About 6 years ago, when I was 25 (I'm male), my training went off the rails. I was running 70-80 miles a week and completely exhausted all the time. I was sleeping, no joke, 12-14 hours a day. Could not function. Beyond that, I had all the classic symptoms of hypothyroid.
The first doc ran some bloodwork. My TSH came back in the 3-4 range. The doc said everything was normal and sent me on my way.
I went to another doc, told him my symptoms, and he ran another thyroid panel PLUS the thyroid antibodies.
My TPO antibodies were through the roof. He immediately put my on 100mcg Levoxyl per day.
Very quickly, I felt like a normal person again, and my TSH dropped to 1.
Moral: even if your TSH is within the lab-defined normal range, you may still have a thyroid related problem. Make sure to get the antibodies tested as well.