Outdoor light at night exposure was associated with hypothyroidism in pregnant women: A national study in China

Background: Evidence on the influence of outdoor artificial light at night (ALAN) on hypothyroidism in pregnant women is scarce. We aimed to investigate the association between outdoor ALAN exposure and hypothyroidism in pregnancy.

Methods: 81,120 pregnant women from the China Birth Cohort Study (CBCS) were analyzed, which recruited from 18 provinces and autonomous regions in China between February 2018 and December 2020. Hypothyroidism was defined based on clinical diagnosis by physicians. Outdoor ALAN exposure (nW/cm²/sr) within 1500 m was estimated using VIIRS/DNB satellite data based on the participants' residential address. The Outdoor ALAN was divided in quartiles, with the lowest quartile (Q1) serving as the reference group. Generalized linear mixed models were employed to estimate the association between ALAN exposure and hypothyroidism.

Results: Among the 81,120 pregnant women, 3902 (4.77 %) were diagnosed hypothyroidism. Pregnant women with hypothyroidism had significantly higher median (IQR, interquartile range) outdoor ALAN levels during pregnancy compared to those without hypothyroidism (30.97 (18.15) vs. 29.14 (20.21) nW/cm²/sr, p < 0.01). After adjusting for covariates, we found that each quartile increment of outdoor ALAN exposure was associated with an adjusted Odds Ratio (aOR of 1.15 (95 % Confidence Interval [95 % CI]: 1.03-1.27), 1.15 (1.05-1.28), and 1.12 (1.00-1.25) for hypothyroidism, respectively from Q2 to Q4. Additionally, stratified analyses revealed that pre-pregnancy BMI was a significant modifier in the association between outdoor ALAN and hypothyroidism in pregnancy, with stronger effects observed among those who were overweight before pregnancy (1.21 [95 % CI, 1.05-1.39] vs. 1.03 [95 % CI, 0.97-1.10], p for interaction = 0.01).

Conclusions: Outdoor ALAN exposure is positively associated with hypothyroidism in pregnancy. To benefit maternal and infant health and well-being, recommendations for mitigating ALAN pollution and effective measures to avoid excessive light exposure should be developed.