18-hydroxycorticosterone (18-OHB) and -cortisol (18-OHF) Tests

18-OHB is an intermediate compound produced in the synthesis of aldosterone from cholesterol in the adrenal cortex (see physiology section and section on Congenital Adrenal Hyperplasia). 18-OHF is formed from circulating cortisol in the adrenal cortex. Their plasma and urine concentrations are elevated in most causes of primary hyperaldosteronism but are highest in glucocorticoid-suppressible aldosteronism (GSA), less high in aldosterone-producing adenomas and least high in idiopathic hyperaldosteronism. This pattern can provide some useful information when used in conjunction with other tests.