BACKGROUND
Thyroid hormone is extremely important in the normal development of a baby during pregnancy. This is most important during the 1st trimester, as the baby gets their thyroid hormone from the mother as their thyroid is not yet developed. Once the baby’s thyroid starts working, the thyroid hormone from the mother becomes less important. If the baby’s thyroid does not develop normally (congenital hypothyroidism), the baby requires thyroid hormone from the mother throughout pregnancy.

Hypothyroidism is common, especially in women, which means that hypothyroidism in the mother during pregnancy is also common. Undiagnosed or under-treated hypothyroidism in the mother during pregnancy has been associated with impaired growth of the baby and both small- and large-for-gestational-age (SGA and LGA, respectively) in newborns. We also know that birth weight is an important predictor of both short- and long-term health of the baby and newborn, and both SGA and LGA have been associated with adverse pregnancy and newborn outcomes. There are multiple factors contributing to birth weight, including function. The placenta is the main connection between the mother and the baby. Placental weight is a marker of placental function and is positively correlated with birth weight.

This study was done to identify the relationship between hypothyroidism in the mother, birth weight and placental function, using placental weight as a marker of placental function.

THE FULL ARTICLE TITLE
Lundgaard MH et al. Birth weight and placental weight in children born to mothers with hypothyroidism. Eur Thyroid J 2025;14(4):e250111; doi: 10.1530/ETJ-25- 0111. PMID: 40570047.

SUMMARY OF THE STUDY
This study was a analysis using large national registers in Denmark. All single live births from 2004 – 2015 were identified using the Danish Medical Birth Register and information on age, # of prior pregnancies, pre-pregnancy weight (body mass index, BMI), and smoking in pregnancy, the pregnancy outcome, last menstrual period, gestational age at birth, birth year, birth weight and sex of the child, and placental weight was obtained.

This was correlated with hospital information to identify mothers with diagnosis of hypothyroidism before or during pregnancy and with prescription registries for information on redeemed medications during the same time frame. A total of 694,734 single live births were identified, of which 98% included information on placental weight. In addition, a subset of births from a northern region of Denmark had information on maternal TSH and free thyroxine at weeks 4-20 (average week 10).

When hypothyroidism was diagnosed during pregnancy, there was a slightly higher risk of SGA (12.1% vs 9.1% in pregnancies where hypothyroidism was known prior), and an increased risk of LGA. When hypothyroidism was known prior to pregnancy but went untreated (as defined by absence of prescriptions being refilled during that time frame), the risk of SGA was also higher than when hypothyroidism was treated (11.6% vs 8.6%). In the small subgroup with thyroid function levels available, higher maternal TSH and higher maternal FT4 were both associated with lower birth weights. However, there was no differences in placental weight in any of these groups.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
While this study confirmed the relationship of hypothyroidism with SGA births, it did not identify placental weight as a mediator between thyroid status and fetal growth. This suggests there are other factors affecting placental function independent of placental weight. Since there was not universal screening during the years of this study, we don’t know if hypothyroidism was missed in the control patients, thus confounding the comparisons. In fact, the prevalence of hypothyroidism in this study (1.5%) was lower than that identified in other studies (4-6%). In addition, the differences in outcome when hypothyroidism was treated or not could be confounded by other socioeconomic factors such as nutrition, iodine status or access to prenatal care.

For patients, this study still confirms the importance of treating hypothyroidism during pregnancy. However, further studies are needed to definitively answer questions regarding treatment doses and timing and their relationship to pregnancy and fetal outcomes.

— Marjorie Safran, MD

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