Does levothyroxine increase live births in women with thyroid peroxidase antibodies?

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Presence of thyroid peroxidase antibodies has been associated with an increased risk of miscarriage and preterm birth (PTB) and a number of previous trials have indicated that levothyroxine may reduce incidence of these birth complications. However, a recent study, appearing in The New England Journal of Medicine, has found that levothyroxine usage does not impact live birth rates. 

The double-blind, placebo-controlled trial enrolled 940 euthyroid British women and randomly assigned participants to receive either 50 µg once daily of levothyroxine or placebo. Women were eligible for enrollment if they were 16 to 40 years old, had a history of miscarriage or infertility, and were trying to conceive in the subsequent 12 months. They were given oral capsules before conception through the end of pregnancy. The primary outcome of the study was live birth after at least 34 weeks’ gestation. 

A total of 266 women in the placebo group (58.3%) and 274 women in the levothyroxine group (56.6%) became pregnant. In the levothyroxine group, the live birth rate was 37.4% (176 of 470 women) and in the placebo group, it was 37.9% (178 of 470 women) (relative risk, 0.97; 95% CI 0.83-1.15, P= 0.74; absolute difference, -0.4 percentage points; 95% CI -6.6-5.8). Furthermore, 5.9% of women in the levothyroxine group experienced serious adverse events compared to 3.8% in the placebo group. There were no significant differences between the two groups in terms of other pregnancy outcomes, such as pregnancy loss or PTB, or in neonatal outcomes. 

Based on their findings, the authors suggest that using levothyroxine does not improve live birth rate for women with thyroid peroxidase antibodies.