Assessing a Novel Thyroid-Stimulating Antibody Bioassay as a Predictor of Radioactive Iodine Therapy Efficacy in Graves' Disease.

OBJECTIVE: No studies have reported the use of the Biosensor TSAb assay (TSAb (Bio)) for predicting the therapeutic efficacy of radioactive iodine therapy (RAIT) for Graves' disease (GD). Therefore, we evaluated the usefulness of this novel thyroid-stimulating antibody (TSAb) bioassay kit, which uses a cyclic adenosine monophosphate (cAMP) biosensor, in predicting the therapeutic efficacy of RAIT in 20 patients with GD. METHODS: We defined the RAIT outcome using two criteria: (i) thyroid weight reduction to <8 g and (ii) thyroid weight reduction to >80% at 12 months post-RAIT. For criteria (i) and (ii), regression analysis was performed for thyroid-stimulating hormone (TSH) receptor autoantibody (TRAb), TSAb (Bio), TSAb (Bio)/TRAb, I-131 absorbed dose/TSAb (Bio), and I-131 absorbed dose/(TSAb (Bio)/TRAb). After evaluating the significance of each factor, receiver operating characteristic (ROC) analysis was also performed. RESULTS: Among the five aforementioned anti-TSH receptor antibody parameters, only I-131 absorbed dose/(TSAb (Bio)/TRAb) showed statistical significance in the multivariate analysis (p = 0.004 and p = 0.021 for each criterion). Although I-131 absorbed dose/(TSAb (Bio)/TRAb) correlated significantly with the thyroid weight (r = -0.54, p = 0.015), it did not correlate with thyroid weight reduction rates at 12 months post-RAIT. ROC analysis also demonstrated a good predictive value (area under the curve of approximately 0.8 with a cut-off value of 3.9) for I-131 absorbed dose/(TSAb (Bio)/TRAb) in predicting thyroid weight at 12 months post-RAIT. CONCLUSIONS: TSAb (Bio) is a useful predictor of RAIT efficacy when evaluated as I-131 absorbed dose/(TSAb (Bio)/TRAb). This index was defined by dividing a factor that positively affects RAIT outcomes (I-131 absorbed dose) by a factor negatively affecting it (i.e., (TSAb (Bio)/TRAb)). The therapeutic efficacy of RAIT can be expected with an I-131 absorbed dose/(TSAb (Bio)/TRAb) value of ≥3.9. Adjusting the I-131 dosage based on this index may lead to more effective treatment outcomes.