21-hydroxylase deficiency is a disorder of steroid metabolism (the buildup or breakdown of steroids). Steroids are types of lipids (fats) that include cholesterol, vitamin D, and many hormones. The 21-hydroxylase deficiency results in the inability of the body to produce cortisol and aldosterone (steroid hormones that affect metabolism and salt balance, respectively) from cholesterol. This condition is also known as congenital adrenal hyperplasia. When the precursors of these hormones build up in the adrenal glands, which sit on top of the kidneys and produce hormones, they are converted to androgens (male sex hormones). These androgens can affect the growth and development of both males and females. There are three types of 21-hydroxylase deficiency. Two types of 21-hydroxylase deficiency are classic forms, known as the simple virilizing and salt-loss types. Simple virilizing 21-hydroxylase deficiency causes a buildup of potent androgens that leads to the masculinization (development of male characteristics) of external genitalia in females at birth. Salt-loss 21-hydroxylase deficiency results from an extremely severe loss of enzyme activity. In these cases, so little aldosterone is produced that the kidneys do not reabsorb sodium (in the form of salt). In non-classic 21-hydroxylase deficiency, there are moderate levels of functional 21-hydroxylase enzyme. In this type, both males and females can display signs and symptoms of androgen excess after birth.