Background: The approach not to screen thyroid function of all pregnant women is mainly based on conflicting evidence of whether treatment of women with mild hypothyroidism is beneficial. However, there is consensus that all women with overt hypothyroidism (OH) and those with a thyrotropin (TSH) >10 mIU/L should be treated immediately, but data on these conditions are scarce. We assessed the prevalence of OH and a TSH >10 mIU/L during the first trimester of pregnancy. Methods: Thyroid function was assessed at 10-12 weeks gestation in 4199 Dutch Caucasian healthy pregnant women from three studies conducted in 2002, 2005, and 2013 from the same iodine sufficient area in the southeast of The Netherlands. We defined the first trimester specific cutoffs (2.5th-97.5th percentile) for TSH and free thyroxine (fT4) in thyroid peroxidase antibody (TPO-Ab) negative women in each study to determine the prevalence of women with OH and those with a TSH >10 mIU/L. We extrapolated these figures to the pregnant population of 2012 in The Netherlands, the United Kingdom, and the United States. Results: The prevalence of OH or a TSH >10 mIU/L in these 4199 women was 26 (0.62%) of whom 96% had (highly) elevated TPO-Ab titers. Based on the birth figures of 2012, if all pregnant women from The Netherlands, the United Kingdom or the United States were screened, the conservative annual number of cases would be 1000, 4500, and 25,000 respectively. However, the United Kingdom and parts of the United States have recently been demonstrated to be iodine deficient, which will result in even higher numbers. Conclusion: Our findings show that the discussion concerning thyroid screening during pregnancy should be based on data of overt hypothyroidism in healthy pregnant women. Screening of thyroid function is not expensive because all pregnant women have a standardized blood sample test at 8-12 weeks' gestation. Positive patients largely benefit from a cheap, safe, and effective treatment.