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Outcome of Thyroid Function in Graves’ Patients Treated with Radioiodine: Role of Thyroid-Stimulating and Thyrotropin-Blocking Antibodies and of Radioiodine-Induced Thyroid Damage¹

Institute of Endocrinology, University of Pisa, 56124 Pisa, Italy

Address all correspondence and requests for reprints to: Luca Chiovato, M.D., Institute of Endocrinology, University of Pisa, via Paradisa 2, 56124 Pisa, Italy.

We investigated the interrelationship and the influence of thyroid-stimulating antibodies (TSAb), TSH-blocking antibodies (TSHBAb), and of radioiodine (¹³¹I)-induced thyroid damage in the early (within 1 yr) outcome of thyroid function in hyperthyroid patients with Graves’ disease (GD) treated with ¹³¹I. TSAb, TSHBAb, and ultrasound thyroid volume (as an index of thyroid damage) were simultaneously measured before and at 1, 3, 6, and 12 months after ¹³¹I in 31 GD patients. One year after radioiodine, 9.7% of patients were hyperthyroid (Hyper-group), requiring methimazole; 12.9% were euthyroid (Eu-group); and 77.4% were hypothyroid (Hypo-group). Pretreatment thyroid volume in the Eu-group and Hyper-group was significantly greater (P = 0.009) than in the Hypo-group. Pre-¹³¹I TSAb levels were higher in the Hyper-group vs. the Hypo-group (P = 0.01) or the Eu-group (P = 0.03). A significant post-¹³¹I increase in TSAb levels occurred in 66% of patients developing hypothyroidism but not in those remaining hyperthyroid. After ¹³¹I, TSHBAb appeared in 7 patients, in all but one associated with high levels of TSAb. One year after radioiodine: 1) the mean percent reduction in thyroid volume was greater in the Hypo-group (80.7%) or the Eu-group (83.5%) than in the Hyper-group (35.7%) (P = 0.007 and 0.033, respectively); 2) hypothyroid patients had smaller (P = 0.0058) post-¹³¹I thyroids than hyperthyroid patients; and 3) TSAb were still elevated in 75% hypothyroid patients, but all of them had a thyroid volume 8 mL, indicating major postradioiodine gland damage. In conclusion: 1) the early outcome of thyroid function after ¹³¹I for GD is mainly related to pretreatment thyroid volume and to the degree of its reduction after therapy; 2) high TSAb levels before ¹³¹I are associated with a relative resistance to therapy; 3) a postradioiodine increase in TSAb levels is related to the development of hypothyroidism; and 4) the concomitant appearance of TSHBAb and disappearance of TSAb are not frequent after ¹³¹I and play a role in the development of early postradioiodine hypothyroidism only in a minority of patients.

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