Very interesting "meta-analysis " on studies on Iodine and intelligence. Follow the link for complete text:

http://www.gwern.net/Iodine



"Iodine deficiency is interesting from the ethical standpoint as one of the most cost-effective - yet obscure - public health measures ever devised which deserves a name like nootropics: a few pennies of Iodine added to salt eliminates many cases of mental retardation & goiters.

BACKGROUND

Supplementation of Iodine in salt, water, or oil increases body iodine levels and reduces iodine deficiency disorders (Wu et al 2002). Supplementing, during pregnancy or infancy, can raise average IQs in the worst-off regions by as much as 13 IQ points1 (close to a full standard deviation). In an additional bonus for our post-feminist society, females benefit more from iodization than males2. Because salt production is generally so centralized as a bulk commodity extracted from a very few areas, iodization is almost trivial to implement. (Although humans being humans, there are obstacles even to successful iodization programs3.)

Cretinism is only the most extreme form of iodine deficiency, although a major & worthwhile humanitarian task; iodine correlates with IQ in non-deficient children, eg. Japan is simultaneously one of the longest-lived & highest IQ countries in the world and one of the greatest consumers of iodine4 (from seaweed, principally, levels so high they suggest that current recommendations are overly conservative), and even weak hypothyroidism impairs mental performance (in the old). School is directly impacted in randomized trials; from a review of Poor Economics (Banerjee & Duflo 2011):
Providing iodine capsules to pregnant mothers is an intervention that helps brain development in fetuses. It costs around 51 cents per dose - and leads to kids who stay in school about five months longer because they are cognitively better able to learn.

The original waves of iodization caused large-scale changes in: numbers of people going to school5, working at all6, their occupation7, how they voted, or even how many recruits from a region are accepted to selective flight schools."...