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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 out number male cases by a ratio of 3 :1 , and the birth prevalence for Hispanic fem ales 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 mislead in g if sex is not taken into account. These factors are important when screen in g tests, such as the serum T 4 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.