Persistent Thyrotropin Receptor Antibodies Positivity Despite Prolonged Biochemical Euthyroidism in Graves’ Disease Associated With Down Syndrome: A Case Report

Down syndrome (DS) is associated with a markedly increased prevalence of autoimmune diseases, reflecting immune dysregulation related to trisomy 21.Although Hashimoto's thyroiditis (HT) is the predominant thyroid disorder, Graves' disease (GD) remains relatively uncommon in this population.The optimal duration of antithyroid drug (ATD) therapy in patients with DS remains uncertain, particularly when thyrotropin receptor antibodies (TRAb) remain positive despite prolonged biochemical euthyroidism during treatment.A 31-year-old man with DS presented with GD characterized by severe thyrotoxicosis (thyroid-stimulating hormone (TSH) <0.005 mIU/L; free thyroxine (FT4), 48 pmol/L) and markedly elevated TRAb, 69 IU/L).Carbimazole therapy achieved rapid clinical and biochemical control, allowing progressive dose reduction to a maintenance dose of 5 mg/day.After more than 40 months of continuous treatment, the patient remained clinically asymptomatic and biochemically euthyroid, while TRAb remained positive (4.3 IU/L).Definitive treatment options were considered but deferred after taking into account the patient's clinical and functional context, treatment tolerance, and family preference.Long-term low-dose carbimazole was, therefore, continued with regular clinical and biochemical follow-up.This case illustrates persistent TRAb positivity despite prolonged biochemical euthyroidism during carbimazole therapy in GD associated with DS.Persistent conventional TRAb positivity may indicate ongoing TSH-receptor autoimmunity, but it does not by itself establish the functional activity of the antibodies or immunological remission.Because treatment withdrawal was not attempted, clinical remission after discontinuation of antithyroid therapy cannot be assessed.Treatment duration should, therefore, be individualized according to biochemical control, TRAb evolution, treatment tolerance, patient-specific functional considerations, and patient or caregiver preferences.