Public Release: 

Levothyroxine treatment in women with thyroid antibodies may not increase live birth rate

The Endocrine Society

NEW ORLEANS--Treating women who have thyroid antibodies, but normal thyroid function, with a medicine called Levothyroxine does not make them more likely to deliver a live baby, new research from the United Kingdom suggests. The research will be presented Saturday, March 23 at ENDO 2019, the Endocrine Society's annual meeting in New Orleans, La., and published in The New England Journal of Medicine.

"Levothyroxine treatment, started before pregnancy, in women with normal thyroid function and thyroid peroxidase antibodies who have a history of miscarriage or infertility, does not improve the chances of live birth," said lead author Rima Dhillon-Smith, Ph.D., M.R.C.O.G., M.B.Ch.B., an academic clinical lecturer at the University of Birmingham in the United Kingdom. "We were surprised because previous small studies suggested there could be a benefit with levothyroxine treatment in women with thyroid antibodies."

The authors conducted a double-blind study of women with normal thyroid function between 16 and 41 years of age who were positive for thyroid peroxidase antibodies, had a history of miscarriage or infertility, and were trying to conceive naturally or with assisted conception. Between 2011 and 2016, the researchers randomized and followed-up 470 women who received 50 mcg daily levothyroxine and 470 who received daily placebo, through 34 or more weeks of gestation in 49 hospitals throughout the U.K. The participants began taking the medication before they conceived and continued until the end of their pregnancy.

Outcomes in both groups were similar: 266 of the 470 (56.6 percent) women who received levothyroxine, and 274 of 470 (58.3 percent) who received placebo, became pregnant; 176 women (37.4 percent) taking levothyroxine, and 178 (37.9 percent) taking placebo, had live births (relative risk 0.97, p=0.74; absolute risk difference, -0.4 percent).

Other pregnancy and neonatal outcomes were similar in both groups, including pregnancy loss and preterm birth. Serious adverse event rates were also similar: 6 percent of women in the levothyroxine group and 4 percent in the placebo group (p=0.14).

"Thyroid peroxidase antibodies are found in the blood in approximately one in 10 women who have normal thyroid function, and they have been linked to increased risk of miscarriage and preterm birth," Dhillon-Smith said.

"The American Thyroid Association and other international guidelines currently recommend the consideration of levothyroxine treatment for women with thyroid antibodies, as there is thought to be minimal chance of harm and a potential to help increase the chance of having a live birth," she said.

"As our study was large and of high quality, we can be confident that levothyroxine does not improve pregnancy success for women with thyroid antibodies and normal thyroid function and therefore should not be recommended or used in clinical practice," Dhillon-Smith said. "This will mean no longer providing unnecessary medication to women who do not need it."

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The study was funded by the National Institute for Health Research (NIHR) in the U.K.

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