21-Deoxycortisol
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. Marker of 21-hydroxylase deficiency. 21-deoxycortisol is a marker of congenital adrenal hyperplasia due to 21-hydroxylase deficiency, even in mild (non-classic) cases. The deficiency of the 21-hydroxylase enzyme leads to excess of 17α-hydroxyprogesterone, a 21-carbon (C) steroid. This excess is accompanied by the accumulation of other C steroids, such as 21-deoxycortisol, which is formed by  the 11β-hydroxylation of 17α-hydroxyprogesterone via 11β-hydroxylase (CYP11B1). 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". Unlike 17α-hydroxyprogesterone, 21-deoxycortisol is not produced in the gonads and is uniquely adrenal-derived. Hence, 21-deoxycortisol is a more specific biomarker of 21-hydroxylase deficiency than is 17α-hydroxyprogesterone. The corticosteroid activity of 21-deoxycortisol is lower than that of cortisol. As 21-deoxycortisol can be at high levels in congenital adrenal hyperplasia, and it has structural similarity to cortisol, it  can cross-react in immunoassays, resulting in a falsely normal or high cortisol result, when the true cortisol is actually low. 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. Hence, the use of LC-MS/MS instead of immunoassays in cortisol measurement aims to provide greater specificity. Besides 21-deoxycortisol, another C steroid, 21-deoxycorticosterone (11β-hydroxyprogesterone), has been proposed as a marker for 21-hydroxylase deficiency, but this marker did not gain acceptance due to the fact  that testing for the levels of this steroid is not routinely offered by diagnostic laboratories.