What Causes Severe Fatigue Inspite Of Being Diagnosed With Thyroid Disorder?

I just received the results of my blood test for my thyroid. It came back below 0. My Dr. is having me take a new blood test to make sure that the test is correct. I can barely carry my body around, pure exhaustion even though I get plenty of rest. Is this a direct cause of a malfunction in my thyroid gland? Thank You