To study the interaction between TSH, autoantibodies and the amino-terminal half of the TSH receptor extracellular region (amino acids 1-260; domains ABC), we constructed 20 LH/CG chimeric receptor cDNAs. The prototypic receptor for modification contained domains ABC of the TSH receptor and domains DE of the LH/CG receptor. Segments (6-13 amino acids) within the ABC domains were replaced with the corresponding amino acids of the rat LH/CG receptor. Fifteen of the 20 chimeric receptors could be expressed functionally in Chinese hamster ovary cells. Twelve retained both high affinity TSH binding and normal signal transduction. These 12 receptors were tested with a panel of 10 patients' immunoglobulin G (IgG) samples containing potent TSH receptor stimulatory activity. With 11 of the receptor variants, the cAMP responses were similar to those with the prototype receptor (TSH-LHR-6). However, the -A1 variant of TSH-LHR-6 (Ser25-Glu30) responded poorly to 6 of 10 IgGs. The same pattern was observed when the IgGs were tested for their ability to inhibit [125I] TSH binding to the receptor variants, suggesting qualitative differences between the different stimulatory TSH receptor autoantibodies. Therefore, we examined the dose-response relationship of 2 IgGs that were approximately equipotent when tested with TSH-LHR-6 and its -A1 variant and another 2 IgGs that displayed greatly diminished potency with respect to the -A1 variant. Despite dilution to nearly undetectable levels, the relative potencies of the 4 IgG samples for both types of receptors remained similar. These data demonstrate directly that stimulatory TSH receptor autoantibodies do not all recognize the same components of the TSH receptor. The segment of the TSH receptor discriminated by these autoantibodies is between amino acids Ser25-Glu30.