Birth prevalence of primary congenital hypothyroidism by sex and ethnicity.

Over 5 million infants have been screened for primary congenital hypothyroidism in California since 1980. This large number along with the multi-ethnic nature of California's population allows for a detailed analysis of the effects of ethnic origin, sex, and their interaction on birth prevalence. Sex is the most important factor, with at least a 2:1 (female:male) ratio across all major ethnic groups except blacks. The sex ratio among Hispanics is more striking; female cases outnumber male cases by a ratio of 3:1, and the birth prevalence for Hispanic females is 1 in 1886 births. Previously published rates for Asians and blacks are suspect because of small sample sizes, and Hispanic rates also may be misleading if sex is not taken into account. These factors are important when screening tests, such as the serum T4 test, are used as a statistical prescreening before thyroid stimulation hormone levels can be determined and before the influence of ethnic group and sex can be taken into account, because other factors may prevent high-risk groups (such as Hispanic females) from being declared positive.