Background: The effect of subclinical hypothyroidism (SCH) on adverse perinataloutcomes is unclear, and thyroid-stimulating hormone (TSH) reference values inpregnancy are controversial. Objective: To evaluate the effects of thyroid peroxidaseantibody (TPOAbs) negative SCH on perinatal outcomes according to the differentTSH reference values. Methods: A total of 554 pregnant women, including 509euthyroid and 45 subclinical hypothyroid (TSH > 3 mIU/L) pregnant women, wereincluded in this prospective case-controlled study. All pregnant women were inthe third trimester and were TPOAbs negative. Thyroid functions were evaluatedusing trimester-specific reference values recommended by the American Collegeof Obstetrics and Gynecology (ACOG) (TSH > 3 mIU/L) and the American ThyroidAssociation (ATA) (TSH ≥ 4 mIU/L) guidelines. Results: Neonatal mortality insubclinical hypothyroidism with a ...