It’s a trace element; big deal for two thyroid hormones, T3 +T4. Vegans might not be hollering at enough.
Travels from gut → blood → thyroid. How much you already have affects how much special juice* your Auntie Pootie*releases (inside you). In turn, her juices also control T3 + 4’s release (inside you)
When replete, most is let loose in tinkle + fece, but measuring the former (“UI”; mcg/L) only directly reflects what you’ve eaten in preceding hours. It’s not a reflection of your true background iodine spirit.
When dry, the 1st hormone to dip out is T4, but Auntie P gets the message loud and clear, and in no time at all she’s blowing more and more loads into you, trying to compensate for T4’s loss (low T4 precipitates elevated TSH). She (your pituitary) helps. Really, she does. But not forever**. That sweet, cool TSH, it’s little more than a band-aid here. And you need more. Soon enough you’re grabbing at every bit you can; from your blood, maybe even your intestines, dude. You’re a fiend***. You need help.
Outside iodized salt (most dietary salt’s from processed foods, which don’t use iodized salt), which is all the rage in < 120 countries, it’s mostly obtained from seafood and dairy.
So vegans are basically (not entirely) left with seaweed. The variations in their iodine content are fucking substantial though, so it’s not really recommended. Sea plant-dependence for iodine has led to both deficiency and toxicity.
The key is:
keeping abreast of relative monster-loaders [Kombu/kelp is usually highest, then it’s hijiki, usually, followed by wakame, mostly]
controlling content [cooking lowers iodine content; increase in moisture post-boil with resultant solubilization. Also, avoid uncooked cruciferous vegetables, sweet potaters, cassava, lima beans and maybe millet].