Maternal euthyroid hyperthyroxinemia may reduce preterm birth rate in multiparous women

Objective:

To determine whether L-Thyroxine (L-T4) replacement, sustaining pregnancy-induced maternal euthyroid hyperthyroxinemia by keeping free thyroxine level in the high normal reference range during the whole pregnancy, might reduce preterm birth rate in singleton gestation. Maternal euthyroid hyperthyroxinemia is defined as elevated serum thyroxine either total or free in the absence of hyperthyroidism and preterm birth is defined as delivery before completed 37 weeks of gestation.

Materials and methods:

The study group encompassed 918 pregnant women below 12 weeks of gestation, 383 primiparous and 535 multiparous. They were prospectively enrolled and retrospectively analysed. The control group included 6414 pregnant women, 4338 primiparous and 2076 multiparous from the „Baby Care” surveillance program, who had completed a questionnaire about preterm birth risks and medication intake during pregnancy with particular consideration of L-T4 administration. Since some women already ingested L-T4 before pregnancy, two groups A and B were formed. Preterm birth rates were determined in group A, women without L-T4 administration before pregnancy, and in group B, women already administered L-T4 prior to conception. Analysis was made by means of the Chi-Square test with a p-value < 0.05 considered statistically significant separately for primiparous and multiparous.

Results:

The preterm birth rates in group A declined by 51% (p = 0.01) and in group B by 87% (p = 0.001) in multiparous women, while in primiparous the preterm birth rate was similar in the study and the control groups.

Conclusion:

Sustaining pregnancy-induced maternal euthyroid hyperthyroxinemia with L-T4 may significantly reduce preterm birth rate in multiparous women.