I have looked up synonyms for lethargy online and found that fatigue is definitely one of them, which is stated dozens of times in my records, dating back to December 2010, when my kidney condition was discovered. Just look up fatigue and the first word listed is lethargy.
Quote from 38 cfr " §4.115a Ratings of the genitourinary system—dysfunctions - or, generalized poor health characterized by
lethargy, weakness, anorexia, weight loss, or limitation of exertion"
Quote from BVA - "The Board is affording the Veteran the benefit of the doubt that he now suffers from lethargy as his NOD and his December 2016 VA examination report both document that symptoms. Based on his symptoms of lethargy, the Board finds that the Veteran is entitled to an 80 percent rating, effective the date of the NOD which alleged symptoms which approximate the requirements of that level."
Quote from Doctor Letter - "Patient has stage 3B chronic kidney disease and anemia. Both of these conditions can cause chronic fatigue/tiredness/lethargy, which the patient has reported to me and other providers."