The main job of the red blood cells is to carry oxygen from the lungs to the rest of the body. If you don't have enough red blood cells, or if they aren't carrying enough oxygen, your fatigue may be caused by anemia. A simple blood test will show whether you have anemia. If you do, your doctor will determine what is causing it. It could be due to blood loss, damage to your bone marrow caused by anti-HIV medications or vitamin deficiencies, or by a low level of the hormone erythropoietin which helps make red blood cells.
Medications can correct anemia.
Serious cases might be treated with a blood transfusion. Low hormone levels . Especially in men, low levels of the sex hormone testosterone can cause fatigue and lack of interest in sex and other normal activities. Hormone levels can be checked with blood tests. Pills, patches, creams, or injections can restore hormone levels to normal.