Renin is an enzyme hormone produced by the kidneys and helps control the release of aldosterone. Renin is usually compared to aldosterone when lab testing is used. If aldosterone by itself is low, it can point to an adrenal issue. If both aldosterone and renin are low, it can point to a pituitary issue. For the latter, see this study.
***To read what thyroid patients have learned in interpreting this test, go here.