I haven’t checked my thyroid but I have some hypo symptoms, which isn’t surprising after next to no iodine for 6-months or more (I actually gave up table salt a year ago). Iodine deficiency shares a lot of symptoms and anyone fighting an infection needs more of it, so supplementing kelp definitely can’t hurt. It’s cheap too.
Iodine was second behind omega-3 for deficiency according to that woman’s data (more than 70% of people), and people get no omega-3 nowadays. I don’t think salt covers it, especially now everyone has a phobia of it, and definitely for someone fighting an infection.
As for “bottom threshold” thyroid levels, I’d look into that. It’s a bit strange that you’ve dropped from the high end to the low end, developed symptoms, and your doctor sees no problem with that. I know for adrenal deficiencies “bottom threshold” means you’re running on empty, but it’s perfectly fine as far as medical interpretation goes. It’s the same for a lot of vitamin/mineral deficiencies.