Associations between Oxidative/Nitrosative Stress and Thyroid Hormones in Pregnant Women-Tainan Birth Cohort Study (TBCS)

Oxidative and nitrosative stress have been linked to thyroid function in both animal and human studies. In the present study, the associations between oxidative and nitrosative stress and thyroid hormones were investigated. Measurements were obtained from 97 Taiwanese pregnant women at the first, second, and third trimesters. Levels of five oxidative and nitrosative stress biomarkers (8-hydroxy-2'-deoxyguanosine [8-OHdG], 8-nitroguanine [8-NO₂Gua], 4-hydroxy-2-nonenal-mercapturic acid [HNE-MA], 8-isoprostaglandin F2α [8-isoPGF_2α], and malondialdehyde [MDA]) were measured using urine samples, and levels of five thyroid hormones (triiodothyronine [T₃], thyroxine [T₄], free T₄, thyroid-stimulating hormone [TSH], and T₄-binding globulin [TBG]) were measured in blood samples. Multiple linear regressions and linear mixed-model regressions were conducted to determine the associations between oxidative or nitrosative stress biomarkers and thyroid hormones in pregnant women. We found that TSH was negatively and significantly associated with 8-NO₂Gua (-14%, 95% CI [-26.9% to -1.1%]) and HNE-MA (-23%, 95% CI [-35.9% to -10.0%]) levels. However, T₄ (3%, 95% CI [0.2%-5.8%]) and free T₄ (4.3%, 95% CI [0.8%-7.8%]) levels were positively and significantly associated with 8-NO₂Gua. The T₄ to TBG and free T₄ to TBG ratios were positively and significantly associated with 8-NO₂Gua level (T₄/TBG: 3.6%, 95% CI [0.5%-6.7%]; free T₄/TBG: 5.6%, 95% CI [0.2%-11.1%]). However, the TSH to T₄ ratio was negatively and significantly associated with 8-NO₂Gua level (-17.3%, 95% CI [-30.4% to -4.3%]). The T₃ to TSH ratio was positively and significantly associated with HNE-MA level (25.2%, 95% CI [11.2%-39.2%]). However, the TSH to T₄ and TSH to free T₄ ratios were negatively and significantly associated with HNE-MA level (TSH/T₄: -21.2%, 95% CI [-34.5% to -7.8%] and TSH/free T₄: -24.0%, 95% CI [-38.3% to -9.6%]). Our findings suggest that an imbalance of oxidative and nitrosative stress may alter thyroid hormone homeostasis during pregnancy. Disruption of the maternal thyroid homeostasis during pregnancy would affect embryonic and fetal development.