Reference intervals of trimester‐specific thyroid stimulating hormone and free thyroxine in Chinese women established by experimental and statistical methods

Background As a result of physiological and metabolic changes during pregnancy, thyroid hormones can be affected significantly throughout entire three trimesters. According to the guidelines published by American Thyroid Association in 2017, it is strongly recommended to establish population‐based trimester‐specific and assay method‐specific reference intervals ( RI s) using local population. Methods A total of 1209 pregnant women without personal or family history of thyroid disease were recruited from July 2015 to April 2017 at Beijing Obstetrics and Gynecology Hospital. Those initially selected patients were further tested for TSH , FT 4 and thyroid peroxidase antibody ( aTPO ), performed on the chemiluminescent platform Siemens ADVIA Centaur ® XP . Only patients tested negative for aTPO were included in reference interval establishment. RI s for both TSH and FT 4 were determined as 2.5th percentile to 97.5th percentile on the data distribution. Results The TSH and FT 4 trimester‐specific RI s were as follows: 0.59‐3.54 mIU /L, 11.8‐18.4 pmol/L (n = 188, 1st trimester); 0.80‐4.46 mIU /L, 11.6‐17.4 pmol/L (n = 133, 2nd trimester); 0.72‐4.19 mIU /L, 9.7‐15.1 pmol/L (n = 157, 3rd trimester). The RI s of TSH and FT 4 determined by Hoffmann method for first trimester outpatient pregnant women were 0.33‐3.96 mIU /L (n = 9924) and 11.7‐17.5 pmol/L (n = 10039), respectively. Conclusion Trimester‐specific thyroid function tests RI s are distinct from those provided by assay manufacturers. The RI s determined by direct sampling and Hoffmann indirect calculation showed no statistical difference.