Thyroid hormone treatment is associated with a decreased risk of pregnancy loss among women with subclinical hypothyroidism, but is linked to an increased risk of other pregnancy-related adverse outcomes, according to a study published in BMJ.

Rozalina G McCoy, MD, from the Mayo Clinic in Rochester, Minnesota, and colleagues conducted the first national study to evaluate the effectiveness and safety of thyroid hormones for pregnant women with subclinical hypothyroidism. The study included 5,405 pregnant women with subclinical hypothyroidism, defined as untreated thyroid-stimulating hormone concentration between 2.5 and 10 mIU/L.

A total of 853 pregnant women with a mean pre-treatment thyroid-stimulating hormone concentration of 4.8 mIU/L were treated with thyroid hormone, and 4,562 women with a mean baseline concentration of 3.3 mIU/L were not treated.

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Women who were treated with thyroid hormone had lower adjusted odds of pregnancy loss (odds ratio, 0.62) compared with women who were not treated. However, women who received treatment had higher odds of preterm delivery (odds ratio, 1.60) gestational diabetes (odds ratio, 1.37), and preeclampsia (odds ratio, 1.61).

The researchers also found that the adjusted odds of pregnancy loss were lower among women who received treatment if their pre-treatment thyroid-stimulating hormone concentration was between 4.1 and 10 mIU/L (odds ratio, 0.45) but not if the concentration was between 2.5 and 4.0 mIU/L (odds ratio, 0.45).

“To facilitate the decision making process for pregnant women with subclinical hypothyroidism, clinicians are encouraged to use a shared decision making approach,” the study authors wrote. “With this approach, clinicians can discuss with patients the uncertainty behind our treatment recommendations and explore what is important to them when making decisions about their health with the goal of reaching a decision about treatment that best fits their situation.”


  1. Maraka S, Mwangi R, McCoy RG, et al. Thyroid hormone treatment among pregnant women with subclinical hypothyroidism: US national assessment. BMJ. 2017. doi:10.1136/bmj.i6865