Iodine deficiency has been linked with child mortality as well as permanent cognitive debilitation. Some references are on page 25 of the 2004 Copenhagen Consensus report on malnutrition; a WHO report paints a more vivid qualitative picture (page 6):

In areas of iodine deficiency, where thyroid hormone levels are low, brain development is impaired. In its most extreme form, this results in cretinism, but of much greater public health importance are the more subtle degrees of brain damage and reduced cognitive capacity … the mental ability of ostensibly normal children and adults living in areas of iodine deficiency is reduced … there is little chance of achievement and underdevelopment is perpetuated. Indeed, in an iodine-deficient population, everybody may seem to be slow and rather sleepy.

How do you and I avoid these disorders? The answer isn’t as simple as you might think. Within the last century, iodine deficiency was common in the U.S., and it has been combated using an explicit and sustained public health effort. The following is taken from the Micronutrient initiative’s global progress report:

The element iodine was discovered in 1811, but almost a century passed before it was established that lack of iodine caused the swelling of the thyroid gland commonly known as goitre … In the United States, the alarm was first raised in Michigan in 1918 when it was revealed that over 30% of men medically examined for war service had been found to have an enlarged thyroid. Many were declared unfit for service. By 1923 an Iodised Salt Committee had been formed, including physicians and representatives of the Salt Producers Association …. Later that same year, the Morton salt company began marketing iodised table salt nation-wide … by 1932 iodised salt accounted for 90% to 95% of all sales.

Iodine deficiency disorder is now extremely rare in the U.S., but it is still common in many other parts of the world (see the WHO 2004 report on iodine status worldwide). The effects on cognition and economic growth are widely unknown but potentially disastrous – and this is a problem that isn’t necessarily going away by itself, or going away as soon as wealth increases. (Especially if there’s a direct link between iodine deficiency and productivity.)

Iodizing salt may not have the same visceral appeal as water-related programs, but it’s a proven way to solve a truly debilitating problem and reduce both death and poverty. And it’s an area where there’s arguably no substitute for large-scale public health programs. Note that the Copenhagen Consensus rates it as the 3rd most cost-effective use of funds.