Objective To establish the reference intervals of thyroid hormones in normal pregnant women in Chengdu area of China, and to investigate the effects of thyroid autoantibodies on thyroid function in pregnant women. Methods We included 696 pregnant women who had gestation data from June 2013 to March 2014 in Chengdu Women ＆ Children’s Central Hospital. Every subject under went questionnaire survey, physical examination, thyroid ultrasound and measurement of thyroid hormone and thyroid autoantibodies. The normal reference intervals were established according to the percentiles (P2.5-P97.5) of the healthy pregnant women in the same trimester. Another 50 non-pregnant women were selected as the control group. Results Of the 696 pregnant women, 579 subjects had negative thyroid autoantibodies and 117 subjects had positive thyroid autoantibodies. The positive rate of thyroid autoantibodies was 16.81%. Of the 579 subjects with negative thyroid autoantibodies, 257 were in the first trimester, 202 in the second trimester and 120 in the third trimester. In the first trimester of normal pregnancy, the reference intervals of serum thyrotropin （TSH）, free triiodothyronine （FT3）and free thyroxine （FT4） were 0.02-4.03 mIU/L, 3.85-6.27 pmol/L and 11.93-21.04 pmol/L respectively. In the second trimester, the reference intervals of serum TSH, FT3 and FT4 were 0.02-4.05 mIU/L, 3.51-5.82 pmol/L and 11.23-19.22 pmol/L respectively. In the thirdtrimester, the reference intervals for serum TSH, FT3 and FT4 were
