What are the causes of low serum potassium?

(A.k.a. "What are the causes of 'hypokalemia', or 'hypokalaemia?" Damn this transatlantic spelling thing.)

As usual, any answer to a question beginning with "What are the causes of ... " would keep you writing to the end of time. The key is to have an approach to the problem that helps you remember more of the causes than the 'scattershot approach', and that is hopefully clinically relevant.

A general schema for causes of a low that I use is as follows:

• Too little going in.
• Too much being destroyed / used up.
• Too much hiding somewhere else.
• Too much being lost from the body.

For example, in the case of low platelets (thrombocytopenia) an example from each category would be:

• Ineffective marrow production
• Immune thrombocytopenia (autoimmune destruction of the platelets)
• Hypersplenism (platelets being sequestered in a huge spleen)
• (Hmm, there aren't really any good examples for this category, but you get the picture.)

Now let's use the schema for dealing with low potassium:

• Deficient dietary intake (a rare cause on its own, since potassium abounds in most foods; however, this can still contribute.)
• You can't very well destroy potassium (short of a nuclear explosion) since they are nice 'indivisible' atoms. This category doesn't therefore apply to any of the atoms or ions.
• On the other hand, 'hiding somewhere else' is a major cause of hypokalaemia, in the form of an intracellular shift. Anything that stimulates the sodium/potassium ATPase on cells will do this, and thus certain elevated hormones (insulin, beta-adrenergics) lower the extracellular potassium nicely by hiding some of it inside cells. Other ways to get more potassium inside cells is to make more cells (e.g. some cancers, treatment of B12 or folate deficiency) or to get a metabolic alkalosis (hydrogen ions usually compete for intracellular binding sites; the less of them around, the more potassium gets to go indoors.)
• Excreting too much potassium from the body is also a famous way to have a hypokalaemia. Broadly, the potassium may be lost by the kidney (most commonly secondary to diuretics), the gut (as in diarrhoea) or, less commonly, through sweat.

There you go. Now, into this schema you can insert as many causes as you need. It also suggests a way to work up a person with hypokalaemia too, if you think about it.