The Association Between Perchlorate Exposure And Thyroid Function

The iodine in blood is transported into the thyroid gland by the sodium iodine symporter(NIS),and then further participates in the synthesis of thyroid hormones.Hypothyroidism occurs when synthesis and secretion of thyroid hormones are reduced,which will lead to abnormal metabolism of the body.Perchlorate is an environmental pollutant with high diffusivity and durability.It can be enriched by plants through water and soil and then enter into the human body through the food chain.Since the ionic radius and charge are similar to those of iodide ions,perchlorate can compete with iodide ions for NIS to inhibit iodine uptake in the thyroid gland,thereby interfering with the synthesis of thyroid hormones.Animal experiments have confirmed that perchlorate can cause adverse effects on thyroid function.Meanwhile,relevant studies have also found that environmental pollutants thiocyanate and nitrate can participate in the competition of NIS.These three substances act on NIS in a simple additive manner,which can lead to the reduction in iodine availability.Considering the potential hazards of perchlorate,thiocyanate and nitrate in public health,some population epidemiological studies have been carried out internationally,but the results are controversial so far.At present,the perchlorate exposure data of Chinese people,as well as the assessment criteria of perchlorate exposure in China,are scarce.There are few relevant population studies in China,too.Exploring the exposure levels and potential hazards of perchlorate,thiocyanate and nitrate in Chinese population will provide epidemiological basis for the formulation of prevention and control standards of such environmental pollutants in China,which is of great significance to public health.This paper intends to quantify the exposure levels of perchlorate,thiocyanate and nitrate in Chinese population and focus on exploring the relationship between perchlorate exposure and thyroid function.Study on this subject is divided into two parts:Part Ⅰ Establishment of the quantitative method for perchloratethiocyanate and nitrate in human urineObjective:The aim of this part is to stablish a quantitative method with simple pretreatment,high stability and high reliability using ultra high performance liquid chromatography–tandem mass spectrometry(UPLC–MS/MS)to measure the contents of perchlorate,thiocyanate and nitrate in human urine.Method:In this study,Agilent 1200SL ultra performance liquid chromatography coupled with QQQ6460 tandem mass spectrometer was used for qualitative and quantitative detection of perchlorate,thiocyanate and nitrate in human urine.Chromatographic conditions and mass spectrometric conditions were optimized firstly.And then we validated the current method referring to the European Drug Administration Bioassay Validation Guidelines.Results:Urine samples can be tested by UPLC–MS/MS after dilution and centrifugation simply.The chromatographic separation was performed on a Primesep B4 Column with a gradient elution method.The results of ion pair(m/z)parameter optimization of target analytes and their corresponding isotopic internal standard were:98.9/83.0*(perchlorate),107.0/89.0*(perchlorate IS),58.1/58.1*(thiocyanate),59.0/59.0*(thiocyanate IS),62.1/46.1*(nitrate)and 63.1/47.1*(nitrate IS).The results of method validation indicated that no significant endogenous interference was observed at the retention times of analytes and internal standards.The lower limits of quantification(LLOQ)of analytes were set to be 0.5μg/L(perchlorate),15μg/L(thiocyanate),and 5mg/L(nitrate).The calibration curve of each analyte exhibited good linearity over its concentration range(r~2>0.99).For quality control(QC)samples,all the values of accuracy were less than 10%and all the values of precision were within±10%.All extraction recoveries were greater than 85%and less than 115%.Furthermore,analytes were kept stable when stored at room temperature for 24 h and no obvious reduction was observed after three freeze–thaw cycles.Conclusion:The detection method established by UPLC MS/MS showed advantages of simple pretreatment,strong specificity,high sensitivity and good stability in the quantificatin for perchlorate,thiocyanate and nitrate in human urine simultaneously,which lays a methodological foundation for further large–scale epidemiological researches.Part Ⅱ The Association between Perchlorate Exposure andThyroid FunctionObjective:The aim of this part is to explore the association between perchlorate exposure level and thyroid function.Method:Residents who had physical examinations at two community health service centers in Nanshan District,Shenzhen were selected as the subjects of this study.Questionnaire and physical examination were used to collect the basic data and related information of the subjects.The blood samples and urine samples were collected at the same time.The biochemical indicators measured in this study mainly include serum thyroid–stimulating hormone,thyroid hormones(TT3,TT4,FT3 and FT4),urine creatinine,sensitive indicators of renal injury,common indicators of glucose metabolism and lipid metabolism.Urinary perchlorate,thiocyanate and nitrate levels were determined using the quantification method established before.Analysis of variance,Kruskal-Wallis test and chi–square test were used to analyze the general characteristics of the population,and multivariate linear regression was used to analyze the association between perchlorate exposure and outcomes(thyroid function,etc.)in the population.Results:A total of 1171 subjects were included in the study,including 448 males and 723females,aged between 20 and 70 years.The median average exposure level(quartile spacing)of perchlorate,thiocyanate and nitrate in the population were 4.44(2.77–7.23)μg/g Cr,293.87(175.32–533.27)μg/g Cr and 86.43(55.27–149.04)mg/g Cr.The results of multivariate linear regression showed that after adjusting the confounding factors such as sex,age,BMI,smoking status,alcohol consumption,exercise habits and glomerular filtration rate(eGFR),the level of urinary perchlorate was positively correlated with the level of serum TSH and negatively correlated with the level of serum FT4,TT3 and TT4.The negative correlation between perchlorate exposure and serum FT3 was significant(P<0.05).Meanwhile,there was a significant positive correlation between exposure levels of perchlorate,thiocyanate and nitrate and renal injury sensitive indicators(microalbumin,α1–microglobulin andβ2–microglobulin)(P<0.05).The combined exposure level of perchlorate,thiocyanate and nitrate showed a significant negative correlation with serum FT3,FT4,TT3 and TT4(P<0.05)and a significant positive correlation with urinary microalbumin,α1–microglobulin andβ2–macroglobulin(P<0.005).Conclusion:This study showed that exposure levels of perchlorate,thiocyanate and nitrate in the population were positively correlated with serum thyroid stimulating hormone levels and negatively correlated with serum thyroid hormone levels.They also had significant positive correlations with renal injury sensitive indicators,which suggested that exposures of perchlorate,thiocyanate and nitrate may be risk factors for hypothyroidism and renal injury.