Into concideration I would take the fact that some areas of the US is high in selenium (soil) this may contribute to thyroid problems; as the selenium gets to high in proportion to iodine.
Over here in Scandinavia the soil is very low, and it is a problem, even the government have admitted; but we are saved by grain import, from US.
Anyway; I took up to 800 mcg selenium methionine for some time in the beginning of io supp + brazil nuts. With under 50 mg iodine, it was too much selenium as I see it now. But I survived. What I have seen in a couple of cases was that to much selenium caused hair loss.
Diet influence this too; you have mentioned eating sardines very often, you get a near perfect selenium-sulfur-amino acid food.