gestation; immunoassay; pregnancy; reference interval; thyroid disease

BackgroundTo establish maternal thyroid-stimulating hormone (TSH) reference ranges for first trimester screening from 11 + 0 to 13 + 6 weeks of gestation.MethodsA total of 10 592 singleton and 201 twin consecutive Caucasian pregnant women who underwent simultaneously prenatal first trimester Down's syndrome screening and thyroid function screening from January 2010 to November 2017 were included in the study. Women with positive antithyroid peroxidase antibody (TPOAb) and positive personal history of thyroid disease were previously excluded. TSH was measured by immunochemiluminescent assay on ci 16200 Abbott Architect analyzer. Nonparametric percentile method (also known as CLSI C28.A3) was used for the determination of reference ranges.ResultsWe established reference ranges of TSH for the period of gestation from 11 + 0 to 13 + 6 weeks of pregnancy as 0.16-3.43 mU/L for singleton Caucasian pregnancies and 0.02-2.95 mU/L for twin Caucasian pregnancies. The median (IQR) of TSH for singleton pregnancies was higher than that for twin pregnancies (1.25 mU/L (0.83-1.81) vs 0.84 (0.37-1.47), respectively; P < .0001).ConclusionsEach first trimester screening center should be aware of which type of immunoassay their laboratory uses. TSH reference ranges in women during the first trimester of pregnancy are lower than those for general population. Twin pregnancies have lower TSH than singleton pregnancies.