Trimester Specific Reference Intervals for Thyroid Function (#56)
Objectives: Using non-pregnancy reference intervals in pregnancy can be misleading. We aimed to establish trimester-specific reference intervals for thyroid stimulating hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3) specific for Beckman DxI 800 analytical system, a commonly used method for measuring thyroid function in Australia.
Design/ setting/ participants: Healthy women attending Mercy Hospital for women (a tertiary maternity hospital in Victoria) for antenatal care were followed prospectively.
Main outcome measures: Normal reference intervals for serumTSH, fT4 and fT3 were determined at each trimester and post partum.
Results: One hundred and fifty four women were recruited into this study. After excluding women who had miscarriage, twin pregnancy and women who were thyroid peroxidase antibody positive, 131 women’s results were used for the reference interval determination. For trimester 1 (T1), trimester 2 (T2) and trimester 3 (T3), the median (2.5th 5th, 95th, 97.5thpercentile ) TSH were 0.76 (0.02, 0.05, 2.37, 3.22), 1.16 (0.26, 0.43, 2.70, 3.34) and 1.33 (0.03, 0.34, 2.66, 3.34) mIU/L, respectively. Free T4 (mean±SD) was 10.7±2.4, 8.1±1.6, 7.6±1.5 pmol/L, respectively. Free T3 (mean±SD) was 4.8±0.5, 4.4±0.4, 4.3±0.4 pmol/L, respectively. In T2 and T3, 34.5% and 40.3% of the fT4 values respectively, fell below the manufacturer’s quoted reference intervals.
Conclusions: The trimester specific TSH reference intervals in our cohort are very similar to those put forward by ATA 2011 Guidelines. However, guided by non-pregnancy associated reference intervals for fT4 levels, up to 40% of of pregnant women would be considered inappropriately as having abnormal thyroid function which may lead to confusion and potential mismanagement. This study highlights the need for establishment and use of pregnancy and trimester specific reference intervals for fT4 in addition to TSH.