Quote taken from Lauren Fleshman in regards to handling training loads and a plausible explanation for why certain athletes can survive more extreme training loads for longer periods of time than others:
I was unable to handle the training load that I'd been able to handle without any problem for the past few years. I wasn't resting at night; I was restless ... because of the altitude ... And I'd heard some of his athletes had seen this doctor and they had suppressed thyroids, and I knew these athletes slept in altitude tents too. And I was like, oh man, well maybe that's the problem, maybe my thyroid is messed up and maybe — instead of looking at it like the altitude tent messed up my thyroid — maybe it's something about my thyroid is insufficient to handle this altitude tent. You can see there's a different in that mentality, right? I felt like somehow I deserve to have some superhuman thyroid that can handle anything. So I talked to my coach, Vin Lananna: I think my thyroid is suppressed. He said that could very well be it, you're not responding to the training anymore, you're tired all the time. And I was like people go to this doctor, Alberto knows about him and I'm thinking about reaching out. And this was the most embarrassing moment, but the really important turnaround for me: my coach looked at me and he goes: "Ya know, you could do that, and if you want to do that, it's your career, and I don't think there's necessarily anything illegal about it, but the reason why your thyroid is as messed up as it is because you're not resting enough, you're sleeping in an altitude tent, and you're training too hard. And you have to back off, you have to change those things, you have to take responsibility."
https://www.propublica.org/article/elite-runner-had-qualms-alberto-salazar-asthma-drug-performance