Background. Autoimmune diabetes is usually accompanied by other autoimmune endocrinopathies, most often by autoimmune thyroiditis (AIT), but it is not clear whether diabetes in these patients differs from diabetes without AIT. Methods and Results. Eleven-year follow-up of 47 young adult Type 1 diabetic patients with respect to the presence of AIT (group I – positive antibodies against microsomal peroxidase, antiTPO, and tyreoglobulin, antiTgl, group II – only positive antiTPO, group III – without AIT) showed: a) cessation of endogenous insulin secretion (Cpeptide <0.03 nmol/l) in 100 % of patients with AIT (in group I between year 2 and 9, in group II between year 3 and 11, p<0.05), while in group III without AIT only in 55 % of patients (I,II vs. III, p<0.001); b) higher prevalence of antiGAD values > 5 U/ml in group I when compared to patients without AIT (I vs III, p<0.05); c) tendency toward higher doses of insulin needed for diabetes compensation in patients without AIT; d) the highest prevalence of organ-specific and systemic autoantibodies in group I with the most distinct manifestations of AIT, and the lowest prevalence in group III without AIT (statistically significant). Conclusions. Autoimmune diabetes in adults with AIT compared to diabetes occurring isolated showed differences in the area of autoimmunity against islets of Langerhans, Langerhans islets’ function and in the clinical course of the disease.

        Key words: diabetes mellitus Type 1, autoimmune thyroiditis, antiGAD, antiIA2, Langerhans islets’ function, autoimmune polyglandular syndromes.