Ambient biothermal stress, preconceptional thyrotropin abnormalities, and the risk of preterm birth: a nationwide Chinese cohort study.

Epidemiologic evidence on thermal stress and preterm birth (PTB) is based on ambient temperature rather than a biothermal metric. Thyrotropin abnormalities have been associated with PTB and may increase thermal vulnerability. However, it remains unknown whether thermal stress and thyrotropin abnormalities synergistically contribute to increased risk of PTB. Here we conducted a nationwide cohort study among 6,218,131 singleton live births in China. Biothermal stress was measured using the Universal Thermal Climate Index. We found that both heat stress and cold stress during each trimester were associated with increased risks of PTB, with hazard ratios ranging from 1.06 (95% confidence interval [CI]: 1.05, 1.07) to 2.33 (95% CI: 2.30, 2.35) and from 1.08 (95% CI: 1.07, 1.09) to 1.68 (95% CI:1.67, 1.70), respectively. Participants with subnormal (<0.37 mIU/L) or supranormal thyrotropin levels (≥4.88 mIU/L) and biothermal stress had higher risks of PTB, compared with those with normal thyrotropin levels and non-thermal stress. Additive interactions were also identified between biothermal stress and thyrotropin abnormalities. We estimated that up to 13.52% of PTB were attributable to biothermal stress and thyrotropin abnormalities. Our study showed a synergistic effect of biothermal stress and preconceptional thyrotropin abnormalities, highlighting the importance of climate adaption measures and thyroid management toward pregnant women under climate change scenario.