Everyone I know that has had a thyroid problem, has been struggling with stress of some sort at the same time. It's anecdotal, this is not the result of a scientific study. Yet, everyone I know who has had thyroid issues has had stress. In short, they were not happy people.
Our bodies are amazing, and no one understands them. For example, our hormones react to stress by rebuilding our body, influencing a myriad of functions. Hormones are excreted by our endocrine system. When someone runs hard, your adrenal glands excretes adrenaline into your body, and then people end up calling you an "adrenaline junkie". However, you can only excrete so much adrenaline before you have to let your body rest, by not excreting adrenaline. Where exactly the thyroid fits in, I can't say for sure. But, when someone has problems with the thyroid, he or she runs slow.
The logic is, when a person has stress, because of work, life, family, you ran too much and didn't take the time for your endocrine system to recover, or whatever, you are far more likely to have thyroid problems, and if you have thyroid problems, you will probably run a lot slower. Additionally, iron and B12 play a role in a healthy throid, although how they do, I do not know. All I can say is, take some vitamins to make sure you get enough of both, and more importantly eat a diversity of foods.
What is the cure? Be happy! Seriously, that's it.
Okay, since it's not that easy to "be happy" what concrete steps might one take to increase happiness and endocrine system recovery so that it is possible to perform as well as possible when the day or hour comes? For starters, get your sleep. The hardest (best) rest I know of it sleep. Second, cut out the things, if possible, that create the most stress for you. This is hard for me to give an example because I am pretty content. I suppose, coaching was something that was causing me stress because it was such a large time commitment, and I felt it was actually taking away from my own running and sleeping, and thus happiness, so I quit. Another person I know had thyroid problems, while she was unemployed. Another person I know had thyroid problems while he was counseling way too many people on a weekly basis. Not sure how reversible thyroid problems are. Many biological processes are reversible, when they go poor, a little recovery can lead to them healing, like the average paper cut, or the glycogen depletion after a long run, but more complicated things like a finger, they don't grow back.
It's an old coaching saying, "a happy runner is a fast runner" and it's true. Obviously, this is a huge simplification of endocrinology, but sometimes we make things more complicated than they have to be.