Clinical Thyroidology® for the Public
Summaries for the Public from recent articles in Clinical Thyroidology
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THYROID AND PREGNANCY
Iron deficiency in late pregnancy may be associated with low thyroid levels in pregnant women
BACKGROUND
Low thyroid hormone levels in mothers during pregnancy have been linked to problems with pregnancy such as miscarriage and preterm delivery as well as delay in the child’s development. Hypothyroidism in the mother is well known to be associated with these problems if the hypothyroidism is not diagnosed or is poorly treated. Since the thyroid gland uses iodine to make thyroid hormone, iodine deficiency can also lead to low thyroid hormone levels in both the mother and the baby.
Some recent studies also linked iron deficiency with low thyroid hormone levels in the mother and the baby. A previous study in rats suggested that iron deficiency can interfere with thyroid hormone production. Two previously studies done in Spain and China also suggested iron deficiency is a risk factor for low thyroid hormone levels in pregnant women. The need for both iodine and iron increases during pregnancy to provide for a growing baby. Therefore, pregnant women may be more likely to develop iodine or iron deficiency. This study investigated potential effect of iron deficiency on thyroid hormone levels during pregnancy in mildly iodine-deficient pregnant women.
THE FULL ARTICLE TITLE
Moreno-Reyes R et al 2021 Iron deficiency is a risk factor for thyroid dysfunction during pregnancy: A populationbased study in Belgium. Thyroid 31(12):1868-1877. PMID: 34538131.
SUMMARY OF THE STUDY
A total of 1241 pregnant women in their first or third trimesters in Belgium were recruited into the study. Women who were taking thyroid medications were excluded. Blood thyroid function tests (including TSH, free T4, and free T3 levels), iron levels, and red blood cell (hemoglobin) levels, and urine iodine levels were measured. Free T4 levels below the 10th percentile or Free T3 levels below the 10th percentile were defined as low thyroid hormone levels. Potential effects of iodine and iron deficiencies on the risk of low free T4 or low free T3 levels in the third trimester were assessed, taking into account other factors such as age, gestational age at study participation, and body-mass index (BMI).
Women in this study were found to have mild iodine deficiency, which was the more common in the first trimester. Iron deficiency was about 10-times more common in the third trimester than in the first trimester. Both low free T4 levels and low free T3 levels were about 16-times more common in the third trimester than in the first trimester. Urine iodine levels were not associated with thyroid hormone levels. In simple correlation, both iron deficiency and anemia were associated with low free T4 levels in the first trimester. In the third trimester, only anemia was associated with low free T4 levels. Anemia, but not iron deficiency, was associated with low free T3 levels in both first and third trimesters. Iron or hemoglobin levels were not associated with TSH levels.
In analysis adjusting for age, gestational age, and BMI, women in the third trimester with iron deficiency or anemia had higher risks of having low free T4 and low free T3 levels, compared to women without iron deficiency or anemia.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study concluded that iron deficiency, but not mild iodine deficiency, was associated with a higher risk of low free T4 levels in pregnant women in the third trimester.
One thing to note is that the researchers used a higher cutoff for low free T4 or free T3 levels than used in some other studies, so women with low thyroid hormone levels in this study may only have very mild cases. The adjusted calculations for risks of low free T4 or low free T3 levels could be only done in the third trimester because of the number of women with iron deficiency or anemia in the first trimester was too small. Therefore, it is unclear if the findings of this study has a significant impact in clinical care. The baby’s own thyroid gland is working in the third trimester, and T3 does not cross from mother to baby in significant amount. Thus, a baby may not be as affected by mother’s low thyroid levels at this time. Iron deficiency or anemia was also not associated with TSH, a sensitive screening test for thyroid abnormalities. Further studies on potential effects of iron deficiency or anemia in the first trimester on mother’s thyroid hormone levels would help evaluate clinical importance of this study’s finding. A study on potential benefit of iron supplementation on low thyroid hormone levels in pregnant women would also be useful to determine values of the findings of this study as well as other previous studies.
— Sun Y. Lee, MD, MSc
ATA THYROID BROCHURE LINKS
Thyroid Disease in Pregnancy: https://www.thyroid.org/thyroid-disease-pregnancy/
Iodine Deficiency: https://www.thyroid.org/iodine-deficiency/
ABBREVIATIONS & DEFINITIONS
Miscarriage: this occurs when a baby dies in the first few months of a pregnancy, usually before 22 weeks of pregnancy.
Preterm Delivery: this is when a baby is born too early, before 37 weeks of pregnancy.
Iodine: an element found naturally in various foods that is important for making thyroid hormones and for normal thyroid function. Common foods high in iodine include iodized salt, dairy products, seafood and some breads.
Iron: Iron is a major component of hemoglobin, a type of protein in red blood cells that carries oxygen from your lungs to all parts of the body. Without enough iron, there aren’t enough red blood cells to transport oxygen, which leads to fatigue.
Hemoglobin: the protein in red blood cells that binds oxygen to carry around to all the cells in the body. Hemoglobin levels are low with anemia.
Gestational Age: the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the woman’s last menstrual cycle to the current date. A normal pregnancy can range from 38 to 42 weeks.
TSH: Thyroid Stimulating Hormone — produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.
Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.
Triiodothyronine (T3): the active thyroid hormone, usually produced from thyroxine.
Body-Mass Index (BMI): a measure of body fat based on height and weight that applies to adult men and women.
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