Objective To establish trimester-specific reference intervals of thyroid hormones for pregnancy in normal iodine intake.Methods A total of 716 pregnant women were selected as research subjects ,and another 307 normal women without pregnancy were served as control group .The serum thyroid stimulating hormone (TSH), free thyroxine(FT4),free three iodine thyroid gland originalacid(FT3) were measured by chemiluminescent microparticle immunoassay.The changes of thyroid indicators in different gestational period were analyzed ,and the thyroid reference range during pregnancy was established .Results The levels of TSH,FT3 and FT4 in non-pregnant women and women in different pregnancy were significantly different (F =78.901,249.571,137.090,all P <0.05),in which the TSH increased with the increase of gestational age ,while the FT3 and FT4 decreased with the increase of gestational age.The reference range of TSH in the early ,middle and late gestational weeks were 0.12-4.82mIU/L,0.60-4.25mIU/L,0.37-4.03mIU/L;the reference range of FT3 in the early,middle and late gestational weeks were 3.81-6.10pmol/L,3.40-5.20pmol /L,3.00-5.00pmol /L;and the reference range of FT4 in the early,middle and late gestational weeks were 12.85-22.12pmol/L,10.81-17.66pmol /L,10.96-18.20pmol /L,respectively. The diagnostic rates for thyroid dysfunction of reference values ,non-pregnancy reference values and reference values established by American Thyroid Society (ATA) were 5.17%,5.45% and 6.01%,respectively,which showed no statistically significant difference among the three groups (χ2 =0.498,P >0.05).However,the difference among the three diagnostic criteria for subclinical hypothyroidism was statistically significant ( χ2 =9.661, P <0.05 ). Conclusion The thyroid function indicators of pregnant women are significantly different from those of normal people,and there are differences in different stages of pregnancy .Establishing a region-specific reference range can effectively prevent misdiagnosis or missed diagnosis of thyroid disease and reduce adverse pregnancy outcome .