RAI to obtain euthyroidism instead of hyperthyroidism.