Thyroid Function Reference Ranges During Pregnancy in a Large Chinese Population and Comparison with Current Guidelines
Thyroid function during pregnancy is a critical factor in ensuring both maternal and fetal health. Abnormal thyroid function has been linked to a range of adverse outcomes, including miscarriage, premature birth, and impaired fetal neurodevelopment. Establishing accurate reference ranges for thyroid hormones during pregnancy is essential for the proper diagnosis and management of thyroid disorders. This study aimed to determine the thyroid function reference ranges specific to the Chinese population and compare these ranges with the guidelines provided by the American Thyroid Association (ATA).
The study enrolled 52,027 pregnant women from January 2013 to December 2016 at the International Peace Maternity and Child Health Hospital in Shanghai, China. After excluding women with twin pregnancies, in vitro fertilization, thyroid disease history, and those using thyroid medication, the final study population comprised 46,262 women. The participants’ thyroid stimulating hormone (TSH), free thyroxine (FT4), and thyroid peroxidase antibody (TPOAb) levels were measured during the first and third trimesters of pregnancy. The reference ranges for TSH and FT4 were established using the 2.5th and 97.5th percentiles of the TPOAb-negative population.
The study found that the TSH reference ranges were 0.03 to 3.52 mU/L in the first trimester and 0.39 to 3.67 mU/L in the third trimester. The FT4 reference ranges were 11.7 to 19.7 pmol/L in the first trimester and 9.1 to 14.4 pmol/L in the third trimester. These ranges were compared with the 2011 and 2017 ATA guidelines. Using the 2011 ATA criteria, 7.0% and 4.0% of pregnant women would be overdiagnosed with subclinical hypothyroidism in the first and third trimesters, respectively. In contrast, using the 2017 ATA criteria, 1.2% and 0.8% of pregnant women would have a missed diagnosis in the first and third trimesters, respectively.
The study also examined the relationship between TSH and FT4 levels. It was observed that FT4 levels remained relatively constant (median 14.0 to 15.0 pmol/L) when TSH levels were between 0.5 and 4.0 mU/L. However, when TSH levels exceeded 4.0 mU/L, FT4 levels began to change significantly.
Additionally, the study investigated the impact of TPOAb status on thyroid hormone levels. TPOAb-positive pregnant women had significantly higher TSH levels (1.53 vs. 1.14 mU/L) and lower FT4 levels (14.7 vs. 14.8 pmol/L) in the first trimester compared to TPOAb-negative women. In the third trimester, TPOAb-positive women had significantly higher FT4 levels (11.7 vs. 11.5 pmol/L) compared to TPOAb-negative women. There were no significant differences in T3 levels between the two groups in either trimester. However, T4 levels in the third trimester were significantly higher in TPOAb-positive women (111.1 vs. 108.0 nmol/L).
The study’s findings suggest that population-specific reference ranges for thyroid function are crucial for accurate diagnosis during pregnancy. The upper limit of TSH in the first trimester (3.52 mU/L) was closer to the 2017 ATA criteria (4.0 mU/L) than the 2011 ATA criteria (2.5 mU/L). The study recommends using the local population-based reference ranges for thyroid function during pregnancy in China. If such ranges are unavailable, the 2017 ATA criteria can be used, with a TSH upper cut-off value of 4.0 mU/L for TPOAb-negative women and 2.5 mU/L for TPOAb-positive women.
The study also highlighted the importance of considering TPOAb status in the management of thyroid function during pregnancy. TPOAb-positive women exhibited different thyroid hormone changes compared to TPOAb-negative women, suggesting that they may require a different treatment approach.
In conclusion, this study provides valuable insights into the thyroid function reference ranges during pregnancy in a large Chinese population. The findings underscore the importance of using population-specific reference ranges for accurate diagnosis and management of thyroid disorders during pregnancy. The study’s recommendations align with the 2017 ATA guidelines, emphasizing the need for tailored approaches based on TPOAb status.
doi.org/10.1097/CM9.0000000000000051
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