More Than Adequate Iodine Intake

Although not excessive, studies in more than adequate iodine intake (see table 4) following iodine prophylaxis, also pointed out the possible development of thyroid autoantibodies. Zois et al. [85] investigated the iodine status and the impact of iodine prophylaxis on the prevalence of autoimmune thyroiditis among schoolchildren in a formerly iodine-deficient community in northwestern Greece. The findings were compared to those obtained from a similar survey carried out 7 years previously in the same area. A total of 302 schoolchildren (12-18 years of age) from a mountainous area of northwestern Greece were examined for the presence of goiter, and blood and urine samples were collected for assessment of thyroid function, antithyroid antibodies and urinary iodine excretion. Median urinary iodine concentration in the children was ~200 |xg/l. Thyroid function was normal in all but 7 children, who had subclinical hypothyroidism (2.5%). Antithyroid antibodies (antithyroid peroxidase and/or antithy-roglobulin) were positive in 32 children, including those with subclinical hypothyroidism (10.6%). Twenty-nine of these children (9.6%) also had the characteristic hypoechoic pattern of thyroiditis on ultrasound studies and were diagnosed to have autoimmune thyroiditis (AIT). It was concluded that iodine prophylaxis has resulted in the elimination of iodine deficiency in this region of Greece but this has been accompanied by an increase in the prevalence of AIT. These authors followed up 29 children (12-18 years old) with AIT for 5 years to track its course in the postiodination era [86]. At diagnosis, thyroid peroxidase autoantibodies (TPOAb) were positive in 25 children (86%) and became positive in all children during follow-up. Thyroglobulin autoantibodies (TGAb) were positive in 17 children at diagnosis (59%) and became positive in 3 more children (69%). Both antibody types increased by the end of the observation period. Regarding thyroid function, 7 children (24%) at diagnosis had subclinical hypothyroidism that persisted and 4 more children developed subclinical hypothyroidism during the study period (38%). Only 5 of these children (45%) had positive TGAb. There was an increase in thyrotropin (TSH) so that at the end of the study all children had TSH greater than 2.5 |xU/ml but none developed overt hypothyroidism. Thyroid hypoechogenicity that increased over time was seen in all children, especially in those with subclinical hypothyroidism. They concluded that both antibody types increased in frequency and level, but TPOAb were the predominant autoimmunity marker predictive of impending thyroid failure in children with AIT, as was thyroid hypoechogenicity on ultrasound.

Although the short-term effects of iodine in inducing thyroid autoimmu-nity by enhancing the immunogenicity of thyroglobulin are properly understood, the long-term effects of dietary iodine in modulating the autoimmune process are debated [87]. In this regard, a recent study suggested that thyroid autoimmunity markers may evolve during the course of iodine prophylaxis [88]. In particular, the authors reported a high prevalence of thyroid autoantibodies among schoolgirls 5 years after the introduction of an iodine prophylaxis program in Sri Lanka. The predominant antibodies were against thyroglobulin (TGAb), whereas thyroid peroxidase autoantibodies (TPOAb) were less frequent [88]. Interestingly, 3 years later, a shift in the pattern of autoantibodies was observed with a significant reduction in the frequency of TGAb and the predominance of TPOAb [89]. In a study from Epirus, an area under salt iodization for three decades, overall prevalence of juvenile AIT, as diagnosed by assessment of thyroid antibodies was 3.3% and the goiter specific prevalence was 16.5% [90]. This scenario resembles what is currently occurring in India [91]. In a countrywide study to assess the thyroid status on Indian schoolchildren in the post-salt iodization phase, the authors demonstrated that there was a residual goiter prevalence ranging from ~12 to ~31% (mostly grade 1, WHO definition) [9] in different age groups of boys and girls, striking relationship between residual goiter prevalence and urinary thyocyanate excretion and significantly higher thyroid autoimmunity markers and functional abnormalities among goitrous children when compared to nongoitrous controls. Thus, after the elimination of iodine deficiency, at least in the above mentioned areas, the occurrence of clinically significant iodine-induced AIT appears to be a persistent and progressive phenomenon.

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