The American College of Obstetricians and Gynecologists ( ACOG ) said there is no evidence that identifying and treating pregnant women with subclinical hypothyroidism improves either maternal or infant outcomes. Therefore, ACOG recommended against routine screening.

" The issue has been whether thyroid screening should be a routine test during prenatal care, " said Sarah J. Kilpatrick, chair of ACOG's Committee on Obstetric Practice, which issued the Committee Opinion. " Some groups argue that identifying and treating asymptomatic hypothyroidism ( subclinical hypothyroidism ) will improve outcomes for pregnant women and their infants. With the information we have at this time, there isn't any proven health benefit."

Untreated symptomatic hypothyroidism can lead to preterm birth, low birth weight, and decreased mental ability in infants. For women, it can lead to preeclampsia or placental abruption.

Thyroid hormones, specifically thyroxine, are essential for normal fetal brain development. During the first trimester when fetal brain development begins, the fetus is entirely dependent on maternal transfer of thyroid hormones. When pregnant women are truly clinically hypothyroid with both symptoms and abnormal thyroid tests, including both thyroxine and thyroid stimulating hormone ( TSH ) ( from iodine deficiency or other cause ), throughout pregnancy, it can have a dramatic, negative impact on infant brain development.

A subset of women ( estimated at 2%-5% of all women ) have subclinical hypothyroidism. Subclinical hypothyroidism has no symptoms and while one test of thyroid function is mildly abnormal, the thyroxine level itself is still normal. Several studies have suggested a relationship between subclinical hypothyroidism and preterm delivery and impaired brain development in children, leading some to call for routine screening of pregnant women. However, the benefit of treatment to either mother or fetus has not yet been demonstrated in pregnant women with subclinical hypothyroidism.

The US Preventive Services Task Force says there must be demonstrated improvement in important health outcomes of those individuals identified through screening before recommending routine screening of asymptomatic people.

According to ACOG, thyroid testing is not a routine part of prenatal care and should be limited to women with symptoms of thyroid disease and those with a history of thyroid disease or other medical conditions associated with it, such as diabetes. Pregnant women who have been diagnosed with overt thyroid disease should be treated throughout pregnancy and during the postpartum period.

Source: ACOG, 2007

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