::::B2C, you introduced the notion of meeting WP:COMMONALITY as a matter of degree. I, though, have taken is as a mere hurdle requirement. In this case, both spellings plainly meet that requirement, so it is reasonable to move on to other considerations. Here there are two: consistency within the article, and consistency between similar articles (a desideratum according to policy at WP:TITLE). The article is written in British English (for example with goitre, not goiter); so a spelling that tends the British way gives internal consistency. Do you find that other titles include semantically related forms beginning with "iodiz-", or "iodis-"? I don't. I therefore think there is no problem with consistency between articles, either. 
 ::::As for the definition and individuation of words being a "semantic" matter, I agree. Of course! Much that is important in RMs, and in the proper reading of guidelines and policies, is inevitably  "semantic". That is simply to say that we have to attend to meanings. Surprised by that? Alarmed? Now, WP:COMMONALITY refers to "variant spellings". It does not refer to "distinct words that are exactly similar except in that they differ in spelling, along with differences that are minimally and necessarily consequent on that primary difference". So the guideline is naturally to be interpreted in the way I show above. 
 ::::Noetica