In biology and medicine, virilization refers to the development of changes which make a male body different from a female body. Most of the changes of virilization are produced by androgens.
In the prenatal period, virilization refers to closure of the perineum, thinning and rugation of the scrotum, growth of the phallus, and closure of the urethra to the tip of the penis. In this context masculinization is synonymous with virilization. Prenatal virilization and undervirilization are common causes of ambiguous genitalia and intersex conditions.
Undervirilization can occur if a genetic male cannot produce enough androgen or the body tissues cannot respond to it. Extreme undervirilization occurs when no significant androgen can be produced or the body is completely insensitive to it, and results in a female body. Partial undervirilization produces ambiguous genitalia part way between male and female. The mildest degree of undervirilization may be a slightly small penis with hypospadias. Examples of undervirilization are androgen insensitivity syndrome, 5 alpha reductase deficiency, and some forms of congenital adrenal hyperplasia.
Prenatal virilization (or masculinization) of a genetically female fetus can occur when an excessive amount of androgen is produced by the fetal adrenal glands or is present in maternal blood. In the severest form of congenital adrenal hyperplasia complete masculinization of a genetically female fetus results in an apparently normal male baby boy with no palpable testes. In most cases, the virilization is partial and the genitalia are ambiguous.
Virilization can occur in childhood in either boys or girls due to excessive amounts of androgens. Typical effects of virilization in children are pubic hair, accelerated growth and bone maturation, increased muscle strength, acne, adult body odor, and sometimes growth of the penis. Congenital adrenal hyperplasia or androgen producing tumors (usually) of the gonads or adrenals are common causes, but in a boy, virilization may signal precocious puberty.
Virilization in a woman can manifest as clitoral enlargement, increased muscle strength, acne, hirsutism, frontal hair thinning, deepening of the voice, and menstrual disruption due to anovulation. Some of the possible causes of virilization in women are: