IgG4‐related hypophysitis is highly prevalent among cases of histologically confirmed hypophysitis

IgG4‐related disease is an immune‐mediated disease with manifestations in most organ systems among them the pituitary gland. To date, few cases of histologically confirmed cases of IgG‐related hypophysitis have been reported. The aim of this study was to retrospectively determine the prevalence of IgG4‐related hypophysitis among cases previously diagnosed as primary hypophysitis (lymphocytic hypophysitis, granulomatous hypophysitis and hypophysitis not otherwise specified). Histological and immunohistochemical analysis revealed that 12 of 29 cases (41.4%) previously diagnosed as primary hypophysitis fulfilled the criteria for IgG4‐related disease and, thus, IgG4‐related hypophysitis should always be considered in the differential diagnosis of primary hypophysitis. All cases of IgG4‐related hypophysitis showed a dense lymphoplasmacytic infiltrate with more than 10 IgG4‐positive cells per high power field and a ratio of IgG4/IgG‐positive cells of more than 40%, whereas storiform fibrosis was an inconsistent histological feature and was also seen in few cases of non‐IgG‐related hypophysitis, thus lacking sensitivity and specificity. Obliterative phlebitis was not seen in any case. Thus, histological criteria defined for IgG4‐related disease in other organs should be modified for IgG4‐related hypophysitis, accordingly.