Levothyroxine supplementation in pregnancy not linked to prematurity risk

1. Levothyroxine usage for hypothyroidism in pregnancy was not linked to elevated prematurity risk

Evidence Rating Level: 2 (Good)

While untreated hypothyroidism is linked to obstetrical complications, including prematurity, the impact of levothyroxine therapy during late gestation remains unclear. This population-based cohort study aimed to see if levothyroxine supplementation in late pregnancy affects the risk of prematurity. 9,489 pregnancies with hypothyroidism from the Quebec Pregnancy Cohort (1998–2015). Of these, 70.3% received levothyroxine during the final two months of pregnancy. Prematurity was defined as delivery before 37 weeks of gestation. The study assessed levothyroxine exposure (yes/no), duration (>30 days), cumulative dose (>7125 mcg), and mean daily dose (>125 mcg/day). Additionally, dosage variations after the second trimester were examined. There was no significant association between levothyroxine use and prematurity risk (adjusted RR 0.98; 95% CI 0.81–1.20). Neither longer exposure (aRR 0.99; 95% CI 0.81–1.21) nor higher cumulative (aRR 0.97; 95% CI 0.73–1.27) or daily doses (aRR 0.95; 95% CI 0.72–1.26) influenced prematurity risk. Increasing the levothyroxine dose after the second trimester also showed no effect (aRR 0.84; 95% CI 0.67–1.05), though a 30% or less dose increase was linked to a 28% reduced prematurity risk (aRR 0.72; 95% CI 0.54–0.96). These findings suggest levothyroxine supplementation in late pregnancy is safe and does not elevate prematurity risk. The observed reduced risk with modest dosage increases may reflect better management of hypothyroidism. This study supports current guidelines endorsing levothyroxine use during pregnancy and highlights the need for further research into the complex interplay between thyroid function and prematurity.

Click to read the study in BMC Medicine

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