ObjectiveTo determine the levels of antithyroid antibodies and thyroid hormones in the sera of patients with oral lichen planus (OLP), and to quantify the expression of thyroid proteins in OLP lesions. Subjects and MethodsVenous blood samples were drawn from 110 patients with OLP who had no history of thyroid disease or levothyroxine supplementation (OLP+/LT4-). A random population sample of 657 healthy subjects was used as the control group. Two additional groups were used as comparators. Immunohistochemical and qPCR analyses were performed on tissue specimens collected from the patients with OLP and thyroid disease and healthy subjects. ResultsNo association was found between the presence of antithyroid antibodies and OLP. More patients in the OLP+/LT4- group showed high levels of thyroid-stimulating hormone and low levels of free thyroxine than were seen in the control group. Thyroid-stimulating hormone receptor was more highly expressed in the OLP lesions of patients with thyroid disease than in the healthy oral mucosa. ConclusionsA significant number of patients with OLP who are not previously diagnosed with thyroid disease have thyroid parameters that are compatible with hypothyroidism. The expression of thyroid-stimulating hormone receptor in OLP lesions suggests that mechanisms related to autoimmune thyroid disease are involved in the aetiology of OLP.