Autoimmune thyroid disease in children includes Hashimoto thyroiditis and Graves' disease or toxic diffuse goitre. The immunological mechanisms involved in these diseases are closely related while the clinical picture differs because of the specific type of immunological response that occurs. Both conditions are connected together by their similar thyroid pathology, co-occurrence in family and within the same individual over time. A background inherited predisposition to autoimmunity with additional environmental and hormonal factors contributes to the development of the diseases. These observations help understand their key clinical features as well as their diagnostic and therapeutic approaches.