Thyroid Research Thyroid Research Archive Pregnancy

Subclinical hypothyroidism is associated with premature delivery

(July 2005)

The background of the study. Hypothyroidism in pregnant women is associated with complications of pregnancy and abnormal fetal development. In this study, pregnant women were screened for hypothyroidism, and the outcome of the pregnancy was compared in those with subclinical hypothyroidism (high serum thyrotropin [TSH] and normal free thyroxine [T4] values) and normal women.

How the study was done. Serum TSH was measured in 17,298 women who were ≤20 weeks pregnant and who later delivered a singleton infant. Serum free T4 was measured in those who had a serum TSH concentration >3.0 mU/L; if the value was low the woman was referred for therapy and excluded from the study. The outcomes of pregnancy evaluated in the women with subclinical hypothyroidism and the normal women evaluated included pregnancy-related hypertension, severe preeclampsia, placental abruption, and preterm birth.

The results of the study. Among the 17,298 women, 404 (2.3 percent) had subclinical hypothyroidism, and 15,689 had normal serum TSH values. The 404 women with subclinical hypothyroidism were slightly older than the normal women, but parity, body-mass index, and duration of pregnancy at the time of testing were similar in the two groups.

The frequency of pregnancy-related hypertension and severe preeclampsia was similar in the two groups, but placental abruption (rupture) was more frequent in the women with subclinical hypothyroidism. The mean gestational age at delivery in the two groups was 39 weeks, but delivery at or before 34 weeks was more common in the group with subclinical hypothyroidism. The mean birth weight of the infants in the two groups was similar, as was the frequency of birth weight <2500 g and <1500 g, but more of the infants in the group with subclinical hypothyroidism were admitted to the intensive-care unit and had respiratory distress. There were no differences in major malformations or fetal or neonatal deaths.

The conclusions of the study. Pregnant women who have subclinical hypothyroidism at or before 20 weeks of gestation have an increased risk of preterm delivery.

The original article. Casey BM, Dashe JS, Wells CE, McIntire DD, Byrd W, Leveno KJ, Cunningham FG. Subclinical hypothyroidism and pregnancy outcomes. Obstet Gynecol 2005;105:239-45.