One of the words we don’t use anymore is cretin; it’s long been a derogatory slur rather than a precise description of something. But a century ago cretinism actually meant a specific thing: a person, generally a child, who was severely damaged by a lack of thyroid hormone during early development, particularly fetal development. Now we call the condition congenital hypothyroidism. A few cases still exist, which is why we screen all newborns for thyroid function. But the overwhelmingly most common cause a century ago was hypothyroidism — low thyroid hormone — in pregnant women. The overwhelmingly most common cause of that was deficiency of iodine in the diet.

Congenital hypothyroidism is now rare in the developed world. Why? The reason is iodine supplementation of food, particularly salt. This is a fascinating example of several companies, particularly the giant Morton Salt Company, listening to the advice of medical experts and then just adding iodine to their product. This turned out to be an easy thing to do...