“Do maternal thyroid hormones levels and iodine influence birth weight and the time of birth amongst healthy pregnant women?”


The thyroid gland is a butterfly shaped gland located on the front of the neck, just below the Adam’s apple. It secrets several hormones called thyroid hormones. The main thyroid hormone is called T4. Thyroid hormones act all over the body, mainly helping the body to use energy, stay warm, and keep the brain, heart, and other organs working as they should. They are essential for brain development during prenatal life and childhood. When the function of the thyroid gland is impaired it secretes too little or too much hormones resulting in diseases. Iodine is an essential element for the production of thyroid hormones. The body does not produce iodine therefore we need to obtain it from the diet.During the first part of pregnancy the fetus is dependent on the maternal thyroid hormones. Several previous studies showed that manifest under- and overactive maternal thyroid function can cause delay in the development of the fetus’ nerve system resulting in mental retardation. The INMA Project examined whether maternal thyroid hormones of healthy pregnant women can influence birth weight and change the time of birth.

The study included data from 2178 healthy mother-child pairs from four areas in Spain: Valencia, Sabadell, Asturias, and Gipuzkoa. Women who previously had thyroid disease or had hormone levels outside the normal ranges were excluded from the study. Amongst the participants, two types of thyroid hormones (T4 and TSH) were measured in maternal blood samples before week 24 of pregnancy. Gestational age and birth weight was determined at delivery.

The investigators concluded that higher levels of maternal T4 hormone within the normal range were associated with lower birth weight. The time of birth was not associated with any of the thyroid hormones measured. Although prior studies showed that impaired thyroid function can affect maternal weight gain, and gestational diabetes – diabetes occurring during pregnancy – this study did not find any association between maternal weight gain and gestational diabetes and thyroid function.

The investigators also concluded that there is little evidence about the relationship between mild iodine deficiency without thyroid disease and birth outcomes, such as low birth weight and preterm birth. In a previous INMA study researchers found that pregnant women who had mild iodine deficiency during the 3rd trimester of the pregnancy had a higher chance of having a child with lower birth weight as compared to women who did not have iodine deficiency. However, such an association was not shown in relation to mild iodine deficiency during 1st trimester in the current study.

Overall, the investigators highlighted that maternal thyroid function may have an important role in fetal growth, even in the absence of overt thyroid disease. Although the investigators did not find association between mild iodine deficiency and impaired fetal development, they highlight the importance of maternal diet throughout pregnancy, since iodine deficiency during the second part of pregnancy might influence not only maternal but also the fetal thyroid function.

ReferencE: León G, Murcia M, Rebagliato M, Álvarez-Pedrerol M, Castilla AM, Basterrechea M, Iñiguez C, Fernández-Somoano A, Blarduni E, Foradada CM, Tardón A, Vioque J. Maternal Thyroid Dysfunction during Gestation, Preterm Delivery, and Birthweight. The Infancia y Medio Ambiente Cohort, Spain. Paediatr Perinat Epidemiol. 2015 Jan 7. doi: 10.1111/ppe.12172. [Epub ahead of print]