Association of Isolated Hypothyroxinemia and Subclinical Hypothyroidism With Birthweight: A Cohort Study in Japan.

Context: Isolated hypothyroxinemia (low maternal free thyroxine [FT4] in the absence of thyroid-stimulating hormone [TSH] elevation) and subclinical hypothyroidism (high TSH in the absence of FT4 elevation) during early pregnancy are common. However, there are limited data regarding pregnancy outcomes, particularly their association with birthweight. Objective: We assessed the association between isolated hypothyroxinemia and subclinical hypothyroidism during the first trimester and birthweight. Methods: Analyses were conducted using a database of pregnant women (n = 1105; median age, 35 years) who delivered at the National Center for Child Health and Development, a tertiary hospital in Tokyo. The primary outcomes included the rates of small for gestational age (SGA), large for gestational age (LGA), and low birth weight. Results: Of the 1105 pregnant women, 981 were classified into the euthyroidism group, 25 into the isolated hypothyroxinemia group, and 26 into the subclinical hypothyroidism group during the first trimester. The prevalence of SGA was significantly higher in isolated hypothyroxinemia and subclinical hypothyroidism groups than the euthyroidism group (28.0% and 19.2%, respectively, vs 5.7%; Conclusion: Pregnant women with isolated hypothyroxinemia and subclinical hypothyroidism in the first trimester have an increased likelihood of SGA. Screening and careful perinatal checkups for isolated hypothyroxinemia and subclinical hypothyroidism may help identify pregnant women at high risk for SGA.