Suboptimal thyroid function in pregnancy is associated with adverse obstetric outcomes but it is unclear whether levothyroxine treatment, initiated during pregnancy is beneficial. We investigated whether correction of abnormal thyroid function during pregnancy is associated with improved obstetric outcomes.
Retrospective analysis of the Controlled Antenatal Thyroid Screening (CATS) study with obstetric outcomes obtained through data-linkage in the Secure Anonymised Information Linkage (SAIL) databank. Setting: Welsh participants from CATS. Participants: 13,506 pregnant women; 12,874 women had normal thyroid function, 320 had subclinical hypothyroidism (SCH), 281 had isolated hypothyroxinemia (IH) and 31 had overt hypothyroidism. Main Outcome Measures: Odds of stillbirths (fetal demise after 24 weeks gestation), Caesarean sections, prematurity and abnormal birth-weight by thyroid and treatment status.
Untreated women with SCH had increased odds of stillbirth compared to women with normal thyroid function OR = 5.73 (95%CI 1.74, 18.9) p = 0.003. No stillbirths occurred in women receiving levothyroxine. In analysis of women with IH, untreated women had an increased risk of early (≤37 weeks) caesarean section than those who received levothyroxine (6% vs 0%) p = 0.006. Untreated women with IH also had earlier mean gestational age 38.8 (SD 2.34) weeks vs. 39.7 (SD 1.94) weeks p = 0.002 and lower mean birth-weight 3353 g (SD 639) vs. 3558 g (SD 532) p = 0.004.
Both SCH and IH are associated with adverse obstetric outcomes. In IH levothyroxine treatment is associated with favourable effects on, gestational age at delivery and early Caesarean section rates. Levothyroxine may also have some protective impact on stillbirths in SCH.