Autoimmune Thyroiditis in Children: Incidence and Clinical Features in South Estonia

Background and Objectives: Autoimmune thyroiditis (AIT) is the leading cause of acquired hypothyroidism in children. Published data on its incidence and clinical features in Estonia are limited. The aim of the study was to describe the incidence of paediatric AIT in South Estonia from 2012 to 2024 and to characterize thyroid function, thyroid autoantibody profiles, and associated autoimmune comorbidities in children with AIT. Materials and Methods: A retrospective descriptive study included all children aged 0–17.9 years diagnosed with AIT at Children’s Clinic of Tartu University Hospital from 2012 to 2024. Diagnosis was based on clinical evaluation and the presence of elevated levels of anti-thyroid peroxidase antibodies (TPOAb) and/or anti-thyroglobulin antibodies (TGAb). Age at the presentation of AIT and hypothyroidism, thyroid function and comorbid autoimmune diseases were analyzed. Results: Altogether, 346 patients (84 boys and 262 girls) were diagnosed with AIT over the whole observation period at a mean age of 13.0 years (range 0.97–17.9 years). The average annual incidence of AIT was 37.6 per 100,000 children per year, with corresponding rates of 59.1 in girls and 17.3 in boys. The average annual incidence increased from 30.2 per 100,000 children per year in 2012–2018 to 46.1 per 100,000 children per year in 2019–2024. Coexisting autoimmune diseases were found in 22.8% of patients, with type 1 diabetes being the predominant comorbidity (21.7%), followed by coeliac disease (1.7%) and vitiligo (0.9%). Conclusions: The incidence of AIT in children in South Estonia increased over the study period, with a marked female predominance. Autoimmune comorbidities were frequent, predominantly type 1 diabetes, followed by coeliac disease and vitiligo. These findings highlight the importance of early recognition of AIT and systematic monitoring of thyroid function and associated autoimmune conditions.