A retrospective review of thyroid function tests (TFTs) was performed on 49 young children (aged 4 months to 3 years) with Down syndrome compared with age-matched controls screened for hypothyroidism because of developmental delay or failure to thrive. Three of the 49 children with Down syndrome had congenital hypothyroidism; of the three, one had Hirschsprung's disease and two had duodenal atresia. Thyroiditis was uncommon, with only two children having thyroid antibodies present: one had acquired hypothyroidism and the other acquired hyperthyroidism. Twenty-seven percent of the Down syndrome cohort had mildly increased thyrotropin (TSH) and normal thyroxine levels. When compared with children with Down syndrome who had normal TFTs, no significant differences in sex, growth rate, maternal age, associated anomalies, developmental or specific thyroid symptoms were present. Transient elevations of TSH level were common in children with Down syndrome whether or not TSH values were initially normal or elevated. Routine neonatal and sequential thyroid screening in young children with Down syndrome is warranted.