Our expert says:
It's a very detailed email, I can tell that you have given it a lot of thought, and I think that some of your theories are quite plausible. I'm not as sure about the anaemia, because most people are not deficient in iron, although there are conditions that can cause this. It would be very easy to check out through a simple blood test though, which I would recommend.
The hyperthyroidism is an interesting one, and again, something for which there is a definitive test, so that you don't have to guess but can know for sure if you have the test performed. I don't think that tachycardia would account for it - it's not really a condition, but rather a symptom of having a high heart rate, and so perhaps the tachycardia is just pointing to something else.
The other options that I think of right away are a viral infection - there are some viruses which can cause excessive fatigue for months as a result of recurrences - malaria, eppstein-barr virus are two that come to mind. You might want to check that out. There are other conditions that we don't know too much about at this stage, where the body seems to develop an 'intolerance' to exercise, and this is something that you might have, though I doubt it because of your age and your previous training history. I think that the virus idea might be worth looking into, together with the hyperthyroidism. I'm sorry I can't be of more help, but it sure does seem like a medical consideration, and so perhaps seeing a doctor for some comprehensive tests is the way to go.
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