Study On Appropriate Threshold Value Of Urinary Iodine During Pregnancy

Objective:Pregnant women are the focus of the population in the evaluation of iodine nutritional status.The iodine deficiency occurred during pregnancy may have a serious impact on the growth and development of offspring.WHO/UNICEF/ICCIDD recommends using the median urinary iodine concentration of pregnant women to assess iodine nutritional status of them in an area,and the standard is 150-250 μg/L.At present a number of monitoring data shows that the pro vinces of which pregnant women median urinary iodine less than 150μg/L accounts for more than 50%,but significantly thyroid function damage is not observed.Therefore,that the international standard of"the median urinary iodine concentration of pregnant women should be more than 150μg/L" is suitable for China’s actual situation,is worth discussing.Methods:In this study,we recruited those who met the requirements and underwent their routine antenatal care in Obstetric Clinic of Peking Union Medical College Hospital.And this study was divided into two parts to verify the recommended standerd "the median urinary iodine of pregnant women should be more than 150 μg/L" proposed by WHO/NICEF/ICCIDD international organization.In the first part,pregnant women whose urinary iodine concentration was less than 150 μg/L at the 6th weeks were recruited to investigate the appropriate threshold value of urinary iodine under the normal conditions of iodine nutritional status,thyroid function throughout the pregnancy and the normal conditions of thyroid function of newborn.Those who underwent their first routine antenatal care in Obstetric Clinic of Peking Union Medical College Hospital were chosen,and related information(including name,age,ID number and contact information)were collected.All pregnant women were tested related parameters of thyroid function(including Free Triiodothyronine(FT3),free Thyroxine(FT4),Thyroid-Stimulating Hormone(TSH),Thyroglobulin Antibody(TGAb),Thyroid Peroxidase Antibody(TPOAb))in the early weeks of pregnancy(at the 6th weeks)after 8 hours of fasting.At the same time,spot urine samples were collected for the urine iodine concentration detection.The results of thyroid function and urinary iodine of pregnant women were collected and recorded.The pregnant women whose urinary iodine concentration was less than 150μg/L,TSH is lower than 2.5mlU/L,TGAb and TPOAb were negative,were followed up.The thyroid function and iodine nutritional status were followed up at 12 weeks,24 weeks and 32 weeks,and the basic condition and the TSH level of heel blood of newborns were observed.3 day meal record,24h urine samples,drinking water samples and household salt samples were collected for each follow-up time.Besides the routine blood test,the thyroid function indicators were also measured.In addition to FT3,FT4,TSH,thyroid globulin(Thyroglobulin,TG)and serum iodine(Serum Iodine,SI)were also tested.In the second part,women in early pregnancy wew focused on to investigate urinary iodine distribution and the appropriate threshold value of urinary iodine during early pregnancy by using normal thyroid function as the gold standard.Refering to the U.S.National Academy of Clinical Biochemistry’s requirements for establishing medical reference ranges,we also explore the appropriate range of serum thyroid hormones in early pregnancy in the hospital where this project was developed.Results:1.103 early pregnant women(at the 6th weeks)were recruited,The average age was 32 years old.79 pregnant women were completed all three times follow-up,and 24 pregnant women were lost in varying degrees.2.The serum FT3、FT4、SI levels gradually decreased with the progress of pregnancy,the difference was statistically significant(P<0.05)during each period of gestational weeks.TSH and TG showed a trend of decline at the 12th weeks and then increased,the difference was statistically significant(P<0.05).Strong correlation was found in thyroid function indexes during pregnancy.FT3 and FT4 were positively correlated in the 12th weeks and the 24th weeks(P<0.05).TSH and FT3 were negatively correlated in the whole pregnancy(P<0.05).TSH and FT4 were positively correlated in the 12th weeks(P<0.05).SI was positively correlated with FT3 and FT4,and negatively correlated with TSH in the whole pregnancy(P<0.05).The correlation between TG and other hematological indexes was poor.During three follow-up studies to 79 pregnant women,19 pregnant women were found to have an abnormal thyroid function.One pregnant woman had two abnormal thyroid function,but it did not affect the health of herself and newborn baby.Therefore,according to the criteria of abnormal thyroid function in pregnant women in this study,the group of pregnant women was basically in normal thyroid function throughout pregnancy.3.The subjects consumed iodized salt during the whole pregnancy.The median iodine content of salt was 22.84mg/kg,and the interquartile range was 21.15mg/kg～25.49mg/kg.The median iodine content of drinking water was 2.0 μg/L,and the interquartile range was 0.4 μg/L～2.9 μg/L.There was no significant difference in total iodine intake between different gestational weeks(P>0.05).The median dietary iodine intake during pregnancy was 231.17 μg/d,according with the recommended nutrient intakes of dietary iodine for Chinese residents(230μg/d).4.The median urinary iodine concentration was 101.56μg/L,108.00μg/L and 113.0μg/L,respectively.There was no significant difference between different gestational weeks(P>0.05).The median of total number of urinary iodine concentration was 107.41μg/L.The median urinary iodine excretion was 185.00μg/L,209.10μg/L and 198.50μg/L,respectively.There was no significant difference between different gestational weeks(P>0.05).The median of total number of urinary iodine excretion was 200.11μg.Urine iodine excretion accounted for 90%of total iodine excretion,in consequence the median of total iodine excretion were 222.34μg,in accordance with dietary iodine intake.These pregnant women were in a positive balance of iodine metabolism during the whole pregnancy.5.In this study,the neonatal heel blood TSH were less than the clinical thyroid function screening cut point(≥10 mIU/L),and the neonatal thyroid function were normal.6.The study included 368 early pregnant women(at the 6th weeks).The reference ranges of serum thyroid hormones in early pregnancy were:FT3 3.74-5.88pmol/L,FT4 12.54-20.15pmol/L,and TSH 0.12～3.48mlU/L.In the early stage of pregnancy when thyroid function was normal,the median urinary iodine concentration was 134pg/L,the interquartile range was 89μg/L～200μg/L,and the reference value of 95%was 33～576μg/L.Conclusion:Thyroid dysfunction was not found during the track of the observed 79 pregnant women whose urinary iodine was less than 150μg/L at the 6th weeks and neonates.These pregnant women were in a positive balance of iodine metabolism during the whole pregnancy.There was no significant difference in the distribution of urinary iodine in different gestational weeks.At last,we obtained that the median urinary iodine concentration in pregnant women was 107.41 μg/L.The detection of urinary iodine in 368 pregnant women showed that the appropriate threshold value of median urinary iodine during early pregnancy was 13μg/L by using different thyroid function criteria.Therefore;the standard of "the median urinary iodine of pregnant women should be more than 150 μg/L" proposed by WHO/UNICEF/ICCIDD may not be suitable for the actual situation in our country.As the evaluation indexes of the iodine nutrition status of population in China,the median urinary iodine is probably in the range of 110～130μg/L,and the specific range should be under further validation.