A TSHr-LH/CGr Chimera that Measures Functional TSAb in Graves' Disease

Context: Stimulating thyrotropin receptor (TSHr) autoantibodies (TSAb) are the cause of hyperthyroidism in Graves' disease. In a patient's serum, TSAb can coexist with antagonist TSHr autoantibodies that block TSAb stimulatory activity (TSBAb); both can vary in amount and time. Objective: The objective of the study was to create a functional assay that detects only TSAb, thus having an increased accuracy for diagnosing Graves' disease. Design: A TSHr chimera (Mc4) that retains an agonist-sensitive TSAb epitope but replaces a TSBAb epitope was stably transfected in cells to establish the Mc4 assay. Setting: The study was conducted at the Chieti University (Outpatient Endocrine Clinic) and the University of Pisa (the Department of Endocrinology). Patients: The assay was validated using sera from 170 individuals with Graves' disease, Hashimoto's thyroiditis, and nonautoimmune hyperthyroidism and normal subjects from Chieti University. A second blinded study evaluated sera from 175 patients with autoimmune thyroid disease (mainly Graves' disease) from the University of Pisa. Interventions: Interventions included the assessment of patients' sera using human wild-type TSHr (WT-TSHr), Mc4 chimera, and binding (TRAb) assays. Main Outcome Measures: The Mc4 assay has the best accuracy for diagnosing Graves' disease. Results: The Mc4 assay has a better diagnostic accuracy than WT-TSHr and second-generation TRAb assays. Indeed, the sensitivity of the WT-TSHr, TRAb, and Mc4 assays was 97.3, 86.5, and 100%, respectively, whereas the specificity was 93.1, 97, and 98.5%, respectively. Conclusion: The Mc4 assay is a functional assay with improved sensitivity and specificity for the detection of TSAb and is clinically useful in diagnosing Graves' disease.