I've just found I have an underactive thyroid.
I had been going to the doctor on and off for years in the UK because I was always exhausted and had zero energy, they just said you work, you have kids, you study of course your tired - basically go away.
I went for a physical at the doctor in Canada a couple of weeks ago and within 3 days I get a call to go back in, blood test have shown I have an underactive thyroid and very very low levels of vitamin B - both of which cause tiredness. One little pill a day and I'm alraedy feeling much better, I'm not falling asleep by 4 in the afternoon.