Approximately 1 in 4000 newborn infants has a severe deficiency of thyroid function, while even more have mild or partial degrees

Studies of populations from North America, Europe, Japan, and Australia, indicate that congenital hypothyroidism affects 1 in 3,000 to 4,000 newborns. For reasons that remain unclear, congenital hypothyroidism affects more than twice as many females as males.

Congenital hypothyroidism is observed in all populations. The prevalence at birth is increased in Hispanics, particularly in Hispanic females, who have a birth prevalence of 1 in 1886 births.11 Black infants have about one third the prevalence rate of white infants.

 Most studies of congenital hypothyroidism suggest a female-to-male ratio of a 2:1.The sex ratio for Hispanics is more striking, with a 3:1 female-to-male ratio. The ratio is lower among black infants. Twin births are approximately 12 times as likely to have congenital hypothyroidism as singletons.

 By definition, congenital hypothyroidism is present at, or before, birth. Children who develop primary hypothyroidism when aged 2 years or older