Stimulating and blocking thyroid-stimulating hormone (TSH) receptor autoantibodies from patients with Graves' disease and autoimmune hypothyroidism have very similar concentration, TSH receptor affinity, and binding sites

Objective: The distinct biological properties of TSH receptor ( TSH-R) autoantibodies ( TRAbs) from patients with Graves' disease ( GD) are yet unexplained on the molecular level. Here we compare serum concentration, affinity to the TSH- R, and binding sites on the TSH- R of stimulating ( TSAb) and blocking ( TBAb) TRAbs. Methods and Patients: Four-step affinity purification using human recombinant TSH- R was performed with 22 TRAb- positive sera from GD patients ( 11 with only TSAb and 11 with only TBAb) and five control sera. Antibody concentration, TSH binding inhibition ( TBII), and TSAb/ TBAb activity of the purified TRAb were assessed. Labeled purified TRAbs were used for displacement studies with TRAb and an additional 30 patients and 10 control sera. Results: TRAbs could be purified to 80 - 93% purity with recovery of the TBII and TSAb and TBAb activity. No TRAbs could be purified from healthy individuals. The mean +/- SD concentration of TRAb was 17.3 +/- 5.4 mu g/IU for the TSAb sera ( range, 9.6 - 25.9) and 18.2 +/- 8.5 mu g/IU for the TBAb sera ( range, 4.6 - 29.2), respectively ( P = 0.79). Affinity was in the picomolar range for both TRAb subtypes with mean +/- SD dissociation constant of 167 +/- 109 pM ( 60 - 410 pM) for TSAb and 253 +/- 132 pM ( 80 - 410 pM) for TBAb ( P = 0.12). Purified and labeled TSAb and TBAb showed a very similar binding pattern to the TSH- R in displacement studies with unlabeled TSAb/ TBAb or unpurified patients sera, indicating binding sites on the TSH- R in close proximity to each other. Conclusion: TSAbs and TBAbs in the serum of patients with GD have similar characteristics. They are of low concentration with high affinity and have also similar binding epitopes on the TSH- R.