From infertility to high baby birth weight – the impact of maternal hypothyroidism

Hypothyroidism and pregnancy

Thyroid disorders are 3–5 times more common in women than in men in any population, and so their potential effects during pregnancy are important.

While much has been written about maternal hyperthyroidism on maternal and fetal outcomes, there has been far less attention on hypothyroidism, partly because maternal hypothyroidism is associated with infertility.

The fetus starts to synthesize its own thyroid hormones at roughly 12 weeks’ gestation; up until this point, it is entirely dependent on the thyroid hormones of the mother. Thus, any abnormality in maternal thyroid function impacts the growing fetus.

The growing brain

Maternal hypothyroidism has a negative impact on brain development and the subsequent cognitive function of the child. It is also associated with an increased risk of fetal death in utero and with gestational hypertension.

Birth weight

A recent meta-analysis revealed that maternal hypothyroidism during pregnancy is associated with increased birth weight in infants. High birth weight causes   several complications, particularly during delivery.

The same review, however, failed to show a significant association between maternal hypothyroidism and the risk of preterm delivery or birth of a baby either large or small for gestational age.

Maternal thyroid hormones regulate fetal growth by acting as a signal for the nutrient and oxygen supply to the fetus, and have both direct and indirect effects on fetal metabolism.

The direct effect is related to fetal oxygen and glucose consumption.

Indirectly, they affect the expression of the sodium-potassium-ATPase pump, or act on the electron transport chain and oxidative phosphorylation process in the mitochondria.

The bioavailability of fetal hormones and growth factors, such as the insulin-like growth factors, are another indirect effect of maternal thyroid hormones.

Conclusion

Given the large impact that maternal hypothyroidism has on the growing fetus and the newborn infant, it is imperative that this condition is identified pre-conception and treated with replacement hormones before pregnancy.

References

Hou J, et al. Gynecol Endocrinol 2015 Nov 3:1–5.

Forhead AJ and Fowden AL. J Endocrinol 2014;221:R87–R103.