To ascertain whether thyroxine (T₄)-based screening programs for congenital hypothyroidism (initial measurement of total T₄ [tT₄] followed by thyroid stimulating hormone [TSH] measurement in patients with tT₄ <10th percentile) identifies congenital hypothyroidism in all neonates with Down syndrome.

Retrospective cohort study of 159 neonates with Down syndrome, born during the period 1998-2007 were included. Screening test results were compared with those of the general population. All primary care physicians of these infants were contacted and infants' thyroid status verified.

tT₄ concentrations in children with Down syndrome were significantly lower, and TSH higher than those in the general population; tT₄ concentrations did not correlate with screening TSH concentrations. Twenty children with Down syndrome were treated with L-thyroxin within the first month of life although only 10 babies had been identified by the routine screening test.

T₄-based screening does not identify many cases of congenital hypothyroidism in neonates with Down syndrome. We recommend that neonates with Down syndrome be screened by simultaneous measurements of both tT₄ and TSH.