Iodine excess in relation to decreased free thyroxine levels during pregnancy in an iodine-sufficient region, and potential interactions with iron and selenium.

BACKGROUND: Optimal intake of iodine is fundamental to maintain an adequate thyroid function during pregnancy. Selenium and iron are also involved in thyroid homeostasis and could modify the effect of iodine in thyroid hormones synthesis, but the interaction between these minerals has been scarcely studied. We aimed to evaluate the association between urinary iodine concentration (UIC) and the levels of thyroid stimulating hormone (TSH), triiodothyronine, and thyroxine during the first half of pregnancy, as well as the interaction with selenium and iron intake. METHODS: We conducted a cross-sectional analysis of data from 370 pregnant Mexican women with gestational age ≤ 18 weeks. TSH, triiodothyronine and thyroxine were measured by ELISA and UIC by ammonium persulfate digestion method. The association between UIC and each hormone adjusting for potential confounders was assessed via multiple linear regression models. RESULTS: The median of UIC was 180.3 µg/L, within the adequate category as stated by the WHO for pregnant women (150-249 µg/L). The majority (58.3%) of the participants had UIC into this range while 28.7% were below 150 and 13% above 249 µg/L, respectively. After adjusting by the confounders, participants with concentrations ≥ 250 μg/L showed free thyroxine levels significantly lower than women with adequate UIC (β = -0.07, 95% CI = -0.13, -0.01). Selenium and iron intake did not modify this association. CONCLUSION: These results suggest that UIC above requirements could disrupt thyroid function during pregnancy.