steroid hormone biosynthetic pathway C21-steroid hormone biosynthetic pathway C21-steroid hormone biosynthetic process PW:0000070 Those metabolic reactions involved in the synthesis of C21-steroid hormones. Pregnenolone, the first C21 steroid derived from cholesterol, and progesterone, to which pregnenolone can be converted, provide the starting materials for the biosynthesis of C21, C19 and C18 steroid hormones. The C21 class includes glucocorticoids such as cortisol and mineralocorticoids such as aldosterone. Glucocorticoids and mineralocorticoids are collectively referred to as corticosteroids. corticosteroid biosynthetic pathway pathway