A significant source of declining iodine levels in humans can also be traced to increased bromide found in food and medications. Most notably potassium iodate was used as a bread conditioner until the 1950s when it was substituted for potassium bromate. [0]

Excess bromide levels displace the iodide stored in the thyroid and is a contributing factor to the described increase of groiter. Bromide also has the added drawback of increased cancer risk and has now been banned in California [1] [2].

[0] https://pmc.ncbi.nlm.nih.gov/articles/PMC3916868/

[1] https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml...

[2] https://web.archive.org/web/20131203041405/http://www.oehha....