If it were common enough in available foods then they wouldn't have added it to table salt.

IIRC this was added long ago, when Americans ate less meat, dairy, and seafood and were more prone to malnutrition.

And yet, levels have been measurably going down, and Europeans are already fairly deficient since they don't have as many iodine supplementation programs:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9459956/

https://www.cdc.gov/nchs/data/hestat/iodine.htm