The effects of high levels of fluoride and iodine on intellectual ability and the metabolism of fluoride and iodine.

Abstract

The authors carried out a study on the intellectual abilities and fluoride/iodine metabolism of children living in a high fluoride-high iodine area. Among the results: the percentage of the general population living in this fluoride/iodine-contaminated region that suffered from goiter (clinical thyroid enlargement) was 3.8%, the rate of children already showing some thyroid enlargement was 29.80%. Similarly, the rate of dental fluorosis for the general population was 35.48%, while for children it was 72.98%. Student subjects had average IQs of 76.67±7.75, with 16.67% of the IQs in the “low” category. The iodine content and fluoride content of the children’s urine were 816.25±1.80 ug/L and 2.08±1.08 mg/L, respectively, significantly higher than the control. The thyroid glands of the subjects showed a markedly lower uptake rate of iodine-131 when compared with the control, the values after 3 hr and 24 hr were 9.36±1.55% and 9.26±4.63%, respectively, and the blood serum levels of thyroid stimulating hormone (TSH) were significantly higher than the control. The results indicate that high levels of fluoride and iodine have a serious damaging effect on the body, and should be given greater attention.

INTRODUCTION
The subjects of this study were residents of the Huimin and Dezhou regions of
Shandong Province located on the lower reaches of the Yellow River. In 1976, in a
response to increasingly bitter, salty water, much deeper wells were dug. However,
the fluoride and iodine content of this deep well water was found to be much higher
than the standards for drinking water, resulting in a high prevalence of iodine goiter
and fluoride poisoning. The area is known locally as the “twin contamination zone.”
In order to investigate the effects of high iodine and fluoride on child intellectual
ability and the metabolic characteristics of iodine and fluoride, we carried out a
comparative study of two villages in Qingyun County with the following results.
MATERIALS AND METHODS
1. Lidian village of Qingyun County was selected as the site of our study; its
drinking water was tested to have an iodine content of 1,100 ?g/L and a fluoride
content of 2.97 mg/L. The non-disease control was Dading village of the same
county; its drinking water showed an iodine content of 128.6 ?g/L and a fluoride
content of 0.5 mg/L.
2. The diagnoses of goiter and dental fluorosis were both carried out according to
national standards for endemic disease control.1,2 Intelligence testing was done using
the Chinese Comparative Scale of Intelligence Test (Third Edition), as revised by Wu
Tianming.3
3. The determination of iodine and fluoride levels in the drinking water was
accomplished by conventional physicochemical analysis techniques, with their
values expressed in units of ?g/L and mg/L, respectively. The incineration method
was used to determine urinary iodine, again expressed as ?g/L, while the electrode
aThe authors work at the Shandong Provincial Institute of Endemic Disease Control, Jinan,
250014, PR China.

method was used for urinary fluoride, expressed as mg/L. The measure of the thyroid
gland absorption rate of iodine-131 was done on site using a type 44-1 thyroid
function indicator from the Hefei Wireless Electronics Factory. T3, T4, and thyroid
stimulating hormone (TSH) were measured using radioimmunoassay.
RESULTS
I. Rates of goiter and dental fluorosis:
1. The prevalence of goiter and dental fluorosis in the studied populations are
shown in Table 1.
As seen in Table 1, the subject-study group ingesting water with high levels of
fluoride and iodine show significantly increased disease rates for goiter and dental
fluorosis when compared with the control group, and the rates are sufficient to
classify these diseases as endemic to the area.
2. For children 15 or younger, the rate of thyroid swelling was 29.8% (96/322), and
the rate of dental fluorosis reached 72.98% (235/322). In the control group, the rates
were 16.13% (15/93) and 18.28 (17/93), respectively, with P<0.01 in all cases,
indicating that the harm caused by a high fluoride-high iodine environment is
particularly serious in the case of children.
II. Intelligence testing for 8–14-year-old children:
The average IQ scores of children in the high fluoride, high iodine area and the
control area were 76.67±7.75 and 81.67±11.97, respectively. This difference is not
statistically significant; however, as seen in Table 2, the number of children showing
moderately low IQ scores in the subject population is significantly higher than the
control.b

III. Urinary iodine and urinary fluoride test results:

As seen in Table 3, the school-age children in the endemic region have urinary iodine and urinary fluoride levels significantly higher than in the control group (P<0.01), indicating that they are absorbing large quantities of iodine and fluoride from the drinking water, and that the body load of these two substances is high. IV. Thyroid gland iodine absorption rate of children in the two groups: When tested after 3 and 24 hr, the average values of the thyroid gland iodine-131 uptake rate for children in the endemic disease area was 9.36±1.55 and 9.26±4.63%, respectively, which are significantly lower than the averages of the control area of 13.34±2.88 and 22.79±5.29% (P<0.01). The nearly identical values at 3 and 24 hr bEditorial comment: The number of subjects in the <69 IQ groups (5/30 vs. 3/30) are too small for reliable statistical analysis. When the children in the two lowest IQ groups are combined (5 + 18)/30 = 23/30 vs. (3 + 8)/30 = 11/30) the result is statistically significant by the appropriate chisquare
test (with the Yates correction): chi-square, ?2 = 8.22,P<0.01.

after exposure in the endemic area group suggest a backwards shifting of the peak value.

V. Blood serum hormone levels: As shown in Table 4, there was a general increase in the average levels of T3, T4,
and TSH in children from the endemic area compared to the controls. However, only
the TSH value was significantly higher (P<0.01).

DISCUSSION

Iodine is a necessary trace element for organic life, and fluoride is similarly regarded, but if the ingestion level is too high or too low, especially for iodine, disease is the result. In this study, the prevalence of goiter in the subject population from a high iodine-high fluoride village area was 3.8%, with thyroid swelling present in 29.81% of the children age 15 and under. In addition, the rate of dental fluorosis in the endemic study population was 35.48%, reaching 72.98% in the children age 15 and under. In all cases the rates for the endemic group were significantly elevated compared to the control group. This suggests that the separate harm caused by an excess of each of the two elements is compounded when iodine and fluoride occur together in a single region, with their ingestion leading to a serious array of health problems.

An excess of fluoride and a lack of iodine in the same environment has been shown to have a marked effect on child intellectual development, causing a more significant intellectual deficit than lack of iodine alone.4 In our study the study group of children from the high fluoride-high iodine village area had an average IQ of 76.67±7.75, which was somewhat lower than the control (IQ 81.67 ±11.9), although the difference is not statistically significant (P>0.05). However, as seen in Table 2, the percentage of children in the low range (16.67%) is higher in the endemic group than in the control group (10.0%), suggesting that a high iodine-high fluoride environment also has a definite negative influence on child intellectual ability.

The urinary iodine and urinary fluoride levels for children living in the high iodine high fluoride area were 816.25±1.80 ?g/L and 2.08±1.03 mg/L, respectively, clearly elevated by comparison to the control, reflecting the high body load of iodine and fluoride. Elemental iodine is a key component of thyroid hormones, and also influences several stages of their formation and excretion.5 Fluoride, on the other hand, is toxic to living cells, and is a powerful inhibitor of certain enzymes. Excessive uptake of fluoride can cause decreased functioning of the thyroid gland in direct relation to the blood level. Of course, iodine is the primary factor here; what exact role that fluoride is playing requires further study.

The serum levels of T3 and T4 for the children from the high iodine-high fluoride zone were only slightly higher than the control (P>0.05), but the level of TSH was clearly elevated (P<0.01). That the thyroid gland excretion of T3 and T4 for the “twin contamination zone” children was in the normal range, but the pituitary gland secretion of TSH was significantly elevated making it probable that reverse feedback is promoting the hypothalamus excretion of TRH (thyroid releasing hormone), causing a corresponding increase in the excretion of TSH, which stimulates compensating production of T3 and T4 by the thyroid gland.

In summary, the results of this study indicate the following:

(1) Areas that have long-term, serious iodine and fluoride contamination can cause goiter and fluoride poisoning in the population; this problem should be taken seriously.

(2) The clinical characteristics of children from this region include high urinary iodine, high urinary fluoride, poor thyroid iodine-131 absorption with similar values at 3 and 24 hr (possibly a backward peak value shift), and high TSH values.

(3) With regard to the diseases endemic to this high iodine-high fluoride zone, the only fundamental means of control is to change the source of water, limit the ingestion and absorption of iodine and fluorine, and promote their excretion from the body.

REFERENCES

1 The North China Professional Council on Iodized Salt as a Means of Controlling Endemic Goiter. Standards for work to control endemic goiter (Trial Version). Chinese Journal of Preventive Medicine 1978;12(1):62.

2 Dai Guojun. Endemic fluoride poisoning. Hohot: The Inner Mongolian People’s Publishing House; 1985. p. 383-7.

3 Wu Tianmin. The Chinese Comparative Intelligence Test Guidebook (3rd revision). Peking University Press; 1983. p. 1-91.

4 Ren Dali, Li Kecheng, Liu Dawei. Study on the intellectual development of 8–14 year-old children in high fluoride, low iodine zones. Chinese Journal of Control of Endemic Diseases 1989;4(4):351. [A translation of this paper is in this issue of Fluoride on p. 319-20.]

5 Yang Yingkui, Zhang Xining, Zhang Huifang, et al. Research on iodine metabolism, pituitary/thyroid function, and the pathology of goiter in patients exposed to excess iodine. Chinese Journal of Control of Endemic Diseases 1988;3(6):338.

6 Liu Zhenqi, Hu Min, Wu Hanwen, et al. Investigation of the relationship between serum thyroid hormone and fluoride levels in fluoride-poisoned patients. Chinese Journal of Control of Endemic Diseases 1988;7(4):216. An editorial comment on the statistical testing is given as a footnote on p. 337.

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Translated by Julian Brooke for Fluoride Action Network and published with the permission of the Chinese Journal of Endemiology 1994 October;15(4):296-8 in the journal Fluoride.