Objective: To understand the throid function and intelligence among the schoolchildren in three rural areas with different iodine intakes in China to probe the side-effect of excessive iodine intake on intelligence of schoolchildren.
Methods: Chinese version of Raven's test was made among rural schoolchildren, 190 from Panshan County, Liaoning Province, a low iodine intake area with the median urinary iodine (MUI) of 99 microgram/L among its inhabitants, 236 from Zhangwu County, Liaoning Province, a moderate iodine intake area with the MUI of 338 microgram/L, and 313 from Huanghua County, Hebei Province, an excessive iodine intake area with the MUI of 631 microgram/L. Thyroid function, thyroid autoantibody (TAA), and MUI were determined among 116, 110 and 112 from the above-mentioned schoolchildren from these three areas respectively.
Results: There was no significant difference among the prevalence rates of overt hyperthyroidism, subclinical hyperthyroidism and overt hypothyroidism in the areas of Panshan, Zhangwu and Huanghua. However, a significant difference was found among the prevalence rates of subclinical hypothyroidism in theses three areas (P = 0.001). The prevalence rates of subclinical hypothyroidism in Huanghua and Zhangwu were 4.76 and 3.37 times higher than that in Panshan (P = 0.001). TAA was negative among all of the schoolchildren with subclinical hypothyroidism except for one. No significant difference was found among the positive rates of thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TGAb) in these three areas. Serum thyroglobulin (TG) values in Huanghua were markedly higher than those in the other two areas (P = 0.015 7). Serum TG value in Zhangwu was higher than that in Panshan but with no significant difference. The IQ value of the schoolchildren in Huanghua was markedly higher than that in Zhangwu (P = 0.001 2). The IQ value of the schoolchildren in Panshan was lower than that in Huanghua and higher than that in Zhangwu but with no significant difference.
Conclusion: The increase of iodine intake may increase the risk of subclinical hypothyroidism among schoolchildren. In the area with excessive iodine intake, most of the subclicical hypothyroidism cases among schoolchildren are not of autoimmune origin. No obvious effect of excessive iodine intaue is found on mental development of schoolchildren.