Role of thyroid‐stimulating blocking antibody in patients who developed hypothyroidism within one year after ¹³¹I treatment for Graves' disease

<jats:sec><jats:title>OBJECTIVE</jats:title><jats:p>We recently reported that thyroid‐stimulating blocking antibody (TSBAb) may not contribute to the development of hypothyroidism more than six years after <jats:sup>131</jats:sup>I treatment. In the present study, we attempted to determine whether hypothyroidism that develops within a shorter period of time following <jats:sup>131</jats:sup>I therapy is associated with TSBAb.</jats:p></jats:sec><jats:sec><jats:title>DESIGN</jats:title><jats:p>Retrospective study.</jats:p></jats:sec><jats:sec><jats:title>PATIENTS</jats:title><jats:p>Sera were obtained from 8 patients who developed hypothyroidism within 6 months after <jats:sup>131</jats:sup>I therapy (Group 1), 8 patients who became euthyroid one year after <jats:sup>131</jats:sup>I therapy (Group 2), and 7 patients who developed transient hypothyroidism (Group 3).</jats:p></jats:sec><jats:sec><jats:title>MEASUREMENTS</jats:title><jats:p>Thyroid stimulating antibody (TSAb) activity was measured as the amount of cyclic adenosine monophosphate (cAMP) produced by cultured FRTL‐5 cells, and TSBAb activity as the inhibition of cAMP produced in response to 100 mU/l bovine TSH.</jats:p></jats:sec><jats:sec><jats:title>RESULTS</jats:title><jats:p>At about 3 months after <jats:sup>131</jats:sup>I treatment, TSAb activity increased significantly in Groups 2 and 3, but did not change in Group 1. In contrast, TSBAb activity in Group 1 increased significantly and was positive in 6 patients at that time. At 12–18 months after <jats:sup>131</jats:sup>I treatment, TSBAb activity tended to decrease and remained positive in 3 patients but became negative in 3 patients. It did not change in the patients in Groups 2 and 3. The patients in Group 1 were treated with levothyroxine, 75–125 μg/day. Levothyroxine was discontinued in the 3 patients whose TSBAb activity disappeared. Two of them remained euthyroid, and 1 became hypothyroid.</jats:p></jats:sec><jats:sec><jats:title>CONCLUSION</jats:title><jats:p>Results indicate that the hypothyroidism that develops within a short time after <jats:sup>131</jats:sup>I treatment may be caused by TSBAb activity. Thyroid function may be recovered when TSBAb activity disappears.</jats:p></jats:sec>