(239) Association Between Levothyroxine Supplementation for Hypothyroidism in Late Pregnancy and Risk of Preterm Delivery

Background: Hypothyroidism in pregnancy is associated with obstetrical and fetal complications. However, whether its management by levothyroxine (LT) supplementation is beneficial to reduce the risk of preterm delivery (PD) is not yet clear.

Objectives: To quantify the risk of PD in pregnancies with hypothyroidism, associated with LT supplementation in late pregnancy.

Methods: A cohort study was conducted within the Quebec Pregnancy Cohort including pregnancies with hypothyroidism from January 1, 1998, through December 31, 2015. To identify pregnancies, a hypothyroidism diagnosis and/or a LT prescription in the year before pregnancy until the end of the first trimester were considered. Follow-up was done until delivery date or the end of the 36th completed week of pregnancy, whichever occurred first. In primary analyses, LT exposure (y/n), total duration, and cumulative dose in the last 2 months before end of follow-up were considered. In secondary analyses, LT dosage before and after the beginning of the second trimester were compared and pregnancies were categorized in increased or constant dosage groups until end of follow-up. Sensitivity analyses considered time-varying LT exposure from the 14th week of pregnancy until the end of follow-up. PD was defined as a delivery before the 37th week of pregnancy. Generalized estimating equations and time-to-event regressions, adjusted for potential confounders, were used to calculate adjusted relative risks (aRRs) and hazard ratios (aHRs), respectively.

Results: Among the 10,742 pregnancies with hypothyroidism, 9,821 eligible individuals with a mean (SD) maternal age of 29.63 (5.52) years were included. Among them, 6,926 (70.52%) were exposed to LT during the last 2 months of follow-up. Adjusting for potential confounders, no association between LT exposure (aRR, 0.98; 95% CI, 0.77–1.24) and the risk of PD was observed, compared to non-exposure. Also, no association between LT duration ( > 30 days: aRR, 1.00; 95% CI, 0.78–1.29) or cumulative dose (> 7,000 μg: aRR; 0.94, 95% CI, 0.69–1.28) in the 2 months before end of follow-up and the risk of PD was observed, compared to non-exposure. Finally, the risk of PD did not vary between those who had increased or constant dosage throughout pregnancy (aRR, 0.85; 95% CI, 0.69-1.06). Similarly, time-to-event analysis did not show any association between LT exposure and the risk of PD (aHR, 0.98; 95% CI, 0.85–1.14).

Conclusions: Levothyroxine supplementation in late pregnancy was not associated with the risk of PD; duration of use and dosage did not impact the risk of PD. Levothyroxine may be considered in pregnancies to reduce the complications of hypothyroidism.