Context: Suboptimal thyroid function in pregnancy is associated with adverse obstetric outcomes but it is unclear whether levothyroxine treatment, initiated during pregnancy is beneficial.
Design & Participants: 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. We studied 13,224 pregnant women; 12,608 had normal thyroid function, 340 had subclinical hypothyroidism (SCH), 305 had isolated hypothyroxinemia (IH). 518 women with abnormal thyroid function were randomized to receive levothyroxine (N=263) or no treatment (N=255) at the end of the first trimester.
Main Outcome Measures: Composite measure (primary outcome) of stillbirth, neonatal death, preterm delivery <34 weeks, APGAR score at 5 minutes <7, length of hospital stay >5 days. Secondary analyses included early gestational age (<37 weeks), early caesarean sections (<37 weeks).
Results: In individuals with abnormal thyroid function randomized to treatment or control, treatment had no discernible effect on the composite outcome. 29 events occurred in the untreated group vs 22 in the treated. OR (treated) = 0.75 95%CI (0.40, 1.40). Untreated women with SCH had increased odds of stillbirth compared to women with normal thyroid function OR=4.37 (95%CI 1.04, 18.3). No stillbirths occurred in women on levothyroxine. Untreated women with IH had increased odds of an early gestational age at delivery (<37 weeks) than women with normal thyroid function OR=1.58 (95%CI 1.04, 2.50). Women with IH randomized to receive treatment with levothyroxine had reduced odds of early gestational age at delivery OR=0.37 (95%CI 0.14, 0.99) and early caesarean sections (0% vs 4%) p=0.04 than untreated women.
Conclusion: Both SCH and IH were associated with key adverse obstetric outcomes. Although there was no difference in composite outcome there were some benefits observed with levothyroxine therapy. Larger studies are required to confirm the benefits of screening and treatment in pregnancy.