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Reports that maternal hypothyroxinemia during pregnancy might be associated with lower intelligence and impaired psychomotor development in offspring prompted investigators to compare various methods for evaluating thyroid function in 111 pregnant women and in 107 nonpregnant controls. All participants were thyroid peroxidase–antibody negative and iodine sufficient.
Measurement of free thyroxine (FT4) with two different immunoassays failed to reveal the expected physiologic human chorionic gonadotropin (HCG)–mediated rise during the first trimester. Moreover, FT4 values for 57% to 68% of women during their second and third trimesters were lower than the manufacturer’s lower limit of normal. In contrast, FT4 index (which is calculated from tw…