Objective To establish the trimester-specific reference ranges of thyroid hormone for normal pregnant women, and to provide clinical evidence for early diagnosis and treatment of thyroid disease in pregnancy. Method Inclusion criteria were established according to the National Academy of Clinical Biochemistry (NACB) recommended standard, 360 healthy pregnant women whose age are between 20 and 35 were selected as the study group, and were divided into three groups according to gestational ages, 135 healthy non pregnant women were chosen as the control group. The serum levels of T3, T4, FT3, FT4 and TSH in all groups were detected. Results The thyroid hormone levels between different pregnancy stages had statistical significance compared with non pregnancy groups (P< 0.01). The reference ranges of thyroid hormones in non pregnant women were as follows TT3：1.26–2.07 nmol/L，TT4：83.34–136.21 nmol/L，FT3：4.27–6.29 pmol/L，FT4：9.35–15.23 pmol/L，TSH：0.60–5.99 μIU/mL. In the 1st、2nd and 3rd trimester pregnancy, the serum TT3 reference values were 1.54–2.91 nmol/L, 1.48–2.85 nmol/L and 1.48–2.65 nmol/L respectively; the TT4 reference values were 96.74–178.53 nmol/L, 84.44–172.07 nmol/L and 84.60–151.29 nmol/L; the FT3 reference values were 3.86–5.81pmol/L, 3.57–5.54pmol/L, 3.47–5.13pmol/L; the FT4 reference values were 6.87–14.42pmol/L, 6.27–10.86pmol/L, 6.63–11.05pmol/L; the serum TSH reference values were 0.60–3.98μIU/mL, 0.22–4.69 μIU/mL and 0.44–5.02 μIU/mL. Conclusion There are significant differences in the thyroid hormone levels between pregnancy and non pregnancy, and significant differences also exist between different stages of pregnancy, thus to establish trimester-specific reference values of thyroid hormones during normal pregnancy must be meaningful for clinical practice.