Objective To evaluate the iodine nutritional status and thyroid function of pregnant women in different periods of pregnancy,analyze the relationship between iodine nutritional status and thyroid function and provide a basis for scientific supplementation of iodine to pregnant women.Methods In 2013 and 2014,using stratified random sampling method,six counties were selected in the city;around 30 early,middle and late pregnant women each in every county were selected;a total of 545 (173 early,203 middle and 169 late) pregnant women were investigated in the 6 counties;instant random urine samples of 25 ml were collected and urinary iodine was tested.A total of 151 (52 early,55 middle and 44 late) pregnant women were selected to collect 3 ml venous blood samples for determination of thyroid hormone and thyroid autoantibodies.Results Median urinary iodine levels of early,middle and late pregnant women were 132.2,128.9 and 113.5 μg/L,respectively,under the condition of iodine deficiency.Prevalence rates of low frce thyroxine FT4,subclinical hypothyroidism,hypothyroidism and hyperthyroidism in pregnant women were 0.66％ (1/151),15.23％ (23/151),0.66％ (1/151) and 0.66％ (1/151),respectively.The positive rate of TgAb and TPOAb was 15.23％ (23/151) and 11.92％ (18/151),respectively.There were significant differences in the levels of free three iodine thyroid (FT3) in different periods of pregnancy (F =7.723,P ＜ 0.05);FT3 in late pregnancy was lower than that in early and middle pregnancy (P ＜ 0.05).There were significant differences in the levels of FT4 in different periods of pregnancy (F =3.762,P ＜ 0.05);FT4 in late pregnancy was lower than that in early and middle pregnancy (P ＜ 0.05).There were significant differences in the levels of thyroid stimulating hormone (TSH) in different periods of pregnancy (F =13.199,P ＜ 0.05);TSH in early pregnancy was higher than that in middle and late pregnancy (P ＜ 0.05).Subclinical hypothyroidism prevalence rate in late pregnancy was higher than that in middle pregnancy (x2 =3.912,P ＜ 0.05).The positive rate of TgAb in early and middle pregnancy was higher than that in late pregnancy (x2 =9.883,3.906,all P ＜ 0.05).Urinary iodine ≥250 μg/L was a risk factor for subclinical hypothyroidism in pregnant woman [odds ratio (OR) =5.076,P ＜ 0.05].Conclusions The iodine nutrition of pregnant women in Tianjin is insufficient.Excessive urinary iodine is an increased risk of subclinical hypothyroidism.We should monitor the urinary iodine and thyroid function in pregnant women.