First you want to get him tested to see. Usually you see an endocrinologist for that, but make sure they do thorough testing. Holistic nutritionists often do more thorough testing. Your regular doc is unlikely to do thorough tests, but he can if you make him. What he'll do is test for Tsh, which won't help you -- you need to get him to test for at least T3 and T4 for levels and conversion from one to the other. A really thorough test would also test for T1 and T2, but usually only holistic nutritionists test for that. Now, you can have borderline hyper or hypo thyroid which to a doctor isn't a problem but to a person can be a problem. But it's hard to tell. The symptoms of hyper and hypo are pretty much the same quite often. I'm still guessing it's nerves, but that's just me. But again, if he trains as hard as he competes and doesn't have this problem while he's training, it's very hard to see how it's being caused by his activity. You explain him as being incredibly hard on himself, and it still sounds like a more reasonable explanation, but again, what do I know?