['To assess levels of vitamin D and of immunoglobulin G subclasses in children and adolescents with type 1 Diabetes Mellitus with or without autoimmune thyroiditis.', 'Among 213 patients with type 1 diabetes, the cases with thyroid-specific autoantibodies formed Group 1 [n=19, M/F: 7/12, median age 13 years (10.1-14.7)]. Nineteen age-, gender-, and diabetes duration-matched cases with type 1 diabetes without any other systemic disease were designated as controls [Group 2, M/F: 7/12, median age 12.9 years (10.5-14.9)].', 'Levels of thyroid hormones, vitamin D, total IgG and IgG subclasses, as well as IgG subclasses/total IgG ratios were similar between the groups. Five cases (26%) in Group 1 had IgG4 levels > + 2 SDS, whereas there were no such cases in Group 2 (p=0.046). These five patients had similar clinical features but higher median IgG4 levels and IgG4/Total IgG ratios compared to the subjects with IgG4 levels < + 2 SDS in Group 1 and Group 2.'] Is autoimmune thyroiditis in children and adolescents with type 1 diabetes mellitus associated with elevated IgG4 but not with low vitamin D?