A picture is worth a thousand words. There are many, many reasons for fatigue. When you have deep fatigue that persists, it is seldom due to a single cause. Refer back to the page about how I think as a health detective. This is a map of the common reasons you might be struggling with fatigue (this is a simplified map). Basic lab tests, and a thorough history can help navigate how your story fits into this map.
One of the big differences between conventional doctors and naturopathic doctors is that conventional doctors will often only look at basic screening tests for one or two possible causes of a symptom. Naturopathic doctors (especially Health Detectives!) will use detailed history and look deeply into functional tests to find all the potential causes and how they relate to each other.