Implications of High Free Thyroxine (FT4) Concentrations in Euthyroid Pregnancies: The FaSTER Trial

Context: Lower birth weight has been reported in conjunction with high maternal free T-4 (FT4) in euthyroid pregnancies, raising concerns for suboptimal outcomes. Objective: The objective of the study was to explore the relationships between high maternal FT4 and pregnancy complications in euthyroid women and to further examine the relationships among maternal size, FT4, and birth weight. Design: This was an observational multicenter cohort study. Setting: The study was conducted at prenatal clinics. Study Subjects: A total of 9209 euthyroid women with singleton pregnancies participated in the study. Interventions: There were no interventions. Main Outcome Measures: Relationships between second-trimester high maternal FT4 and pregnancy/delivery complications and, among FT4, maternal weight and birth weight were measured. Results: Women in the highest FT4 quintile are younger and weigh less than women in quintiles 1-4; gestational diabetes and preeclampsia occur less often (P = <.001, P < .001, P < .001, and P = .05, respectively). Lowest median birth weight occurs among women in the highest FT4 quintile (P = <.001), but deliveries less than 37 weeks' gestation are not increased. Labor/delivery complications do not differ by FT4 quintile. Restricting analyses to maternal weight-adjusted small-for-gestational-age deliveries yields similar results, except for preeclampsia. In the highest maternal weight decile, adjusted median birth weight is 266 g higher (8.3%) than in the lowest weight decile; adjusted median FT4 is 0.91 pmol/L lower (6.8%). Among women in the highest FT4 decile, adjusted median birth weight is 46 g lower (1.3%) than in the lowest FT4 decile. All three relationships are statistically significant (P < .001, P < .001, and P = .004, respectively). Conclusions: Lower median birth weight among euthyroid women with high FT4 is not associated with adverse pregnancy outcomes. Further investigation is indicated to determine how the variations in thyroid hormone concentration influence birth weight.