The scar tissue seen in the selenium - deficient children affected with myxedematous endemic cretinism
further supports the notion that a lack of the antioxidant selenium leads to the
destruction of thyroid tissue
due to inability to neutralize the hydrogen peroxide.
(Notably, the tougher breeds of Metroids in the Prime series so
far have all been explicitly wiped out: Fission Metroids were confined to the Impact Crater, which blew up; Dark Metroids could not survive outside of (and consequently perished with) Dark Aether; Phazon Metroids and all their variants died out with all other Phazon - based lifeforms in the universe
due to the
destruction of Phaaze; and all the modified Big Metroids were in the Talvania breeding lab when it exploded.)