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Early pregnancy iron deficiency may predict hypothyroxinemia in second, third trimester

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July 11, 2018

Pregnant women with iron deficiency in the first trimester are more likely to have lower levels of free thyroxine and an increased risk for hypothyroxinemia in the second or third trimester vs. those without iron deficiency in early pregnancy, according to findings from a retrospective study conducted in China.

“Overt hypothyroidism and isolated hypothyroxinemia in pregnancy have been shown to be associated with an increased risk of both adverse pregnancy complications and poor fetal neurocognitive development,” Xiaochun Teng, MD, of the department of endocrinology and metabolism, Endocrine Institute, The First Hospital of China Medical University, Shenyang, China, and colleagues wrote. “Currently, a case-finding approach to identify women at high risk of thyroid dysfunction during pregnancy is recommended. According to our study, we suggest that pregnant women with [iron deficiency] in the first and second trimesters be regarded [as a] high-risk group for maternal hypothyroxinemia.”

Teng and colleagues analyzed data from 723 pregnant women (mean age, 29 years) and 1,645 non-pregnant women (mean age, 29 years) without history of thyroid disease participating in the Subclinical Hypothyroid in Early Pregnancy (SHEP) study, aiming to evaluate thyroid insufficiency on maternal health or infant outcomes in China, conducted between 2012 and 2015. Researchers measured soluble transferrin receptor, serum ferritin and total body iron, as well as serum thyrotropin, free T4, thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb). Iron deficiency was defined as a serum ferritin concentration of less than 12 µg/L, a soluble transferrin receptor level of at least 4.4 mg or total body iron level (calculated from soluble transferrin receptor and serum ferritin concentrations) of less than 0 mg/kg. Researchers used multiple linear regression analysis to test the association between free T4 and its related risk factors, and to analyze the risk factors for hypothyroxinemia.

Within the cohort, 675 women and 309 women participated in the second and third trimester, respectively, and 261 pregnant women were followed in both the second and third trimesters. According to trimester-specific reference ranges for serum free T4 and thyrotropin, 13 women were diagnosed with hypothyroxinemia in the first trimester, 20 women were diagnosed in the second trimester and nine women were diagnosed in the third trimester.

Researchers found that iron deficiency status was positively associated with free T4 levels in the first and second trimester, but there was no association found between iron status and free T4 levels in the third trimester of pregnancy. In logistic regression analysis, iron deficiency was an independent risk factor for hypothyroxinemia in the first trimester (OR = 14.86; 95% CI, 2.31-95.81) and in the second trimester (OR = 3.36; 95% CI, 1.01-11.21).

In a prospective data analysis to assess whether iron status in an earlier trimester of pregnancy would affect serum free T4 levels or rate of hypothyroxinemia in a later trimester, researchers found that pregnant women with iron deficiency during the first trimester had lower serum free T4 levels and a higher rate of hypothyroxinemia in the second or third trimester vs. those without iron deficiency. There were no significant differences found in third trimester free T4 levels between pregnant women with or without previously diagnosed iron deficiency in the second trimester, according to the researchers. – by Regina Schaffer

Disclosures: The authors report no relevant financial disclosures.