Treating Subclinical Hypothyroidism in Pregnancy

pregnancy, subclinical hypothyroidism, treatment, miscarriage, preeclampsia
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A study published in the British Medical Journal has shown that treating subclinical hypothyroidism during pregnancy can reduce the risk of miscarriage. At the same time, the researchers found that women being treated for subclinical hypothyroidism face an increased risk of pregnancy complications that include premature delivery, preeclampsia, and gestational diabetes.

About the Study

The study evaluated more than 5,000 women who were sub-clinically hypothyroid, with thyroid stimulating hormone (TSH) levels between 2.5 and 10 mIU/L.

The researchers found that the women treated with thyroid hormone replacement drugs had a 38 percent lower risk of miscarriage, compared to the untreated group. Importantly, the results only applied to those women who had a TSH level of 4.1 mIU/L of higher before treatment.

The reduced miscarriage risk was not seen in the women who had TSH levels from 2.5 to 4.0 mIU/L. In fact, these women had a significantly higher risk of developing gestational hypertension—a condition that can lead of pre-eclampsia.

Preeclampsia is a condition that can develop in pregnancy that causes high blood pressure. Preeclampsia can lead to full eclampsia, which can cause liver or kidney failure, heart failure, seizures, convulsions, and can be fatal to both mother and baby.

Changes to the Guidelines

The study highlights a change in recommendations for women during pregnancy. In the past, treatment has been recommended for pregnant women with hypothyroidism whose thyroid stimulating hormone (TSH) levels fell between 2.5 and 4.0 mIU/L.

The American Thyroid Association (ATA) also released new guidelines in 2017 that echo the recommendations of the British Medical Journal study. According to the ATA, because of evidence that pregnancy outcomes can be adversely affected, the guidelines experts are recommending treatment in women who have overt hypothyroidism, defined as a TSH level above 4.1 mIU/L.

Treatment can be considered for women with subclinical hypothyroidism—a TSH between 2.5 and 4.0 mIU/L—if they have elevated thyroid peroxidase (TPO) antibodies that are evidence of autoimmune Hashimoto’s thyroiditis.

According to the study’s lead author, Spyridoula Maraka, MD:

Continuing to offer thyroid hormone treatment to decrease the risk of pregnancy loss is reasonable for women with TSH concentrations of 4.1-10.0 mIU/L. However, given the smaller magnitude of effect in women with lower TSH levels of 2.5-4.0 mIU/L, and in light of the possible increased risk of other adverse events, treatment may need to be withheld in this group.

As noted, however, the ATA guidelines recommend that physicians consider a woman’s TPO antibody status in making a decision to treat subclinical hypothyroidism. Treatment can still be offered to women who are TPO-positive and who have a TSH level between 2.5 and 4.0 mIU/L.

What Is Subclinical Hypothyroidism?

Subclinical hypothyroidism affects around 15 percent of American women while pregnant. Having sufficient levels of thyroid hormone is essential for the healthy neurological development of a fetus, especially during the first trimester, when a mother provides thyroid hormone to the developing fetus.

After the first trimester, the fetal thyroid has developed and starts producing its own thyroid hormone, to supplement the maternal thyroid hormone.

Maternal hypothyroidism during pregnancy is associated with a variety of negative pregnancy outcomes, including miscarriage, prematurity, low birth weight, stillbirth, pre-eclampsia, gestational diabetes, and lowered IQ levels in children.

Your Next Steps?

If you are pregnant and have subclinical hypothyroidism—but you are TPO-negative—the researchers recommend a discussion with your physician. According to the study findings:

To facilitate the decision making process for pregnant women with subclinical hypothyroidism, clinicians are encouraged to use a shared decision making approach. 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.

A Word From Verywell

It’s important to point out that the study was observational, and not a random, controlled clinical trial. To that end, further research is needed to determine whether or not giving thyroid hormone replacement to pregnant women improves the chance of having a healthy pregnancy, or to define a more specific cut-off point for offering treatment to women with subclinical hypothyroidism during pregnancy.

Another issue that deserves further study is the timing of treatment. Miscarriage most commonly occurs during the first trimester, the same period when the fetus relies on the mother as a sole source of thyroid hormone. The researchers speculated that it's possible that treatment of subclinical hypothyroidism may be needed only during the first trimester of pregnancy. More research on these issues will help to clarify these issues further.


Elizabeth AE, et. al.  "2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum." Thyroid, Volume 27, Number 3, 2017. Online:

Spyridoula M et. al. “Thyroid hormone treatment among pregnant women with subclinical hypothyroidism: US national assessment.” British Medical Journal. J 2017;356:i6865 doi: 10.1136/bmj.i6865 2017. Online: