The transtubular potassium gradient (TTKG) is used to gauge renal potassium secretion by the cortical collecting duct, indirectly assessing mineralocorticoid bioactivity in patients who have hypo- or hyperkalemia. TTKG values <6 indicate an inappropriate renal response to hyperkalemia, whereas values >2 during hypokalemia point to renal loss. Hypokalemia is not addressed here. Studies supporting the usefulness of the TTKG in hyperkalemia are limited to case series. This calculation may be most useful in distinguishing hyperkalemic patients who have mineralocorticoid deficiency versus resistance by observing a change in TTKG values after physiologic or pharmacologic doses of mineralocorticoids.