Very-early-onset autoimmune hypothyroidism: a report of two cases with STAT3 gain-of-function variant.

Autoimmune hypothyroidism occurs rarely before 3 years of age. Two siblings were diagnosed with autoimmune hypothyroidism at age 5 and 16 months, presenting with classic symptoms of hypothyroidism, abnormal thyroid function tests (TSH: 200 and 660 mU/L; reference range (RR): 0.73-8.4 mU/L; Free T4: 5.9 and <1.3 pmol/L; RR: 11.9-25.6 pmol/L), and high thyroid peroxidase antibody levels. Thyroxine medication alleviated their symptoms. Apart from mild infections, the siblings exhibited no other major disorders. Whole exome sequencing identified a pathogenic STAT3 gain-of-function variant, most commonly associated with infantile-onset multi-organ autoimmune disorder. Genetic testing for early-onset hypothyroidism may reveal specific etiologies, impacting follow-up and treatment.