Endocrine Abstracts

Introduction: Maternal thyroid hormone during early pregnancy is important for proper fetal growth and development. Thyroid hormone receptors are widely expressed in placental tissue. Interestingly, thyroid dysfunction and suboptimal placental function have both been associated with pregnancy complications including preeclampsia, premature delivery and fetal growth restriction. First, we studied the association of the maternal thyroid and placental function and second, we investigated whether the placental function changes have an effect in the association of the maternal thyroid with pregnancy complications.

Methods: Maternal thyroid-stimulating hormone (TSH)/free thyroxine (FT4) at intake (median 13.2 weeks, 95% range 9.6–17.6 weeks) were measured in 3556 pregnant women from the Generation R cohort. Placental function in the 2nd and 3rd trimester was estimated by vascular resistance measurement with Doppler ultrasound. For that purpose umbilical artery pulsatility index (UMPI) and uterine artery resistance index (UTRI) were calculated. As the placental vascular resistance normally drops during gestation, we examined the change in values between the two times points which was defined as delta UMPI and delta UTRI. A mediation analysis was performed in order to investigate whether the placental function has a role in the association of thyroid with preeclampsia, premature delivery and fetal growth restriction.

Results: There was a negative linear association of TSH and FT4 with the delta UMPI (P=0.03; P=0.005 respectively). There was a positive linear association of FT4 with the delta UTRI (P=0.03) whereas there was no association for TSH. Placental function showed a partial mediating role with second trimester UTRI in the associations of maternal thyroid with preeclampsia and birth weight (P=0.04; P=0.001 respectively).

Conclusion: Maternal thyroid hormones are associated with placental function. Low FT4 levels were associated with differential changes in the physiological decrease in placental vascular resistance: a large decrease in the umbilical compartment but a smaller decrease in the uterine compartment. 2nd trimester UTRI showed a limited mediating role of placental function in the association of maternal thyroid with the adverse outcomes.