I imagine they've already eliminated the really obvious explanations of that sort.

IIRC, genetics and a high consumption of seafood, especially shell fish and other things high in iodine, seem to be likely factors.

If you tolerate seafood well, that's potentially of interest. People could look into studies relevant to that angle.

In western countries, a pack a day used to be considered moderate. Some people still smoke a ton more than that, chainsmoking every hour they're awake, but it's not as common as it used to be.