Thyroid function in autoimmune thyroid disease (AITD) among patients with Down syndrome (DS) sometimes exhibits fluctuations over time, but the underlying cause of this variability remains unknown. Thyrotoxicosis is often associated with Graves disease (GD) and Hashitoxicosis. Thyroid-stimulating hormone (TSH) receptor antibody (TRAb) includes thyroid-stimulating antibody (TSAb) and TSH-stimulation blocking antibody (TSBAb), causing GD or hypothyroidism, respectively. We report two patients of DS showing discordance between thyroid function and TRAbs. Case 1: A 16-year-old boy with DS was diagnosed with Hashimoto thyroiditis at age 6, with positive TSAb and TSBAb. At age 7, GD was diagnosed based on thyrotoxicosis and elevated I