Objective To establish reference interval of thyroid hormones in pregnant women in Urumqi,standardize the diagnostic criteria of thyroid diseases in pregnancy,assess the iodine nutrition and thyroid function at different stages of pregnancy,and provide evidence to guide iodine supplementation.Methods A cross-sectional survey was performed in 3 731 pregnant women in Urumqi from May 2015 to June 2016.1 206 of them were in the first trimester,1 125 in the second and 1 400 in the third.500 non-pregnant women were recruited as controls.Levels of serum thyrotrophin (TSH),free triiodothyrunine (FT3),free thyroxine (FT4),anti-thyroid peroxidase (anti-TPO) and antithyroglobulin (anti-TG) were measured,and urinary iodine levels were detected by arsenic-cerium catalytic spectrophotometry.Results There were statistically significant differences between the pregnant groups and the control group in FT3,FT4,and TSH levels (P＜0.05).The serum thyroid concentrations in each group were FT3 values:(4.64±1.15),(4.36±0.89),(3.89±0.92),(5.24±0.65) pmol/L;FT4:(16.49±2.78),(15.06±3.76),(12.38± 1.65),(17.56± 1.12) pmol/L;TSH:(1.98±0.65),(2.43 ±0.83),(3.15± 1.25),(2.13 ± 0.75) mU/L.The reference intervals of thyroid hormones in early,middle and late pregnancy (P2.5 to P97.5) were:FT3:3.24-7.58,3.16-5.47,3.05-4.28 pmol/L;FT4:13.36-22.58,12.04-19.77,12.78-20.03 pmoL/L;TSH:0.24-3.78,0.51-3.91,0.55-4.55 mU/L.The medians of anti-TG and anti-TPO at different stages of pregnancy were significantly different,with the first trimester being the lowest and the third trimester being the highest (P=0.07,0.04).The medians of urinary iodine in all four groups were 235.78 μg/L (controls),198.25 μg/L (first trimester),175.36 pg,/L (second trimester) and 141.24 μg/L (third trimester),showing a significant gestational age-dependent decrease (P =0.036).Changes in serum levels of TSH,FT3,FT4,anti-TG and anti-TPO seemed to be in accordance with changes of urinary iodine levels;yet the correlations were not statistically significant (P＞0.05).Conclusions Iodine nutritional status was closely related to gestational age.Abnormal TSH levels were mainly observed in the second and third trimesters,abnormal serum levels of FT3,FT4,anti-TG and anti-TPO in the first trimester,and iodine deficiency in the third trimester.Thyroid function and urinary iodine should be monitored at each trimester during pregnancy.