Objective:The degree of involvement of extraocular muscles varies across different regions of retrobulbar tissue in patients with thyroid eye disease, but the mechanism is unclear. This study aims to explore the relationship between differential expression of thyroid-stimulating hormone receptor(TSHR) in different parts of the extraocular muscles and the varying degrees of muscle involvement.Methods:The medial, lateral, superior, and inferior rectus muscle were separated from the retrobulbar tissue of rats, and the expression level of TSHR in four extraocular muscles was detected by immunofluorescence and qPCR. Extraocular muscle tissue of patients with strabismus was collected to detect the expression of TSHR and the cell types expressed by fluorescence.Results:The results of qPCR showed that the expression of TSHR in the medial rectus muscle was significantly higher than that in the lateral, superior, and inferior rectus muscle(medial rectus vs lateral rectus, P=0.012; medial rectus vs superior rectus, P=0.015; medial rectus vs inferior rectus, P=0.013), but there was no difference in insulin-like growth factor 1(IGF-1R) expression. Immunofluorescence showed that TSHR was co-expressed with PAX7, a molecular marker of muscle satellite cells, and the expression level in the medial rectus muscle of rats and humans was significantly higher than those in the other three extraocular muscles. Conclusion:The high specific expression of TSHR in the satellite cells of the medial rectus muscle may be the reason why the medial rectus muscle is most susceptible to involvement in thyroid eye disease.