Anemia (vitamin B12 or iron deficiency) Autoimmune (rare) Adrenal insufficiency Atrophic gastritis with pernicious anemia Diabetes mellitus type 1 Rheumatoid arthritis Chronic kidney disease Liver disease Menopause Mental health (i.e., depression, anxiety, or somatoform disorders) Obstructive sleep apnea Viral infections (e.g., mononucleosis, Lyme disease, HIV)