ObjectiveTo establish the reference intervals of thyroid hormones in pregnant women from Sanming city and compare them with those from other areas in China. MethodsThe study recruited 605 pregnant women and 229 non-pregnant healthy women who visited Sanming First Hospital between March 29 and June 28， 2023. Blood samples were sequentially collected from the participants to determine the serum levels of thyroid stimulating hormone （TSH）， free triiodothyronine （FT₃） and free thyroxine （FT₄）. The reference intervals of thyroid hormones were established by using a percentile range of P_2.5 to P_97.5 and their changes in different periods of pregnancy were analyzed and compared with those from other areas in China. ResultsThere were significant differences in levels of TSH， FT₄ and FT₃ at different periods of pregnancy （all P <0.05）. Compared with non-pregnant women， women in first trimester had significantly decreased levels of TSH and FT₄， but similar level of FT₃. Since the second trimester， TSH level displayed rising tendency， while FT₃ and FT₄ levels showed gradual decrease. The reference intervals of thyroid hormones in the first， second and third trimester of pregnancy in Sanming city were TSH （0.068， 2.943） μU/mL， FT₃ （4.302， 6.888） pmol/L， FT₄ （8.240， 14.719） pmol/L； TSH （0.419， 3.274） μU/mL， FT₃ （4.074， 6.629） pmol/L， FT₄ （6.726， 11.980） pmol/L； TSH （0.422， 3.570） μU/mL， FT₃ （3.741， 5.850） pmol/L， FT₄ （6.103， 10.347） pmol/L， respectively. Significant differences were found in the reference intervals of thyroid hormones during pregnancy among different areas in China. ConclusionsThe levels of TSH， FT₃， FT₄ during pregnancy are different from those during non-pregnancy， and also significantly differ in different periods of pregnancy. Reference intervals of thyroid hormones in pregnant women are affected by various factors such as geographic location， ethnicity and laboratory test method， etc. Therefore， establishing the population-specific reference intervals of thyroid hormones will benefit for the diagnosis and treatment of thyroid disease in pregnant women from Sanming city.