Clinical Thyroidology® for the Public
Summaries for the Public from recent articles in Clinical Thyroidology
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THYROID AND PREGNANCY
How does mothers’ low thyroid hormone level affect pregnancy?
BACKGROUND
Thyroid hormone is essential for normal development of the baby during pregnancy. It is important for pregnant women to have enough thyroid hormone during pregnancy for normal pregnancy and babies’ normal development. Severe hypothyroidism in pregnant women is known to cause problems in pregnancy such as premature delivery, miscarriages and poor growth in babies. However, the effects of mild hypothyroidism during pregnancy are still unclear. Some studies showed poor pregnancy outcomes while others did not. There is especially limited data regarding the effects of isolated maternal hypothyroxinemia (IMH) on pregnancy. IMH is a condition where the mother’s free T4 level is low but the TSH level is normal. This study aimed to study the effects of IMH on pregnancy outcomes.
THE FULL ARTICLE TITLE
Nazarpour S et al 2021 Effects of isolated maternal hypothyroxinemia on adverse pregnancy outcomes. Arch Gynecol Obstet. Epub 2021 Sep 5. PMID: 34482473
SUMMARY OF THE STUDY
The researchers recruited 2406 pregnant women in Iran and measured their thyroid hormone, thyroid peroxidase (TPO) antibody, and urine iodine levels in the first trimester. Among these women, 1842 women with normal TSH levels and not taking thyroid medications were included in this study. IMH was defined as having normal TSH and low FT4 levels. Risks of poor pregnancy outcomes, including preterm delivery (defined as delivery before 37 weeks of pregnancy), miscarriage, low birth weight (LBW) of baby, third-trimester excessive bleeding, baby’s admission to intensive care unit, and baby’s birth weight, birth height, and birth head circumference. Researchers accounted for potential impacts of the number of previous pregnancies, urinary iodine levels, and TPO antibody status in assessing these risks.
Among 1843 women in the study, 142 (7.7%) women had IMH. There were no significant differences in baseline characteristics and proportion of poor pregnancy outcomes between women with normal thyroid function (euthyoid group) and women with IMH (IMH group). However, when adjusted for potential impact of number of prior pregnancies, urinary iodine levels, and TPO antibody status, women in IMH group had about 2.5-fold higher risk of having babies with LBW compared to women in euthyroid group. Babies of women in the IMH group also had a slightly smaller head circumference and slightly lower birth weight at birth compared to babies of women in the euthyroid group when accounted for these factors.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study shows that women with IMH had a slight increase of poor pregnancy outcomes. However, the overall number of poor pregnancy outcomes were small, which may have limited its ability to study full effects of IMH on certain pregnancy outcomes. Overall, this study adds to the currently available research on the effects of IMH on pregnancy outcome, but larger studies that similarly include measurement of iodine and TPO antibody status would be helpful to confirm and further expand the findings.
— Sun Y. Lee, MD
ATA THYROID BROCHURE LINKS
Thyroid Disease in Pregnancy: https://www.thyroid.org/thyroid-disease-pregnancy/
Thyroid Function Tests: https://www.thyroid.org/thyroid-function-tests/
ABBREVIATIONS & DEFINITIONS
Euthyroid: a condition where the thyroid gland as working normally and producing normal levels of thyroid hormone.
Hypothyroidism: a condition where the thyroid gland is underactive and doesn’t produce enough thyroid hormone. Treatment requires taking thyroid hormone pills.
Miscarriage: this occurs when a baby dies in the first few months of a pregnancy, usually before 22 weeks of pregnancy.
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.
TPO antibodies: these are antibodies that attack the thyroid instead of bacteria and viruses, they are a marker for autoimmune thyroid disease, which is the main underlying cause for hypothyroidism and hyperthyroidism in the United States.
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.
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