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Background: Iodine deficiency (ID) remains common in Europe, and may be especially detrimental during pregnancy. The aim of our study was to assess iodine status and thyroid function in healthy pregnant women in the Lyon metropolitan area. Methods: In a cross-sectional study, healthy pregnant women ( n=228) with no history of thyroid disease were consecutively recruited from an obstetric clinic during all trimesters. Thyrotropin (TSH), free thyroxine (FT4), anti-thyroid peroxidase (anti-TPO) antibodies, thyroglobulin (Tg), and urinary iodine concentration (UIC) ( n=100) were measured. Thyroid functions were compared with those in a control group of nonpregnant adults. Results: The median (range) UIC was 81 (8-832) μg/L, and 77% of pregnant women had a UIC <150 μg/L, indicating inadequate iodine intake. Overall, 11% of women had abnormal TSH or anti-TPO. The median FT4 (pmol/L) was 14.9, 12.6, and 11.5 in the first, second, and third trimesters, respectively. The median Tg in pregnant women was 16.2 μg/L, did not differ across trimesters, and was significantly higher than in the control group of nonpregnant adults (11.7 μg/L) ( p=0.02). Controlling for maternal age and week of gestation, UIC was not a significant predictor of any of the thyroid function tests. Conclusions: Pregnant women in the Lyon area are iodine deficient and have increased serum Tg concentrations compared with nonpregnant controls, likely due to physiological thyroid hyperstimulation during gestation exacerbated by ID. [ABSTRACT FROM AUTHOR]