Treating Pregnant Women for Subclinical Thyroid Disease Is Unlikely to Improve Cognition in Their Progeny

The possibility that subclinical thyroid disease during pregnancy can impair the offspring's cognitive development has long been postulated (N Engl J Med 1999; 341:549). To explore this issue further, investigators screened >97,000 women with singleton pregnancies before 20 weeks' gestation to identify those with subclinical hypothyroidism (defined by thyrotropin levels ≥4.00 mU/L and normal levels of free thyroxine [T₄]) or hypothyroxinemia (normal thyrotropin levels and free T₄ levels <0.86 ng/dL). In two parallel trials, 677 women with subclinical hypothyroidism and 526 with hypothyroxinemia were randomized to receive levothyroxine or placebo beginning at a mean 17 or 18 weeks' gestation. Their progeny underwent yearly neurodevelopmental…