As a keen runner, I've been struggling to find much in the way of conclusive information about how exercise affects thyroid function and vice versa. It seems that not much research has been done, however I have found the following articles which I thought might be of interest to others.
This page has links to several journal articles:
If I'm reading it right, this article explains why TSH can decrease with exercise - because of an impaired response to TRH:
This link is part of a blog but has lots of references:
I hope this helps someone else,