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

Abstract: Objective: To determine using a simplified study design trimester-specific FT4 reference intervals in pregnancy with the Roche Modular immunoassay in routine use. Design and methods: Surplus 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). Results: After 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). Conclusions: The observed trimester-specific FT4 reference intervals progressively decline with advancing gestation and differ significantly from one another. [Copyright &y& Elsevier]