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No birth rate gains from levothyroxine in pregnancy



Treatment with levothyroxine does not improve the live birth rate in women with thyroid peroxidase antibodies before conception, according to data presented at the annual meeting of the Endocrine Society.

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Until now, the evidence for the use of levothyroxine in pregnant women with thyroid peroxidase antibodies but normal thyroid function has been inconclusive, Rima K. Dhillon-Smith, MBChB, PhD, of the University of Birmingham (England), and her coauthors said in a paper published simultaneously with the meeting presentation March 23 in the New England Journal of Medicine.

Previous studies have shown that women with thyroid peroxidase antibodies but normal thyroid function have a nearly fourfold higher risk of miscarriage and twofold higher risk of preterm birth, compared with women who don’t have the antibodies.

In the new double-blind study, 952 women with thyroid peroxidase antibodies, normal thyroid function, and a history of miscarriage or infertility were randomized either to daily 50 mcg levothyroxine or placebo, taken from conception to the end of pregnancy.

The rate of pregnancy was similar in the levothyroxine and placebo groups (56.6% vs. 58.3%, respectively), as was the live birth rate (37.4% vs. 37.9%), despite the observation that the levothyroxine group had consistently lower serum thyrotropin and higher free T4 concentrations than did the placebo group.

There were also no significant differences between the two groups in secondary outcomes of miscarriage, preterm birth, or neonatal outcomes such as birth weight.

Researchers also saw no statistically significant differences in the rate of serious adverse events or in the number of women who showed abnormal results on thyroid function tests.

The authors noted that the dosage of levothyroxine used in the study was fixed, leaving the possibility that “the dose may need to be adjusted depending on the participant’s body weight, thyroid peroxidase antibody level, or thyrotropin concentration.”

Existing guidelines from the American Thyroid Association acknowledge the lack of evidence in favor of levothyroxine decreasing the risk of pregnancy loss. However, the guidelines also state that it can be considered in antibody-positive, euthyroid women with a history of loss, “given its potential benefits in comparison with its minimal risk.”

The study was supported by the National Institute for Health Research. No conflicts of interest were declared.

SOURCE: Dhillon-Smith R et al. N Engl J Med. 2019 March 23. doi: 10.1056/NEJMoa1812537

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