OUTCOME OF THYROID-FUNCTION IN GRAVES PATIENTS TREATED WITH RADIOIODINE - ROLE OF THYROID-STIMULATING AND THYROTROPIN-BLOCKING ANTIBODIES AND OF RADIOIODINE-INDUCED THYROID DAMAGE

We investigated the interrelationship and the influence of thyroid-sti mulating antibodies (TSAb), TSH-blocking antibodies (TSHBAb), and of r adioiodine (I-131)-induced thyroid damage in the early (within 1 yr) o utcome of thyroid function in hyperthyroid patients with Graves' disea se (GD) treated with I-131. TSAb, TSKBAb, and ultrasound thyroid volum e (as an index of thyroid damage) were simultaneously measured before and at 1, 3, 6, and 12 months after I-131 in 31 GD patients. One year after radioiodine, 9.7% of patients were hyperthyroid (Hyper-group), r equiring methimazole; 12.9% were euthyroid (Eu-group); and 77.4% mere hypothyroid (Hypo-group). Pretreatment thyroid volume in the Eu-group and Hyper-group was significantly greater (P = 0.009) than in the Hype -group. Pre-I-131 TSAb levels were higher in the Hyper-group us. the H ypo-group (P = 0.01) or the Eu-group (P = 0.03). A significant post-I- 131 increase in TSAb levels occurred in 66% of patients developing hyp othyroidism but not in those remaining hyperthyroid. After I-131, TSHB Ab 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, respectiv ely); 2) hypothyroid patients had smaller (P = 0.0058) post-I-131 thyr oids than hyperthyroid patients; and 3) TSAb were still elevated in 75 % hypothyroid patients, but all of them had a thyroid volume less than or equal to 8 mL, indicating major postradioiodine gland damage. In c onclusion: 1) the early outcome of thyroid function after I-131 for GD is mainly related to pretreatment thyroid volume and to the degree of its reduction after therapy; 2) high TSAb levels before I-131 are ass ociated with a relative resistance to therapy; 3) a postradioiodine in crease 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 I-131 and play a role in the development of ea rly postradioiodine hypothyroidism only in a minority of patients.