Ok, really hope that doesn't happen. Let's let sleeping dogs lie.

Okay, that's a very clear answer. And it seems, at least from Wikipedia, that tyrosine and iodine are a good combo because the iodation reaction with elemental iodine is pretty consistent and doesn't really need a enzyme... right? I could be totally wrong. Thank you for your time!

Thank you for your answer!

Does that mean that we, by the same token, are one retrovirus away from going back to egg-laying ?

I know this answer: It's far more prevalent in the sea than anywhere else, and diminishes in concentration as you move inland. Many inland regions, especially mountainous regions were traditionally so deficient in iodine that many people suffered from extreme deficiency.

https://en.wikipedia.org/wiki/Congenital_iodine_deficiency_syndrome

Thank you for your answer. Kinda cool how isolated the thyroid is...yet how tightly controlled the process must be.

Thank you for your answer. I'll try to read that article soon. Thyroids are turning out to be really interesting biochemically!

Elemental iodine? In my thyroid?? Wow, that's really cool. Wikipedia said that the iodation of tyrosine by elemental iodine is a really streamlined process, indicating how T3 and T4 evolved.... Thank you for your answer!

Thank you for your response. I'll try to look over that article. The reactivity/antioxidant part is especially interesting.

Why not use a big ol' peptide like insulin ? In any case, that's an interesting observation -- you gotta make your metabolic signaling unambiguous.

Thank you for emphasizing that. Localization was my original question, and I added the others for support.