Objective: To analyze the correlation between iron status and thyroid hormone in pregnant women during their second trimester of pregnancy. Design: A hospital-based case-control study. Population: A total of 550 pregnant women were recruited from Pingguo Maternity and Child Health Hospital. Method: Data was grouped by SF concentration, and was performed by correlation and regression analysis. Main outcome measure: Iron nutritional status and thyroid hormones. Results: A significant difference was found in thyroid hormone level, including FT3, FT4, and TSH (all P < 0.01) between two groups (Mann-Whitney U test), and the levels of FT3 and FT4 were significantly lower in the iron deficiency group. Spearman’s correlation analysis showed that both of SF and Hb were correlated with FT3, FT4, and TSH (all P < 0.01). Canonical correlation was used to analyze the interrelationships between iron status and thyroid hormones, showing SF was positively associated with the FT3 level. The elastic network regression indicated that the coefficient of Hb was greater than that of SF. Conclusion: Our study indicated that iron nutritional status is associated with thyroid hormone levels during the second trimester of pregnancy, suggesting that ID may be an important risk factor in thyroid hormone levels. Involving two common endocrine diseases in pregnancy, the relationship between ID and thyroid hormone deserves further investigation. Tweetable abstract: Iron nutritional status is associated with thyroid hormone levels during the second trimester of pregnancy. Key-words: Iron status, thyroid hormones, the second trimester of pregnancy, canonical correlation, elastic network regression