Discordance Between Thyroid Function and Thyroid-Stimulating Hormone (TSH) Receptor Antibodies in Down Syndrome Patients With Autoimmune Thyroid Disease: A Long-Term Follow-Up Study of Two Cases

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 I¹²³ uptake. His TRAb became negative after 18 months of methimazole treatment. During follow-up, recurrent thyrotoxicosis with positive TRAb/TSAb improved transiently without intensified treatment. Methimazole was discontinued at age 14, but hypothyroidism with positive TSAb and negative TSBAb became evident. Case 2: An 18-year-old woman with DS was diagnosed with Hashimoto thyroiditis at age 13. She experienced recurrent transient thyroiditis with positive TRAb/TSAb. Although TSAb remained positive and TSBAb negative, hypothyroidism continued without GD. In conclusion, our findings indicate that awareness of the potential discordance between TSAb/TSBAb status and thyroid function is important for deciding appropriate treatment in DS patients with unstable thyroid function and AITD.