21-Deoxycortisol, also known as 11β,17α-dihydroxyprogesterone or as 11β,17α-dihydroxypregn-4-ene-3,20-dione, is a naturally occurring, endogenous steroid related to cortisol (11β,17α,21-trihydroxyprogesterone) which is formed as a metabolite from 17α-hydroxyprogesterone via 11β-hydroxylase.

[1] 21-deoxycortisol is a marker of congenital adrenal hyperplasia due to 21-hydroxylase deficiency,[2][1][3] even in mild (non-classic) cases.

[1] The build-up of 21-deoxycortisol in patients with congenital adrenal hyperplasia have been described since at least 1955, this steroid was then called "21-desoxyhydrocortisone".

Whereas immunoassays can suffer from cross-reactivity due to interactions with structural analogues, the selectivity offered by liquid chromatography-tandem mass spectrometry (LC-MS/MS) has largely overcome these limitations.

[17][18][19] Hence, the use of LC-MS/MS instead of immunoassays in cortisol measurement aims to provide greater specificity.