['To investigate if L-thyroxine (T4) treatment may influence the clinical course of autoimmune thyroiditis (AIT) or prevent progression to subclinical or overt hypothyroidism in euthyroid nongoitrous pediatric patients with type 1 diabetes mellitus (T1DM) and AIT.', 'The study was performed in four Polish pediatric diabetes centers. Of 330 children with T1DM and AIT followed between 2008 and 2012, 101 received L-T4 and 160 underwent clinical observation for 24 months. Thyroid stimulating hormone (TSH), free T4 (fT4), anti thyroid peroxidase antibody (anti-TPO), anti thyroglobulin antibody (anti-TG), glycosylated hemoglobin (HbA1c) levels, and lipid profile were assessed in all patients. Ultrasonographic evaluation was also performed in all children at each examination.', 'Patients treated with thyroid hormones had higher TSH levels (3.99; interquantile 3.5 to 4.52 vs. 2.09 mIU/L; interquantile 1.55 to 3.06; pp<0.0001). A fall in TSH level (0.87 mIU/L 95% CI 0.43-1.30; pp<0.0001) was documented after the first year of treatment. FT4 level did not differ between the groups at baseline (p=0.7434), but rose in the treatment group and fell in the control group [mean difference 0.78 95% CI-0.22-1.53 pmol/L (p=0.02) after 12 months and 0.98 95% CI 0.04-1.76 (p=0.005) after 24 months]. Higher levels of anti-TPO were initially found in the treated patients (pp<0.0001) and significantly decreased over the 24-month period (pp<0.0001). Children in the treatment group had higher anti-TG levels (pp<0.0001), which showed a borderline decrease (p=0.08) in time. In the control group, anti-TG levels rose marginally (p=0.06) during the study.'] Does l-thyroxine stabilize autoimmune inflammatory process in euthyroid nongoitrous children with Hashimoto 's thyroiditis and type 1 diabetes mellitus?