Yes, a minority of people (less than 2%) have Hashimoto's Thyroditis, some without knowing about it, and a large dose of iodine could cause damage to their thyroid, I think through some sort of auto-immune process. The selenium helps prevent that.

It's a shame because the remaining 98% could get a lot of benefit and (setting aside the above) iodine is supposed to be the least toxic pure element. In the past Lugol's iodine was used as an antiseptic to put on cuts so I guess a fair amount got absorbed that way.

Not dozens but thousands!