Levothyroxine usage for hypothyroidism in pregnancy not linked to elevated prematurity risk: Study
A new study published in BMC Medicine found that levothyroxine supplementation during late pregnancy in individuals with hypothyroidism does not significantly alter the risk of prematurity. Hypothyroidism in pregnancy is known to be associated with obstetrical and fetal complications, including preterm birth. However, whether levothyroxine therapy influences the risk of premature delivery has remained uncertain. This study was conducted by Maya L. and fellow researchers.
The Quebec Pregnancy Cohort was used to conduct a cohort study comparing data for hypothyroid pregnancies from January 1, 1998, through December 31, 2015. The principal analysis determined levothyroxine exposure by presence or absence, total duration, mean daily dose, and cumulative dose during the two months before delivery (preterm gestations) or prior to 37 weeks of gestation (term gestations).
Levothyroxine dosage was contrasted prior to and after the initiation of the second trimester, grouping pregnancies into rising or stable dosage categories. A second method viewed levothyroxine as a time-varying daily exposure from gestational week 14 through delivery or 37 weeks, whichever occurred first. Prematurity was defined as delivery before 37 weeks of gestation, and term pregnancies were censored at this time point because they were no longer at risk of preterm delivery. Statistical models were performed using generalized estimating equations and Cox proportional hazard models with adjustment for potential confounders.
Key Findings
9,489 hypothyroid pregnancies were included in the analysis.
Of these, 6,667 (70.3%) participants were exposed to levothyroxine during the two months prior to delivery.
Adjusted analysis demonstrated no considerable correlation of levothyroxine exposure with the risk of prematurity (aRR, 0.98; 95% CI, 0.81–1.20).
No substantial difference in risk of prematurity was observed according to levothyroxine duration (>30 days: aRR, 0.99; 95% CI, 0.81–1.21), cumulative dose (>7,125 mcg: aRR, 0.97; 95% CI, 0.73–1.27), or mean daily dose (>125 mcg/day: aRR, 0.95; 95% CI, 0.72–1.26).
No greater or lesser risk of prematurity was identified between groups with increased or stable dosing (aRR, 0.84; 95% CI, 0.67–1.05).
Time-varying exposure analysis replicated no statistically significant relationship between use of levothyroxine and prematurity risk (aHR, 0.95; 95% CI, 0.81–1.11).
The study authors concluded that levothyroxine supplementation in pregnancy among patients with hypothyroidism was not linked to lower prematurity risk. These results add to clinical practice guidelines by affirming that levothyroxine is safe to administer during pregnancy and does not have to be discontinued just to avert preterm birth.
Reference:
Laham M, Sheehy O, Bérard A. Association between levothyroxine supplementation for hypothyroidism in late pregnancy and risk of prematurity: a population-based cohort study. BMC Med. 2025 Feb 21;23(1):105. doi: 10.1186/s12916-025-03934-1. PMID: 39985026; PMCID: PMC11846315.
