Perinatal consequences of maternal hypothyroidism in early pregnancy and inadequate replacement

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Abstract
Maternal hypothyroidism may be associated with a variety of pregnancy complications. We have evaluated the perinatal consequences of maternal hypothyroidism in early and late gestation. Retrospective study of pregnant women, with quasi-experimental design comparing different subjects. Forty-three pregnancies in 42 women with hypothyroidism--either biochemically hypothyroid or with a history of hypothyroidism on adequate replacement--and no other preexisting medical conditions. Free thyroxine index (FTI), TSH, and haematocrit at initial antepartum presentation and near term gestation. Pregnancy outcome variables including: rate of Caesarean section performed for fetal distress in labour, neonatal weight percentile, and gestational age at birth. Of 42 hypothyroid pregnancies, six were complicated by fetal distress in labour leading to Caesarean section. Five of these six were severely hypothyroid (defined as FTI < or = 0.6 (normal range 1.1-4.4)) on initial antepartum presentation. In contrast, of the 36 pregnancies without fetal distress, only four initially presented severely hypothyroid (P < 0.001). Conversely, 56% (5/9) of pregnancies which initially presented severely hypothyroid were subsequently complicated by Caesarean section for fetal distress in labour. This compared to 3% (1/33) among those who presented either mildly hypothyroid or euthyroid on replacement (P < 0.0001). Fetal distress correlated with low FTI (P < 0.001) and high TSH at initial presentation. However, it was independent of FTI near term. A relation between fetal distress and TSH near term did not reach statistical significance. Fetal distress also correlated with low maternal haematocrit on admission to labour and delivery (P < 0.05), but was independent of haematocrit and gestational age at initial presentation, neonatal weight percentile, and gestational age at birth. Severe maternal hypothyroidism early in gestation is strongly associated with fetal distress in labour. This suggests that (1) inadequate maternal replacement leads to fetal distress and (2) maternal thyroid status in early gestation may exert irreversible effects on the fetus, the placenta, and/or on subsequent maternal adaptations to pregnancy. Early adequate replacement therapy is especially prudent in pregnant women presenting with severe hypothyroidism.