In 1990, theWorld Health Organization estimated that 20 million people in the world had varying degrees of preventable brain damage

The 1999 WHO report indicates remarkable progess but nonetheless a large remaining problem: Iodine deficiency Disorder (IDD) ... While remarkable measurable progress is being made through universal salt iodization, there are nearly 50 million people who are estimated to still be affected by some degrees of IDD-related brain damage.
http://www.who.int/inf-pr-1999/en/pr99-wha17.html (fifth paragraph)

In March 2002 the WHO reported, "we are on the verge of eliminating it ... The global rates of goitre, mental retardation and cretinism are falling fast." -- but still reported the 50 million number.
http://www.who.int/nut/idd.htm