Evidence for thyrotropin receptor immunoreactivity in pretibial connective tissue from patients with thyroid-associated dermopathy

Pretibial myxedema (PTM), mainly characterized by the accumulation of glycosaminoglycans in the dermis and subcutaneous tissue, is an extrathyroidal manifestation of autoimmune Graves' disease (GD), almost always associated with Graves' ophthalmopathy (GO). The thyrotropin receptor (TSHR) has been proposed as the common target antigen in GD, GO and PTM, with evidence for receptor transcripts and/or protein in these locations. The aim of this study has been to investigate whether receptor protein is present in the pretibial tissues. Skin biopsies were obtained from two patients with PTM and two normal subjects without thyroid disease. A portion of each sample was fixed to produce semi-thin sections for Toluidine Blue or Periodic Acid Schiff (PAS) staining. The remainder was snap frozen to generate cryostat sections for immunohistochemical analysis using three monoclonal antibodies against TSH-R. In the skin from the two patients suffering from PTM, the dermis was infiltrated by inflammatory cells (lymphocytes, B cells, macrophages, mast cells) and adipocytes. The collagen fibers were dissociated by edema and by the accumulation of a PAS-positive material. Immunodetection of TSH-R produced positive staining on cells localized in the dermis, beneath the epidermis or close to the hypodermis. These cells were elongated and resembled fibroblasts. No immunoreactivity was observed in the dermis from control patients without thyroid disease. In conclusion, we have evidence for TSH-R immunoreactivity in the pretibium. of patients with GD, GO and PTM. Further studies are needed to unambiguously identify the positive cells and determine whether the reactivity is due to the receptor itself or to a cross-reacting protein.