Background. Autoimmune thyroid disease (AITD) represents the most frequent associated manifestation of the autoimmune disease in children with Type 1 diabetes mellitus (T1DM). The aim of the study was to evaluate the prevalence and natural course of AITD in unselected cohort of diabetic children. Methods and Results. TSH, free thyroxin, thyreoglobulin autoantibodies (Ab-hTG), peroxidase autoantibodies (Ab-TPO) and thyroid sonography were prospectively evaluated (1 to 6 years; 323 patient-years) in 110 patients with T1DM (age 2.1–20.8 years; 60 boys). Mild elevation of Ab-hTG and/or Ab-TPO levels (>100, <1000 mIU/l) was found in 19 patients (17 %; boys:girls 1:2.2). Out of these, only 2 had increased thyroid volume and/or abnormal echotexture. In 9 of these patients, levels of autoantibodies subsequently declined below 100 mIU/l, remained unchanged in 9 patients and only in one case increased over 1000 mIU/l („benign form“ of AITD). All patients remained euthyroid. Severe elevation of Ab-hTG and/or Ab-TPO (>1000 mIU/l) was found in 12 patients (11 %; boys:girls 1:1). Out of these, eight had increased thyroid volume and ten had abnormal echotexture. Subclinical hypothyroidism developed within the observation period in nine of them (boys:girls 2:1). Conclusions.AITDwas found in 31 (28%) children and adolescents with T1DM.Whereasmild elevation of Ab-hTG and/or Ab-TPO levels was not accompanied withmorphological changes of the thyroid and did not predict functional disorder, severely elevated levels were associated with the early risk of subclinical hypothyroidism.

        Key words: type 1 diabetes mellitus, autoimmune thyroid disease, subclinical hypothyroidism, thyroid autoantibodies, children.