These are the time frames when a person s brain is developing. The mineral iodine is missing from the soil in many parts of world. This is the same soil used to grow the food for the animals and humans which live within the region. It should not be surprising that such soil would result in plants and vegetables that do not contain iodine. That also applies to weeds, trees, and other vegetation.
Humans consume plants, vegetables, and often the meat from animals.
Since the animals used for food by humans commonly have consumed plants, vegetables, weeds, grasses and tree leaves, it follows that in locales where iodine does not exist within the soil, the normal method for a human to obtain iodine would not work. Humans, and also animals, would be deficient in iodine. If a country or a large area does not have sufficient iodine within its soil, iodine deficiency cannot be eliminated by changing dietary habits or eating certain kinds of foods grown within that area. The correction has to be achieved by supplying iodine through an external source. In the United States, this was done by fortifying the commonly used mineral, salt.