Hypothyroid could definitely be a cause of the fatigue. I remember studying so hard for my CFI and even cup after cup of coffee couldn't keep me from getting tired. I didn't know anything about hypothyroid at the time. Turns out that is what it was. I was put on levothyroxin and after a short period of time I felt substantially better. Something to check with your doctor to see if they can help or at least rule it out.