Thyroid function reference ranges during pregnancy in a large Chinese population and comparison with current guidelines

Background: A correct thyroid function reference range is important for the accurate diagnosis of thyroid disease during pregnancy. However, there is no consensus on whether thyroid function reference ranges in Chinese population should follow the America Thyroid Association (ATA) guidelines. This study aimed to establish a thyroid function reference range more suited to the Chinese population by evaluating the current thyroid function reference range in pregnant Chinese women and comparing it to the ATA guidelines. Methods: A total of 52,027 pregnant women were enrolled from January 2013 to December 2016. Thyroid stimulating hormone (TSH), free thyroxine (FT4), and thyroid peroxidase antibody (TPOAb) levels were tested during the first and third trimesters of pregnancy. Reference ranges of TSH and FT4 were established from the 2.5th and 97.5th percentiles of the TPOAb-negative population of women. The Mann-Whitney U test was used to compare thyroid hormones between the TPOAb-positive and TPOAb-negative groups. Results: We obtained that the TSH reference ranges were 0.03 to 3.52 mU/L and 0.39 to 3.67 mU/L, and the FT4 reference ranges were 11.7 to 19.7 pmol/L and 9.1 to 14.4 pmol/L, in the first and third trimester, respectively. If we used the 2011 ATA criteria about 7.0% and 4.0% pregnant women would be over diagnosed in first and third trimester, respectively, compared with local population thyroid hormone reference. When we compared our local criteria with the new 2017 ATA criteria, about 1.2% and 0.8% pregnant women would have a missed diagnosis in first and third trimester, respectively. Conclusions: Based on our data, which is in line with the current ATA guidelines, a population-based thyroid function reference range would be the first choice for diagnosis of thyroid disease during pregnancy in China. In case such population-based thyroid function reference ranges are unavailable in the east of China, our reference ranges can be adopted, if the same assay is used.