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Subclinical hypothyroidism is defined as elevated thyroid stimulating hormone (TSH) levels along with normal thyroid hormone concentrations; however, exactly what TSH level should be considered as elevated is debatable, particularly during pregnancy, when changes in thyroid physiology result in lower TSH concentrations. The American Thyroid Association has recommended upper TSH thresholds of 2.5 mIU/L during the first trimester of pregnancy and 3.0 mIU/L during the second and third trimesters (Thyroid 2011; 21:1081), but raising the normal cutoff to 4.0 mIU/L has been proposed (Thyroid 2017; [in press]). Some observational studies have suggested adverse pregnancy outcomes in women with subclinical hypothyroidism.
Investigators used a nationa…