The sodium-iodide symporter (NIS) is a novel autoantigen in autoimmune thyroid disease. In the present study we have characterized the antigenic domains on the human symporter which are recognized by autoantibodies from patients with either Graves' disease (GD) or autoimmune hypothyroidism (AH). Deletion derivatives of complementary DNA (cDNA) encoding the Na(+)/I(-) symporter were constructed using polymerase chain reaction (PCR) amplification. These deletion constructs were translated in vitro with the concomitant incorporation of [(35)S]methionine into the protein products. The reactivity of seven GD and six AH sera, which were known to contain symporter-binding antibodies, to each of the radiolabelled modified symporters was then determined in immunoprecipitation experiments. Analyses of the results obtained in the radiobinding assays suggest the existence of multiple antibody binding sites on human NIS (hNIS), including regions between amino acids (aa) 1--134, 191--286, 290--411, 411--520 and 520--588. Computer prediction of the potential B cell epitopes on the symporter revealed that, apart from aa 134--191, all the epitope domains identified overlapped, at least in part, with areas predicted to be highly antigenic. Interestingly, the antigenic domains represented by aa 191--286, 290--411 and 411--520 include regions of the polypeptide which form putative extracellular domains in the secondary structure model of the rat symporter. No correlation between the recognition of specific epitopes on the human symporter and the type of autoimmune thyroid disease was demonstrated.