The effects of pregnancy on the thyroid gland are profound. The gland is required to produce a 50% increase in thyroxine (T4) and triiodothyronine (T3). While a healthy thyroid should respond well to pregnancy, women with limited thyroidal reserve or iodine deficiency who become pregnant can develop hypothyroidism.

“Women in high-risk groups need to be tested as early as possible for hypothyroidism during the first trimester.”

Knowledge about the interaction between the thyroid gland and pregnancy has exploded over that last 15 to 20 years. In response to the emerging data, the American Thyroid Association (ATA) recently created clinical guidelines on the diagnosis and treatment of thyroid disease during pregnancy and postpartum. They were published in the October 2011 issue of Thyroid.

Pregnancies At-Risk for Thyroid Disease

According to the ATA guidelines, about 10% of pregnant women are thyroid peroxidase (TPO)-antibody positive but have normal thyroid function. These women have a two- to four-fold increased risk of miscarriage when compared with women who don’t have the antibody. Among women without the antibody but with slightly elevated thyrotropin (TSH) levels, the risk of miscarriage is increased by 60% when compared with women without hypothyroidism. Women with either the TPO antibody or mild hypothyroidism are also at risk for preterm delivery. The 10% of all women who are TPO-antibody positive have a 50% chance of developing postpartum thyroiditis.
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Thyroid Screening Recommendations

Women in high-risk groups need to be tested as early as possible for hypothyroidism during the first trimester, according to the ATA guidelines. These groups include (but are not limited to) women:

  • With a history of thyroid dysfunction or prior thyroid surgery.
  • Older than 30 years of age.
  • Who have autoimmune disorders.
  • With a history of miscarriage or preterm delivery.
  • With a family history of thyroid dysfunction.
  • Who are infertile.

Effective Treatments for Thyroid Disease

Trimester-specific reference intervals are required for all thyroid function tests, particularly for TSH and free T4. Circulating total T4 and T4-binding globulin concentrations increase throughout the first trimester and remain high throughout pregnancy. Serum TSH decreases during the first trimester. The ATA guidelines recommend that women with subclinical hypothyroidism who are TPO-antibody positive be treated with levothyroxine. Women with subclinical hypothyroidism who are TPO-antibody negative and not initially treated should be monitored during pregnancy for progression to overt hypothyroidism. The majority of women on levothyroxine prior to pregnancy will need to increase the dose of levothyroxine as soon as possible in the first trimester in order to avoid becoming hypothyroid. All women being treated with levothyroxine during pregnancy should have serum TSH and free T4 measured every 4 weeks until 16 to 20 weeks gestation and at least once during weeks 26 to 32. Also, it’s important that all pregnant women take prenatal vitamins that contain 150 mcg of iodine.

More Research Needed on Thyroid Function

A study that screens women before conceiving and randomizes those with subclinical hypothyroidism, isolated hypothyroxinemia, or TPO-antibody positivity to treatment or no treatment would be of great benefit. So too would analyses that assess the cost-effectiveness of screening for thyroid disease in pregnancy. It also behooves us to assess the impact of iodine supplementation in pregnant women with the mildest form of iodine deficiency, as well as on infant thyroid function and cognition. Lastly, we’d like more confirmation that treatment of women with mild thyroid dysfunction decreases the risk of miscarriage and preterm delivery.