Establishment of assay method‐ and trimester‐specific reference intervals for thyroid hormones during pregnancy in Chengdu, China

BACKGROUND: The reference intervals of thyroid hormone will change at different stages of pregnancy because of physiological alterations. On the other hand, the reference intervals of thyroid hormone will also change in different detection systems due to the manufacturer's methodology as well as a different race. The objective of this study was to establish the assay method‐ and trimester‐specific reference intervals for thyroid‐stimulating hormone, free thyroxine and free triiodothyronine for pregnant women in Chengdu. METHODS: A prospective, population‐based cohort study involved 23,701 reference samples of pregnant women during the three trimesters and 8646 non‐pregnant women with pre‐pregnancy clinical and laboratory tests. The 2.5th and 97.5th percentiles were calculated as the reference intervals for thyroid‐stimulating hormone, free thyroxine and free triiodothyronine at each trimester of pregnant women according to ATA Guidelines. RESULTS: The reference interval of thyroid‐stimulating hormone in the 2.5th and 97.5th percentiles has a significant increasing trend from the first trimester, to second trimester and to third trimester, which was 0.08–3.79 mIU/L for the first trimester, and 0.12–3.95 mIU/L for the second trimester and 0.38–4.18 mIU/L for the third trimester, respectively (p < 0.001). However, the reference intervals of free thyroxine and free triiodothyronine in the 2.5th and 97.5th percentiles have significant decreasing trends from the first trimester, to second trimester and to third trimester, which were 11.87–18.83 pmol/L and 3.77–5.50 pmol/L for the first trimester, and 11.22–18.19 pmol/L and 3.60–5.41 pmol/L for the second trimester, and 10.19–17.42 pmol/L and 3.37–4.79 pmol/L for the third trimester, respectively (both p < 0.001). CONCLUSION: It is necessary to establish assay method‐ and trimester‐specific reference intervals for thyroid‐stimulating hormone, free thyroxine, and free triiodothyronine because the reference intervals of these thyroid hormones are significantly different at different stages of pregnancy.