To examine the heterogeneity of autoantibodies to the human TSH receptor (hTSHR), we evaluated 20 sera from patients with Graves' disease for their recognition of prokaryotic (unglycosylated) and eukaryotic (insect cell glycosylated) recombinant hTSHR extracellular domain (ecd) in an unfolded (linear) and a folded (nonlinear) state. With the prokaryotic antigen, 12 (60%) bound folded hTSHR ecd monomer, 8 (40%) bound to the unfolded monomer, and 3 (15%) bound to a tetrameric species. Such binding to different hTSHR antigens was not mutually exclusive. In addition, 7 (35%) sera showed an apparently higher reactivity for the folded than the unfolded monomer.