Establishment of trimester-specific reference intervals for thyroid stimulating hormone and free thyroxine during pregnancy in southwest China by indirect method

Objective A series of physiological changes in thyroid function occur during pregnancy and differ from those non-pregnant women. This study aimed to establish the pregnancy-specific reference intervals of TSH and FT4 using an indirect method based on the healthy pregnant women from southwest China population. Methods Thyroid function test results which available on the Laboratory Information System (LIS) were collected from the pregnancies who visited the Obstetric Clinic or the Department of Gynecology between 1 January 2015, and 30 December 2020. We grouped the data by trimesters to establish the reference intervals (RIs) based on the clinical consensus of different levels of TSH and FT4 at different weeks of gestation. All arrangements were referenced to the document CLSI EP28-A3C. Results A total of 33,040 thyroid function test results of pregnant women, aged 31 (28,33) years were statistical analyzed. Estimated RIs for TSH and FT4 in the first, second and third trimesters corresponding to the 2.5(th) and 97.5(th) percentiles in TPOAb negative were 0.02-5.23, 0.03-5.24, 0.37-5.68 mIU/L, 11.66-20.69, 10.1-18.59, 9.85-16.86pmol/L, respectively. Conclusion This study provides trimester-specific RIs for TSH and FT4 among healthy pregnant women in southwest China which guides clinicians to diagnosis and screen for thyroid disorders in this region.