Several lines of experimental and clinical evidence favor a close etiologic link between Graves' disease and its associated extrathyroidal manifestations, ophthalmopathy and pretibial dermopathy. The human TSHR represents a candidate antigen shared between the thyroid gland and the involved extrathyroidal sites in Graves' disease. Here, we demonstrate that ribonucleic acid encoding exons 1-10 of human TSHR can be detected in fibroblasts derived from the affected orbital and pretibial space in patients with Graves' ophthalmopathy and pretibial dermopathy. RNA prepared from cultured fibroblasts was reverse transcribed and the resulting cDNA amplified by the polymerase chain reaction using primers spanning exons 1 through 10 of TSHR. The predicted transcripts (1890 and 2092 bp, respectively) were obtained with cDNA derived from orbital and pretibial fibroblasts of all patients with GO and PTM, and orbital fibroblasts of one healthy individual, and confirmed by southern hybridization. Sequencing of TSHR transcripts confirmed their identity with the reported nucleotide sequence of the human TSHR. Immunostaining using both monoclonal and polyclonal antibodies directed against the recombinant human TSHR revealed specific TSHR-like immunoreactivity in fibroblasts and adipose/connective tissue derived from the orbital and pretibial space of patients with GO and PTD, but not in normal individuals or control tissues. Detection, within the orbital and pretibial tissues, of RNA encoding nonvariant hTSHR and of immunoreactivity for this important autoantigen in Graves' disease suggests that the pathogenic role of the TSHR may extend beyond the thyroid gland, and may include the associated extrathyroidal manifestations.