Aside from mental retardation, the prevalence of several health problems is higher in individuals with Down’s syndrome than in the general population. Among these problems, hypothyroidism is often noted.¹ At the outset, it is important to distinguish congenital from acquired hypothyroidism. Numerous references continue to state that the prevalence of congenital hypothyroidism is considerably higher in newborns with Down’s syndrome than in the general population.² However, the evidence for this is unconvincing: in Quebec, there was no case of Down’s syndrome in several hundred children with permanent primary congenital hypothyroidism diagnosed through neonatal screening, suggesting that thyroid dysgenesis (ectopy or agenesis, which account for 85% of cases of congenital hypothyroidism) is not more frequent in Down’s syndrome.³ The coexistence of Down’s syndrome with severe persistent primary congenital hypothyroidism with a normally located gland of normal or increased size (“dyshormonogenesis”) has been reported in a few cases but may represent a chance association.⁴ On the other hand, the distribution of thyrotrophin (TSH) and thyroxine (T4) measured on neonatal blood spots are slightly shifted to the right and left, respectively, in newborns with Down’s syndrome compared to the general newborn population.⁵ …