SandsofTime,
HbF has a higher affinity for oxygen than HbA, which means it doesn't let go of the oxygen it carries as easily as HbA does, so that does often result in more fatigue in thal minors.
The vitamin D is great and FYI, I, a non-thal, take 50,000 IU once weekly. It sounds high but is not and has finally got me past winter depression. As I've gotten older, I found myself still having trouble making it through the day and wanting to leave work a couple hours early. After hearing thal minors tell me how much folate improved their energy levels, I added 5 mg L-methylfolate daily. I already take 5 mg B-12 daily. Now, I am making it through the day without feeling so fatigued in the late afternoons. The thal minors were right. L-methylfolate can really help.