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To determine whether treatment of pregnant women identified with either subclinical hypothyroidism or hypothyroxinemia in the first half of pregnancy improves cognitive function in offspring at five years of age.
This multicenter study consists of two randomized, double-masked, placebo-controlled trials run in parallel. Women with a singleton gestation presenting for care before 20 weeks’ gestation underwent thyroid screening using serum TSH and free T4. Women with a TSH ≥ 4.0 mU/L and a free T4 in the normal range (0.86-1.9 ng/dL) were considered to have subclinical hypothyroidism. Those with a normal TSH (0.08 - 3.99 mU/L) but a free T4 < 0.86 ng/dL were considered to have hypothyroxinemia. Eligible and consenting women were randomized to either levothyroxine replacement or an identical placebo. Maternal thyroid function was assessed monthly and study drug adjusted to attain TSH or free T4 goals. Sham adjustments in placebo were used to maintain blinding. Developmental testing of offspring was performed annually including DAS II at age 3-years and IQ score using WPPSI-III at 5 years of age. If an infant was lost to follow-up after 3 years of age, the DAS II was substituted for the primary outcome. If an infant died prior to 3-year testing they were assigned a score of zero. Sample size estimates for each strata were based on 80% power to detect a 5-point IQ difference between treatment groups.
Between October 2006 and October 2009, 97,226 pregnant women underwent thyroid screening. There were 3,058 women (3.1%) identified with subclinical hypothyroidism, of which 677 (22.1%) were eligible and randomized. There were 2,805 women (2.9%) identified with hypothyroxinemia and 526 (18.8%) were eligible and randomized. The mean gestational age at randomization was 17 weeks. WPPSI-III scores at 5 years of age were obtained in 1,110 (92.3%) of the 1203 offspring, 23 children were lost to follow-up after DAS II testing, and 22 infants died prior to 3-year testing.
Treatment of women identified with either subclinical hypothyroidism or hypothyroxinemia during the first half of pregnancy did not result in improved cognitive outcome in offspring at 5 years of age.