Subclinical Changes in Maternal Thyroid Function Parameters in Pregnancy and Fetal Growth

Area of Science:

• Endocrinology
• Maternal-Fetal Medicine
• Reproductive Health

Background:

• Overt thyroid disease in pregnancy poses risks to fetal development.
• The impact of subclinical variations in maternal thyroid function on intrauterine growth is less understood.

Purpose of the Study:

• To investigate the association between maternal thyroid hormone levels and fetal growth parameters.
• To examine the influence of milder thyroid function variations during pregnancy on fetal development.

Main Methods:

• Prospective birth cohort study including 439 pregnant women without diagnosed thyroid disease.
• Repeated measurements of thyroid hormones (free thyroxine, total T4, total triiodothyronine, thyroid-stimulating hormone) and ultrasound-based fetal growth indices.
• Standardization of ultrasound and delivery indices of fetal growth.

Main Results:

• Significant inverse associations were found between free thyroxine (FT4) and birth weight z scores, particularly at 10 weeks gestation.
• A 10% increase in FT4 was associated with an approximate 8.5g decrease in birth weight.
• FT4 was also inversely associated with estimated fetal weight, head circumference, and abdominal circumference; weaker associations were observed for total T4, and a positive relationship for total triiodothyronine.

Conclusions:

• Subclinical variations in maternal thyroid function parameters may influence fetal growth in pregnant women without overt thyroid disease.
• Monitoring thyroid function during pregnancy is crucial for optimizing fetal development.