Serum Negative Chronic Autoimmune Thyroiditis (SN-CAT): A Case Report of Three Siblings Without Familial History of Autoimmune Thyroid Diseases (ATDs).

This case highlights the rare manifestation of a less prevalent form of chronic autoimmune thyroiditis, serum-negative chronic autoimmune thyroiditis, within a sibling cluster without detectable autoantibodies and unclear genetic links, with no familial history, challenging current understanding of autoimmune thyroid disease. Ultrasound findings remain crucial for diagnosis, underscoring the need for clinical vigilance in atypical pediatric hypothyroidism.