Selenium is very unevenly distributed - in parts of China it reaches toxic levels in the soil. The medical archaeologists doing the dig at Soutra Aisle a few miles from where I live found that a particular herb with a high selenium content was prescribed as a spring tonic by the mediaeval monks - probably for its effects on the thyroid as Helen described. So was selenium deficiency a local problem? There is a resource to help answer that, the geochemical atlas of Britain. I've read through relevant parts of it when my wife had patients in a place that seemed to have something endemically toxic that had been poisoning families within an area a few hundred yards across for generations. Unfortunately that was just below the atlas's resolution, but it's much more detailed than you'd expect to be possible. If people lived entirely off their gardens the variations in their mineral intake would be huge.

Another trace element like that is manganese. Robert McCarrison investigated that decades ago in India. Deficiency was a serious problem in Bengal, not at all in Punjab. Bengalis are rice-eating vegetarians, Punjabis are wheat-eating omnivores. So Bengalis had a use for manganese supplementation and Punjabis didn't. And in many parts of the world a normal diet can have toxic levels of the stuff.