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Confusion exists about whether to screen and treat women for subclinical hypothyroidism during pregnancy and even about what constitutes this condition. Investigators conducted a retrospective cohort study of 188,490 women who delivered in Alberta, Canada, with 59.2% having thyroid stimulating hormone (TSH) measured at least once during the pregnancy and 5.7% having free thyroxine measured at least once. The authors considered 4.0 mIU/L as the upper limit of normal.
TSH testing was most commonly performed during gestational week 5 or 6, with thyroid hormone therapy initiated at a median 7 weeks. The first TSH measurement was <4.0 mIU/L in 92.9% of participants. In all, 1709 women had overt hypothyroidism (TSH >10.0 mIU/L on initial testing),…