High fT3 and low fT4 thyroid hormone level in early pregnancy linked to gestational diabetes
Abnormalities in the functioning of the thyroid gland are common in pregnant women and it is associated with labor or delivery complications such as preterm or premature birth, miscarriage and adverse health outcomes (such as hypoglycemia in both newborn child and the mother).
Earlier studies show a limited association between the functioning of the thyroid and the associated risk of gestational diabetes. Some studies have shown the risk of gestational diabetes with thyroid abnormalities such as isolated hypothyroxinemia.
A longitudinal study on women early in pregnancy suggest a direct association between risk of gestational diabetes and free triiodothyronine (fT3) levels and the ratio of free triiodothyronine (fT3) and free thyroxine (fT4 or FTI). The study also shows these risks are independent of other risk factors for the development of gestational diabetes.
Researchers have conducted studies on 107 women with gestational diabetes and 214 women without gestational diabetes, aged between 18 and 40 years. They used the Fetal Growth Studies-Singleton cohort data for the study purpose. Researchers have conducted separate analytical studies during the first and second trimesters for risk of development of gestational diabetes. They measured following thyroid markers at four pregnancy visits during their first and second trimester.
They calculated the fT3:fT4 ratio and determined the gestational diabetes status from their medical record. They accounted thyroid autoimmunity (TAI) status (linked to genetic background and gender) and major gestational diabetes risk factors such as ethnicity, age, and body mass index (BMI) prior to pregnancy in the study results.
The study has found an increased risk of gestational diabetes with the thyroid abnormalities during the first half of pregnancy. The study results show the following thyroid hormone levels during the first and second trimester assessments prior to the diagnosis of gestational diabetes.
In gestational diabetes risk calculation during the second trimester, researchers have found a 14-fold increased risk of gestational diabetes among the women with the highest fT3:fT4 quartile ratio when compared to the lowest fT3:fT4 quartile ratio. The study has found an association between fT3:fT4 ratio and gestational diabetes development even in the first trimester.
The study also shows a direct association between fT3 and the development of gestational diabetes during the first and second trimester. But there was no relationship between the free thyroxine (fT4) and thyroid-stimulating hormone (TSH) and the development of gestational diabetes.
Researchers think increased conversion of free thyroxine (fT4) to T3 increases free triiodothyronine (fT3) levels and causes gestational diabetes in pregnant women.
This was the first study providing longitudinal data on the association between the markers used to asses thyroid functioning (and fT3:fT4 ratio) and gestational diabetes development. Researchers suggest thyroid screening during early to mid pregnancy period to prevent adverse pregnancy outcomes. There are plans to conduct an analytical study on the effects of these hormones in the fetal growth and outcomes. There is a need to conduct another population study before coming to clinical practice guidelines and recommendations.
This study was supported by the Eunice Kennedy Shriver National Institute of Child Health and the Human Development (NICHD) and the American Recovery and Reinvestment Act of 2009 (ARRA, sometimes called the "Recovery Act").
Senior author of the study was Dr. Cuilin Zhang, MD, MPH, Ph.D., Senior Investigator, the National Institutes of Health (NIH). The study findings were published June 7, 2018 in the Journal of Clinical Endocrinology & Metabolism. Title of the article was "A Longitudinal Study of Thyroid Markers Across Pregnancy and the Risk of Gestational Diabetes."
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Published by Jammi Vasista, Chennai, India.