A review of the evidence of the health conse- quences of mild iodine deficiency reveals that: • There is a limited number of published studies
investigating the health consequences of mild iodine deficiency.
• There is reasonable evidence of an association between mild iodine deficiency and sub- optimal neurological development, specially reduced Intelligence Quotient (IQ).
• Of the studies that have attempted to investi- gate the effects of mild iodine deficiency, many have limitations. Consequently, the
current literature does not provide unequivo- cal evidence for significant health effects for populations with urinary iodine in the upper range of mild iodine deficiency.
• There is sufficient evidence to suggest that the known association between neurological out- comes and moderate and severe iodine defi- ciency is likely to extend to mild iodine deficiency. However, there is a suggestion of a dose response relationship with increasing effects on neurological development with higher iodine deficiency levels. Neurological effects associated with mild iodine deficiency include reduced IQ, increased auditory thresh- old and increased rates of attention-deficit hyperactivity disorder. Although the relation between iodine intake during pregnancy, thy- roid function, and child neurodevelopment needs further evaluation, the evidence on the safety and effectiveness of iodine supplemen- tation during pregnancy is needed before it is systematically recommended in iodine- sufficient or mildly deficient areas.
• In constructing a reasonable health-based standard using the precautionary approach, it is clear that urinary iodine levels below 100 μg/L, which are in the mild iodine defi- ciency range, warrant intervention [1, 57, 58]. Iodine requirements are increased ≥50 % dur- ing pregnancy, and iodine deficiency can cause maternal and fetal hypothyroidism and impair neu- rological development of the fetus. The conse- quences depend upon the timing and severity of the hypothyroidism. The most severe manifestation is cretinism. In moderate-to-severely iodine-deficient areas, controlled studies have demonstrated that iodine supplementation before or during early pregnancy eliminates new cases of cretinism, increases birth weight, reduces rates of perinatal and infant mortality and generally increases devel- opmental scores in young children by 10–20 %. Mild maternal iodine deficiency can cause thyroid dysfunction but whether it impairs cognitive and/or neurologic function in the offspring remains uncer- tain. To date, measures to address iodine deficiency in populations with mild-to-moderate deficiencies have been shown to prevent (during gestation)
increases in maternal thyroid volume and in Tg lev- els. On the other hand, the impact of iodine supple- mentation for the mother on the neurological development of the new-born is yet to be ascer- tained given the absence of clinical trials designed to evaluate those outcomes appropriately. In school-age children, iodine supplementation has been associated with modest benefits in terms of cognitive development. Iodine supplementation has proven to be an effective method for reducing goiter rates and improving iodine status in this age group. Moreover, there is some indication of posi- tive effects on the physical and mental develop- ment and on mortality, although the results of the studies have not always been statistically signifi- cant [52, 56]. Recently, WHO proposed a guideline aims to help member states and their partners in their efforts to make informed decisions on the appropriate nutrition actions to achieve the Millennium Development Goals, in particular, reduction of child mortality and improvement of maternal health. This guideline provides global, evidence-informed recommendations on fortifica- tion of food-grade salt with iodine, for the preven- tion and control of iodine deficiency disorders, with the purpose of improving iodine nutrition and preventing iodine deficiency disorders in popula- tions [57–59].
Conclusion
Although substantial progress has been made, iodine deficiency remains a significant public health problem worldwide, even in developed countries. Consequently, good quality popula- tion data on iodine status are required to assess population status and design strategies to cor- rect deficiencies without introducing exces- sive intakes. If programs of iodine prophylaxis are carefully monitored for both iodine defi- ciency and excess, the relatively small risks of iodine excess are far outweighed by the sub- stantial risks of iodine deficiency. In addition to monitoring iodine nutrition, effective sur- veillance systems should also include moni- toring of iodized salt quality at all levels (industrial, retail, and household) to ensure that salt iodization programs are safe and effective in their control of iodine deficiency.
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S.K. Imam, S.I. Ahmad (eds.), Thyroid Disorders: Basic Science and Clinical Practice, DOI 10.1007/978-3-319-25871-3_4