It would seem that there may be a link between endurance running and hypothyroidism (not necessarily causal, though). I've recently started running for the first time and my thyroid function has declined; it may be coincidence. I definitely wouldn't class myself as an endurance runner as I'm only just able to keep going for 28 mins without stopping (thanks, c25k!). Has anyone else had a similar experience? Was it temporary or did you need to adjust the dose of whatever medication you're on for the long term?