Adapted from Cecil’s Essentials of Medicine (I start an ICU rotation in 3 weeks and I’m starting to brush up on this crap).
Hypokalemia differential diagnosis in the adult:
3 major categories – renal, extrarenal, and redistribution losses. So start by looking not only at blood K but urine K as well. If you are shedding potassium into the urine, you have a kidney problem.
Renal losses can be further divided into 3 categories based on the acid/base balance of the blood (metabolic acidosis, metabolic alkalosis, or normal). If you are acidotic, think renal tubular acidosis or organic acidosis with potassium following, or carbonic anhydrase inhibitors. If you are alkalotic, think diurectics, vomiting, low magnesium, hyperaldosteronism, hypercortisolism, or other more rare syndromes. Or hey, even a lot of licorice will do it to you. With normal blood pH, it could be acute tubular necrosis, postobstructive reflex diuresis, or a drug reaction.
Outside of the kidney (extrarenal), think diarrhea or laxative abuse or bulimia, or even decreased intake. Acid-base balance may also be affected.
Redistribution means forcing K into cells, and thus reducing blood levels. This can be done with many drugs, including insulin and albuterol. Also, pheochromocytoma or catecholamine excess will do it, as will regular alkalosis.
That covers most of the common ones. There are others as well, like certain inherited syndromes and other things that are less likely in a previously healthy adult. That said, the ME also mentioned that K was only checked, incredibly, after several rounds of epinephrine (a catecholamine, check the last paragraph!) and lots of IV fluids (which usually don’t contain potassium, just sodium and chloride). This confounds everything anyway, and makes a primary cause impossible to detect, unfortunately.