Study Of The Effect Of The Excessive Iodine On Thyroid Function

Iodine is one of human body's necessary micronutrients.It is one of raw materials for synthesis of thyroid hormone which plays an important role in metabolism and development(both mental and physical) of human body.Lack of iodine in our body can lead to iodine deficiency disorders(IDD),which is characterized by retarded mental and physical development.Mean while,long absorbed iodine may result in iodine excess disorders(lED).Today,after implementing Universal Salt Iodization(USI) for many years,IDD has got the valid control.With the inhabitants' iodine level was enhanced,and it seems that the incident of thyroid diseases increases dramatically.Currently in some area(about thirty million to fifty million people) in China the existence environment iodine is excessive,and the residents are under iodine excess.It has already led to the public hygiene problem.At the time of raising the population character we want to get an adequate iodine intake to avoid the high iodine intake in the crowd and the harm of iodine excess during the process of amending iodine deficiency.The prevented form and strategy is to limit the excessive iodine intake in the people.The important problem of the prevention is to assure what is the iodine intake level that can lead to thyroid dysfunction. The level is called tolerable upper intake level(UL) of iodine.In order to solve the question we choose the healthy volunteers who were divided into seven groups,each group accepted a quantity of iodine respectively. We took advantage of the advanced diagnostic assays to screen for the thyroid hormone and related index to comprise the changes at each group and studied the safe iodine intake level in adults.MethodsAt the result of the physical examination,the thyroid gland antibody examination and urine examination we adopted one hundred and sixty-one people as the objects who were randomly divided in to seven groups.Each group was randomly assigned to receive 500, 750,1000,1250,1500,and 2000 micrograms iodide oral doses/day for four weeks.Blood and urine samples during morning fast were obtained on weeks 0,2 and 4.We used CLIA to measure sTSH,FT3,FT4 and used IRMA to measure T3,T4,TPOAb and TGAb.Urinary iodine was evaluated.Thyroid ultrasounds were also done to the volunteers.The dietary survey was done by recording method to collect food consumption data for 7 days.We also measured the iodine amount in the drinking water,food and iodinized salt.All the investigation data and examination data were input into the database,and used SPSS software to analysis them.Results1.The iodine intake of volunteers was sufficient.MUI was 272.25μg/L.The dietary iodine intake was 338.78～351.30μg/d.The thyroid hormone level and the thyroid volume were normal.Both TPOAb and TGAb were negative.2.We found all the iodide supplemented groups responded in the same way with a significant rise in urinary iodide excretion(p＜0.05) and in serum sTSH levels(p＜0.05),with a small decline in serum free T4 concentration at normal range(p＜0.05).No changes in FT3 were observed(p＞0.05).After 2 weeks each the iodide supplemented groups increased nearly more than one multiple in serum sTSH levels,and nearly more than two multiple after 4 weeks.No significant changes in sTSH were observed(p＞0.05) among groups.The volume of the thyroid gland became significantly decreased after 4 weeks of iodide intake, especially in female(p＜0.05).We found the thyroid hormone at the normal level and serum sTSH levels increased significantly at the 500 micrograms/day doses.Some individuals(15%) of the iodide supplemented subjects serum sTSH levels raised above the laboratory reference range and occured the subclinical hypothyroidism.3.We found some people appeared TPOAb positive in all the iodide supplemented groups after 2 weeks.After 4 weeks the TPOAb positive ratio kept the same level at the 500 micrograms/day and 750 micrograms/day doses,a small declined at the 1750 micrograms/day doses,and increased at the other iodine supplemented groups.The serum FT3,FT4 levels of the people with TPOAb positive were at the normal range.The serum sTSH levels of the people with TPOAb positive significantly increased at the 500 micrograms/day to 1250 micrograms/day doses.And the rate(sTSH＞5.0UI/ml) was respectively 10.74%,11.74%,9.86%,12.86%.4.The subclinical hypothyroidism appeared at the 500 micrograms/day to 2000 micrograms/day doses after 2 weeks.After 4 weeks the prevalence of subclinical hypothyroidism kept the same level at the 500 micrograms/day(15%),a small declined at the 1750 micrograms/day doses,and increased at the other iodine supplemented groups.The subclinical hypothyroidism prevalence at the 750～2000 micrograms/day doses was at the range of 28.00%～47.37%.No clinical hypothyroidism observed among the people during iodide administration.5.When iodide was discontinued after three months we found the thyroid hormone,sTSH and TPOAb returned to baseline levels for the subclinical hypothyroidism subjects.Conclusions1.The volunteers were under good iodine nutrition and MUI was 272.25μg/L.It showed the iodine amount in the iodinized salt was safe.2.Serum FT4 was sensitive to the excess iodine.Serum FT4 decreased significantly and other thyroid hormone at normal level at the 1500 micrograms/day doses and a further doses.3.The subclinical hypothyroidism(sTSH＞5UI/ml,FT4＞10.3pmol/L) is the main form of thyroid dysfunction at the 500～2000 micrograms/day doses.No clinical hypothyroidism observed among the people during iodide administration.4.The excessive iodine supplement can cause the risk of autoimmune thyroid disorders increased for the people with sufficient iodine nutrition. 5.Thyroid autoantibody may lead to thyroid dysfunction.6.The TPOAb of the people with the autoimmunity heredity tendency can be negative in the crowd;it can turn to be positive with high iodine intake.It hinted that TPOAb was not the particular sign of this kind of crowd.7.It can lead to subclinical hypothyroidism in the subjects at 500 micrograms/day doses.In iodine-sufficient area iodine supplemented dose was not appropriate to exceed 500 micrograms/day.We suggested The UL of iodine should below 900μg/d.8.The normal adults can't lead to clinical thyroid dysfunction with exposure excessive iodine in short period(within 1 month).When iodide was discontinued after three months we found the thyroid function returned to baseline levels for the subclinical hypothyroidism subjects.