Iron Deficiency Is a Risk Factor for Thyroid Dysfunction During Pregnancy: A Population-Based Study in Belgium

Area of Science:

• Endocrinology
• Maternal-Fetal Medicine
• Nutritional Science

Background:

• Iron deficiency impairs thyroid peroxidase activity, essential for thyroid hormone synthesis.
• Pregnant women are particularly susceptible to iron deficiency, affecting both maternal and fetal health.
• Mild iodine deficiency is common, but its interaction with iron status on thyroid function in pregnancy requires further investigation.

Purpose of the Study:

• To evaluate the impact of iron status on thyroid function in a large, nationally representative sample of pregnant women with mild iodine deficiency.
• To determine whether iron deficiency or mild iodine deficiency is a stronger determinant of thyroid hormone levels during pregnancy.

Main Methods:

• A nationally representative sample of 1241 pregnant women in Belgium was studied during the first and third trimesters.
• Measurements included urinary iodine concentration (UIC), serum ferritin (SF), hemoglobin, and thyroid function markers (fT4, T4, TSH, Tg).
• Body iron stores (BIS) were calculated, and statistical analyses identified predictors of thyroid function parameters.

Main Results:

• Iron deficiency, indicated by low SF and BIS, was prevalent, especially in the third trimester (39.6% with SF <15 μg/L).
• Iron status (SF, BIS, hemoglobin) significantly predicted free thyroxine (fT4) and total thyroxine (T4) levels, particularly in the first trimester.
• Mild iodine deficiency (median UICs 117-132 μg/L) predicted thyroglobulin (Tg) but not thyroid hormone or TSH concentrations.

Conclusions:

• Iron deficiency, not mild iodine deficiency, is a key determinant of serum fT4 and T4 in pregnant women.
• Maintaining adequate iron status is essential for optimal thyroid function during pregnancy.
• Interventions to correct iron deficiency may improve thyroid health and prevent anemia in pregnant populations.