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The outcome of pregnancy is usually normal in women with low serum free thyroxine concentrations
The background of the study. Women who have subclinical hypothyroidism during pregnancy have an increased risk of preterm delivery and other adverse outcomes of pregnancy, and the development of their infants may be impaired. The development of the infants of pregnant women who have hypothyroxinemia also may be impaired. This study was done to determine whether adverse outcomes of pregnancy and perinatal morbidity were increased in pregnant women with hypothyroxinemia.
How the study was done. Serum thyrotropin (TSH) and free thyroxine (T4) were measured during the first 20 weeks of gestation in 17,298 women with a singleton pregnancy. The women were said to have hypothyroxinemia if they had normal serum TSH and low free T4 concentrations, and subclinical hypothyroidism if they had high serum TSH and normal free T4 concentrations. The adverse outcomes of pregnancy studied included gestational hyperthyroidism and preeclampsia, and the adverse infant outcomes studied included preterm delivery, birth weight, respiratory distress syndrome, and brain hemorrhage.
The results of the study. Among the 17,298 women, 1 percent had hypothyroxinemia and 3 percent had subclinical hypothyroidism. The women with hypothyroxinemia and those with subclinical hypothyroidism differed from normal women several ways (Table 1).
| Table 1. Characteristics and Outcomes in Pregnant Women with Hypothyroxinemia, Normal Thyroid Function, and Subclinical Hypothyroidism. | |||
Hypothyroxinemia |
Normal |
Subclinical Hypothyroidism |
|
| Weight (lb) | 178 |
171 |
176 |
| Gestational hypertension | 8% |
9 % |
9% |
| Severe preeclampsia | 4% |
5% |
6% |
There were no differences in the frequency of preterm delivery in the three groups, except that delivery at 34 weeks of gestation or less was more common in the women with subclinical hypothyroidism. The frequency of birth weight 
1000, 2500, and 
4000 g was similar in the three groups. Among the perinatal outcomes (Table 2), intraventricular hemorrhage was more common in the infants of women with hypothyroxinemia, and the respiratory distress syndrome was more common in the infants of women with subclinical hypothyroidism.
| Table 2. Neonatal Outcomes in Women with Hypothyroxinemia, Normal Thyroid Function, and Subclinical Hypothyroidism. | |||
Hypothyroxinemia (n = 233) |
Normal (n = 16,011) |
Subclinical Hypothyroidism (n = 598) |
|
| Admission to intensive care | 1.3% |
2 % |
4% |
| Respiratory distress syndrome | 1.3% |
1.5% |
2% |
| Intraventricular hemorrhage | 0.4% |
0.1% |
0.2% |
| Fetal and neonatal deaths | 0% |
0.7% |
1.5% |
The conclusions of the study. The frequency of adverse outcomes of pregnancy and perinatal morbidity is slightly higher among women who have subclinical hypothyroidism than among those who have hypothyroxinemia.
The original article. Casey BM, Dashe JS, Spong CY, McIntire DD, Leveno KJ, Cunningham GF. Perinatal significance of isolated maternal hypothyroxinemia identified in the first half of pregnancy. Obstet Gynecol 2007;109:1129-35.
| Next Article: Abnormalities of serum thyrotropin concentrations are often transient | ||
| Thyroid Digest Index | | | July 2007 Thyroid Digest |
