text: I've heard of times when a person's thyroid test came back looking basically okay, but there's a sub-clinical problem that won't show on a blood test. I think they do a thyroid biopsy, and can get answers from that. Also, you could have a sleep disorder- yes, you're sleeping, but maybe not getting enough restorative sleep. Depression can cause fatigue. And chronic fatigue syndrome (which I was diagnosed with) could be the problem, but is not something that there's a test for yet, and there's no real cure for it at this point. I think a vitamin B deficiancy can cause fatigue too. And untreated diabetes, but that would have other symptoms too, and may not cause the tiredness after a few hours of working. For more chronic fatigue info. maybe try this link:\n\n\nhttp://www.nlm.nih.gov/medlineplus/chronicfatiguesyndrome.html