Doctors call it a fatigue panel - There's a set of high-yield blood tests for common conditions that they typically order if you're generally tired. Vitamin D, thyroid, iron levels, testosterone in males. Really quite an easy fix if any of those are significantly off. I've had a couple of times I've felt really run down and got labs done, and I was either anemic or hypothyroid, after a month on a higher dose of levothyroxine or on an iron supplement I feel like a new person.