Background: Graves' disease (GD) is caused by the continuous stimulation of the thyroid gland by autoantibodies directed against the thyroid-stimulating hormone receptor (TSHR). Two frequent assays for the measurement of TSHR autoantibodies (TSHRAb) were compared, one measuring stimulation of cyclic adenosine monophosphate (cAMP) production and one measuring inhibition of TSH binding, with regard to diagnostic accuracy for GD as well as whether there was an existence of their discordant results in patients with GD and painless thyroiditis (PT).
Methods: Using 106 sera from untreated GD and 80 sera from autoimmune PT, we compared the diagnostic performance of two TSHRAb assays that have been recently developed. The first one is a bioreporter assay using chimera TSHR (Mc-4), which detects a stimulation signal of cAMP level in cultured CHO cells (Mc4-TSAb assay). The second is a binding inhibition assay using the extracelluar domain of porcine TSHR and a monoclonal antibody (M22) closely mimicking the binding to TSH (M22-TRAb assay). In addition, we compared both assays by using eight sera from eight GD subjects becoming spontaneously hypothyroid due to appearance of thyroid blocking autoantibodies (TBAb) that were measured with inhibition rates of TSH-stimulated cAMP in porcine cells.
Results: The Mc4-TSAb assay and the M22-TRAb assay were positive in 94.3% and 92.5% of the GD patients, respectively, whereas they were negative in 95.0% and 98.8% of the PT subjects. However, 10 of 106 GD sera (9.4%) showed discordant results. Six of 106 cases with untreated GD (5.7%) were Mc4-TSAb positive and M22-TRAb negative. In contrast, 4 of 106 sera (3.8%) were Mc4-TSAb negative but M22-TRAb positive. Two cases of untreated GD were negative for both Mc4-TSAb and M22-TRAb. In eight GD subjects with TBAb and hypothyroidism, the binding assay was highly positive, although Mc4-TSAb was negative.
Conclusion: Similar and excellent performance was noted for the Mc4-TSAb and M22-TRAb assays in a large group of patients with GD. However, there was 9.4% discordance with regard to false negatives for GD and 3.8% discordance between the two tests with regard to false positives for PT. With regard to the relatively high rate of discordancy, a combination of both assays could reduce the presence of TSHRAb-seronegative GD.