Is there an autoimmune disease that can cause potassium to always be low?

Not typically: Causes of hypokalemia (low potassium) include use of diuretics (water pills), diarrheal illness, excessive sweating, excess use of laxatives, vomiting, primary aldosteronism, diabetic ketoacidosis. Of all these, diabetes is considered an autoimmune disease. Lupus can lead to renal disease, which can cause loss of potassium through the kidneys and therefore hypoklalemia.

Causes of low K: Are antibiotic, diarrhea, diuretics, bulemia, diseases that affect the ability of the kidney to retain k like liddle, barter and cushing syndrome as examples. Autoimmune disease that attack the kidney like lipus may cause low k and also high k in different stages. Other autoimmune dz that attack the intestine with chronic diarrhea like ulcerative colitis and crohn's disease.