Fibroblasts respond to thyrotropin (TSH) and neutral TSH receptor antibodies (N-TSHR-mAbs) in the same way as thyroid cells. A, Fluorescence-activated cell sorter analysis with control mAbs had 0.4% binding. B, TSHR-mAb (RSR1) showed 2 cell populations totaling 62% expressing the TSHR. C, Rat thyrocytes (FRTL-5) demonstrated 98% TSHR expression. D, N-TSHR-mAbs failed to induce cyclic adenosine monophosphate (cAMP) production in fibroblasts while TSH enhanced cAMP generation and protein kinase A (PKA) inhibitor-H89 showed suppression of cAMP. E, N-TSHR-mAbs and the PKA inhibitor induced reactive oxygen species (ROS) generation while TSH suppressed ROS. F, TSH induced fibroblast proliferation while the N-TSHR-mAbs and PKA inhibitor appeared to suppress fibroblast proliferation while TSH enhanced proliferation. *P less than .05.