Iron Deficiency in Pregnancy Linked to Thyroid Disease

Liam Davenport

July 28, 2016

More than one-third of pregnant women are iron deficient, placing them at increased risk of a thyroid disease that increases the likelihood of pregnancy complications such as miscarriage, warn Belgian researchers.

The research, which was published online in the European Journal of Endocrinology on July 22, indicates that 35% of almost 2000 pregnant women had iron deficiency during the first trimester, and that this increased the risk of thyroid autoimmunity by over 50%.

While previous studies have indicated that iron deficiency during pregnancy can affect from 24% to 44% of women, this is the first to show the secondary effect of an increased prevalence of thyroid autoimmunity.

Senior author Kris G Poppe, MD, PhD, head of the Endocrine Clinic, University Hospital CHU St-Pierre, Brussels, Belgium, told Medscape Medical News that this finding is important because thyroid autoimmunity in pregnant women increases the risk of miscarriage, preterm delivery, and low birth weight compared with unaffected women.

Although he emphasized that women who wish to become pregnant should increase their intake of foods rich in iron, he noted that "many don't plan their pregnancy." But "it's not too late" to have serum ferritin levels measured after becoming pregnant, he said, adding that all women should have their iron reserve checked in any case.

"But many societies don't propose that systematically, often for economic reasons, of course," Dr Poppe said. "It depends on the area; it depends also on the ethnicity of the pregnant women."

Taken together, he said that these new findings show that "there is still a problem with iron deficiency, even in urban areas." Noting that there are parallels with iodine deficiency, he said: "We often thought that it had disappeared, but when we do surveys, it's clear that it hasn't disappeared yet."

35% of Pregnant Women Had Iron Deficiency in First Trimester

To examine the prevalence of thyroid autoimmunity and dysfunction during the first trimester of pregnancy, Dr Poppe and colleagues conducted an analysis of 1900 pregnant women taking part in an ongoing prospective study of obstetric parameters and biological data at a tertiary referral center in Belgium.

None of the women had a history thyroid diseases or having used thyroid medications, and women taking iron supplements at the first antenatal visit were also excluded.

Levels of ferritin, thyroid peroxidase antibodies (TPO-abs), thyroid-stimulating hormone (TSH), and free thyroxine (FT4) were measured during the first antenatal visit, and the women's age and body mass index were recorded.

The team defined iron deficiency as a serum ferritin level of <15 µg/mL, while thyroid autoimmunity was said to be present when the TPO-abs level was >60 kIU/L, and subclinical hypothyroidism was defined as a TSH level >2.5 mIU/L.

The results showed that 35% of the women had iron deficiency, with a mean serum ferritin level of 10 µg/L vs 31 µg/L in women without iron deficiency (P < .001). There was no significant difference in the prevalence of women aged ≥30 years or in the prevalence of obesity between women with and without iron deficiency.

Serum TSH levels were significantly higher in the iron-deficiency group than in women without iron deficiency, at 1.5 mIU/L vs 1.3 mIU/L (P = .015), and FT4 levels were significantly lower in iron-deficient women, at 1.0 ng/dL vs 1.1 ng/dL (P < .001). Serum TPO-abs levels were comparable between the two groups.

The researchers found that women with iron deficiency had a significantly higher prevalence of thyroid autoimmunity than non–iron-deficient women, at 10% vs 6% (P = .011) and had a significantly higher prevalence of subclinical hypothyroidism, at 20% vs 16% (P = .049).

Multivariate logistic regression analysis indicated that iron deficiency was significantly associated with thyroid autoimmunity, at an odds ratio of 1.52 (P = .017), although it was no longer significantly associated with an increased risk of subclinical hypothyroidism after adjustment for the multiple confounders.

Further Study Needed to Join the Dots

The team says: "It is obvious that further prospective studies are needed to investigate whether our data can be confirmed and to try to explain the association between iron deficiency, thyroid autoimmunity, and thyroid dysfunction in more detail and especially in relation to the pregnancy outcome."

To those ends, they are planning a further analysis of the cohort. Dr Poppe said: "We have to evaluate the outcomes of our cohort and then look at whether the iron deficiency had an impact on preterm delivery [and] miscarriage."

They will also examine whether any impact on outcomes is due to iron deficiency only, the thyroid autoimmune process, or both, as "one could fortify the effect of the other," he indicated.

This research did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Dr Poppe received fees for lectures he gave at Merck symposia in 2011 and 2014. The coauthors report no relevant financial relationships.

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Eur J Endocrinol. Published online July 22, 2016. Abstract

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