Hypothyroid women have persistently higher oxidative stress compared to healthy controls

Some studies suggest that hypothyroidism is associated with increased oxidative stress. Urinary excretion of 8-oxo-7,8-dihydroguanosine (8-oxoGuo) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) represents whole-body RNA and DNA oxidation, respectively. These biomarkers have only been explored sparsely in patients with thyroid disorders.

In 45 Danish women with newly diagnosed hypothyroidism, we compared 8-oxoGuo and 8-oxodG before or shortly after initiating levothyroxine with the excretion rates at euthyroidism. We also compared the excretion of 8-oxoGuo and 8-oxodG in the patients after restored euthyroidism with 18 healthy control subjects.

Compared with baseline, none of the biomarkers changed significantly in the patients after becoming euthyroid. The geometric mean of 8-oxoGuo was 1.63 (95% CI: 1.49-1.78) nmol/mmol creatinine at baseline and 1.67 nmol/mmol at euthyroidism (95% CI: 1.53-1.83) (P = 0.39), while that of 8-oxodG was 1.28 nmol/mmol creatinine at baseline (95% CI: 1.14-1.44) and 1.32 nmol/mmol at euthyroidism (95% CI: 1.18-1.48), respectively (P = 0.47). The relative mean differences were 0.97 (95% CI: 0.91-1.04) for 8-oxoGuo and 0.97 (95% CI: 0.88-1.06) for 8-oxodG. At baseline, multiple linear regression revealed a positive association between free thyroxine and both biomarkers (8-oxoGuo, P < 0.001; 8-oxodG, P = 0.04). Furthermore, 8-oxoGuo was positively associated with age (P = 0.04) and negatively associated with thyrotropin (P = 0.02). In the control group, the geometric mean of 8-oxoGuo was 1.23 nmol/mmol creatinine (95% CI: 1.07-1.42), while that of 8-oxodG was 1.04 nmol/mmol creatinine (95% CI: 0.88-1.23). Thus, compared with control subjects, euthyroid patients showed a significantly higher level of both 8-oxoGuo (P < 0.001) and 8-oxodG (P = 0.03).

In hypothyroid women, no significant effect of levothyroxine treatment on the oxidative stress biomarkers 8-oxoGuo and 8-oxodG could be demonstrated. However, the excretion of these biomarkers was significantly higher than in healthy controls.