Gu Y, et al. - In this study involving 6,542 qualifying pregnant women, researchers evaluated the combined impact of serum ferritin (SF) concentration and free thyroxine (fT4) levels on the risk of gestational diabetes mellitus (GDM). From December 2012 to March 2014, women presented for antenatal care in a tertiary hospital in Shanghai, China were included. According to SF and fT4 level, women were divided into six groups. To assess the additive interaction of SF concentration and fT4 level, the relative excess risk of interaction (RERI), the attributable proportion (AP) of the interaction and the synergy index (SI) were applied. Findings suggested an association of only high serum ferritin concentration with an increased risk of GDM in early pregnancy. In early pregnancy, the level of fT4 may not affect the association between high SF and GDM risk.