Background/Aims: Thyroid disease is a common comorbidity in individuals with Down syndrome (DS), but historical studies have multiple limitations. We assessed thyroid abnormalities in a large cohort of children with DS. Methods: Retrospective records review from a single institution. Calculated prevalence of common thyroid abnormalities and associations with common comorbidities. Results: Among 508 patients, 120 (24%) had a thyroid-related diagnosis, the majority having elevated thyrotropin treated with levothyroxine. A Kaplan-Meier estimate projects that 50% have thyroid disorder by adulthood, with 20% of hypothyroidism diagnosed before the age of 6 months. When tested, approximately 50% had positive antithyroid antibodies, though this rate was 100% in overt hypothyroidism. There was no association between congenital or acquired hypothyroidism and common comorbidities. Conclusion: Thyroid disease in DS is more common and occurs earlier than in the general population, and is often transient. Thyroid disease is unrelated to gender, obesity, or other comorbidities. Apart from overt hypothyroidism, much of hypothyroidism in DS appears unrelated to autoimmunity; we recommend checking of antithyroid antibodies only in select cases. An additional screen for thyroid disease between the newborn screen and the 6-month well-child visit will detect early cases of hypothyroidism who passed their newborn screen. (C) 2017 S. Karger AG, Basel