The association between iron status and thyroid hormone levels during pregnancy

Iron deficiency may be a risk factor for thyroid disorder; however, the relationship between iron deficiency and thyroid disorder as well as mechanism involved remain unclear.

A hospital-based cross-sectional study was conducted to analyze the correlation between iron status and thyroid hormone levels in pregnant women. A total of 2218 pregnant women were recruited, and iron status and thyroid hormones were measured. Canonical correlation, Lasso regression, and Receiver operator characteristic (ROC) curve analysis were used to determine the association and related factors.

There were 219 cases with iron deficiency anemia (IDA), 168 cases with iron deficiency (ID), and 1831 subjects with normal iron status. Compared with normal group, free triiodothyronine (FT3) and free thyroxine (FT4) in ID group and IDA group had a significant decreasing trend (P < 0.05), with the lowest levels in IDA group. Thyroid stimulating hormone (TSH) was significantly increased in ID group and IDA group (P < 0.05). Moreover, the proportion of hypothyroidism in both ID group and IDA group was higher than the normal group, meanwhile the proportion of hyperthyroidism was lower in both groups (P < 0.05). Serum ferritin (SF) and hemoglobin (Hb) were positively correlated with FT3 and FT4 but negatively correlated with TSH. Correlation analysis indicated that iron status was associated with thyroid hormone levels (P < 0.05). Lasso regression analysis showed that SF, Hb and other variables could be included in the prediction model of FT4. The variables selected by Lasso model were used for ROC curve analysis, and the prediction accuracy was acceptable (AUC=0.778, P < 0.05).

Our study indicated that there is an association between iron status and thyroid hormone levels in pregnant women, and the level of FT4 may change with iron status. Our findings provide new ideas for regulating the thyroid hormone levels to prevent thyroid dysfunction during pregnancy.