Iodine deficiency of some kind isn’t all that rare actually - even in the first world. Severe iodine deficiency used to be very common in the Midwest, with supplementation in salt increasing IQ in the region by double digits. 70% of UK people tested in a 2011 study were iodine deficient. It remains one of the most common micronutrient deficiencies worldwide.

https://en.m.wikipedia.org/wiki/Iodine_deficiency

I kind of worry about it coming back, as restaurant, fast food and processed food is commonly not using iodized salt, and at home ‘sea salt’ that hasn’t been iodized is trendy. Dairy is another important source of iodine but processing facility changes have reduced milk content. This opens the door to more regional or sub population deficiency