Acquired hypothyroidism is more common in children with Down syndrome than in other children. In children without Down syndrome, acquired hypothyroidism is rare before age 3 years and is not reported to occur frequently until adolescence.1 Previous reports of hypothyroidism in children with Down syndrome have described onset at a similar age. Postneonatal-onset hypothyroidism in the very young children with Down syndrome described herein has not previously been reported, to our knowledge.

At the Child and Parent Resource Institute in London, Ontario, we have a multidisciplinary outpatient developmental program for children with Down syndrome. At the time of this writing, we had approximately 120 children aged between 6 weeks and 17 years in the program.

Patient Reports.Patient 1. This 6½-month-old female infant was born at 37 weeks' gestation to a 39-year-old mother who had a distant family history of thyroid disease. The mother and father were third cousins.