Free thyroxine reference interval in each trimester of pregnancy determined with the Roche Modular E-170 electrochemiluminescent immunoassay

ObjectiveTo determine using a simplified study design trimester-specific FT4 reference intervals in pregnancy with the Roche Modular immunoassay in routine use.Design and methodsSurplus blood from 300 women in each trimester, drawn at documented times in the gestation, and from 40 age-matched nonpregnant women were assayed for FT4, thyroid stimulating hormone (TSH) and antithyroid peroxidase autoantibody (anti-TPO).ResultsAfter excluding women positive for anti-TPO and with abnormal TSH, reference intervals were calculated as 12.5–19.1 pmol/L (nonpregnant group), 11–19 pmol/L (first trimester), 9.7–17.5 pmol/L (second trimester) and 8.1–15.3 pmol/L (third trimester). 3rd trimester FT4 was significantly lower than that of the second trimester (p < 0.001) which, in turn, was lower than that of the first trimester (p < 0.001). FT4 reference intervals in pregnancy were significantly lower than in the nonpregnant women (p < 0.002).ConclusionsThe observed trimester-specific FT4 reference intervals progressively decline with advancing gestation and differ significantly from one another.