Gestation-specific reference intervals for thyroid function tests and the clinical significance for thyroid function monitoring through different periods of pregnancy

Objective To investigate the gestation-specific reference intervals (GRIs) and dynamic changes of thyroid function at different gestational ages in Chongqing. Methods Combining self-sequential longtitudinal with cross-sectional study, the serum samples from 640 pregnant women with different gestational age were collected from June 2014 to September 2015 in the Third Military Medical University. The free triiodothyronine (FT3), free thyroxine (FT4), thyroglobulin (TG), thyroid-stimulating hormone (TSH), thyroid peroxidase antibody (TPOAb), and antithyroglobulin antibody (TGAb) were detected by the direct chemiluminescence method. According to China Guideline for the diagnosis and treatment of thyroid disease in pregnancy and postpartum in 2012, the reference interval of the thyroid function was calculated. The data were analyzed by Chi square test. Results Established GRIs of thyroid function during pregnancy in Chongqing: The GRIs was 3.68-5.59 pmol/L for FT3, 9.34-17.02 pmol/L for FT4, 0.18-5.26 mIU/L for TSH in 6-9+ 6 weeks of pregnancy; the GRIs was 3.69-6.03 pmol/L for FT3, 8.42-15.75 pmol/L for FT4、0.09-4.85 mIU/L for TSH in 10-13+ 6 weeks of pregnancy; the GRIs was 3.24-5.46 pmol/L for FT3, 6.50-14.24 pmol/L for FT4, 0.11-5.13 mIU/L for TSH in 14-27+ 6 weeks of pregnancy; the GRIs was 3.06-5.05 pmol/L for FT3, 6.12-11.69 pmol/L for FT4, 0.75-3.67 mIU/L for TSH in 30-34 weeks of pregnancy; the GRIs was 2.96-5.00 pmol/L for FT3, 6.26-11.36 pmol/L for FT4, 0.84-5.54 mIU/L for TSH in 36-40 weeks of pregnancy. Screening by GRIs, the prevalence of thyroid dysfunction was 8.75% (46), however, the prevalence was 37.07% (195) in according with the guidelines, χ2=120.5, P=0.000. The overdiagnosis rate was 28.32%(149/526). Using the guidelines of thyroid disease and our GRIs, the thyroid disease was found 116 (22.05%) and 30(5.70%) in the first screening. Moreover, the thyroid disease was found 79(19.27%) and 10(3.23%) during the repeat screening in the normal population. Conclusions Using the GRIs for thyroid function tests in normal singleton pregnant women could reduce the risk of over diagnosis. The detection rate of repeat screening of TPOAb negative patients was close to the first screening detection rate, and repeated screening could reduce the risk of missed diagnosis for thyroid dysfunction in pregnancy women.(Chin J Lab Med, 2016, 39: 511-515) Key words: Pregnancy; Reference values; Hypothyroidism; Thyroxine