Micropenis
Micropenis is a medical term that describes an unusually small penis in a human male. 
A common criterion is a dorsal (measured on top) erect penile length at least 2.5 standard deviations smaller than the mean penis size.  The condition is usually recognized shortly after birth. The term is most often used medically when the rest of the penis, scrotum, and perineum is without ambiguity such as hypospadias.

# Causes
Of the abnormal conditions associated with micropenis, most are conditions of reduced prenatal androgen production or effect. Examples include abnormal testicular development (testicular dysgenesis, Klinefelter syndrome, Leydig cell hypoplasia), specific defects of testosterone or dihydrotestosterone synthesis (17,20-lyase deficiency, 5-reductase deficiency), androgen insensitivity syndromes, inadequate pituitary stimulation (gonadotropin deficiency) or other forms of congenital hypogonadism. Micropenis can also occur as part of many genetic malformation syndromes not involving the sex chromosomes. It sometimes is a sign of congenital growth hormone deficiency or congenital hypopituitarism. Finally, several Homeobox genes have been identified which affects penis and digit size without detectable hormone abnormalities.
After evaluation to detect any of the conditions described above, micropenis can often be treated in infancy with injections of various hormones, such as human chorionic gonadotropin or testosterone.
Most eight to fourteen year old boys referred for micropenis do not have the micropenis condition.  Such concerns are usually explained by one of the following:
- a penis concealed in suprapubic fat (extra fat around the mons pubis).
- a large body and frame for which a prepubertal penis simply appears too small.
- delayed puberty with every reason to expect good future growth.

# Treatment

## Hormone treatment
Growth of the penis both before birth and during childhood and puberty is strongly influenced by testosterone and, to a lesser degree, growth hormone, but their value in the treatment of micropenis is mainly limited to conditions of hormone deficiency such as hypopituitarism or hypogonadism.
Regardless of the cause of a micropenis, if it is recognized in infancy, a brief course of testosterone is often prescribed (usually no more than 3 months). This will usually induce a small amount of growth, confirming the likelihood of further growth at puberty, but rarely achieves normal size. No additional testosterone is given during childhood to avoid unwanted virilization and bone maturation. (There is also some evidence that premature administration of testosterone can lead to reduced penis size in the adult.)
Testosterone treatment is resumed in adolescence only for boys with hypogonadism. Penile growth is completed at the end of puberty, similarly to the completion of height growth, and provision of extra testosterone to post-pubertal adults will produce little or no further growth.

## Surgery
Because hormone treatment rarely achieves average size, a number of surgical techniques like phalloplasty for penis enlargement have been devised and performed but are not generally considered successful enough to be widely adopted and are rarely performed in childhood.
In extreme cases of micropenis, there is barely any shaft, and the glans appears to sit almost on the pubic skin. From the 1960s until the late 1970s, it was not unusual for sex reassignment and surgery to be recommended. This was especially likely if evidence suggested that response to additional testosterone and pubertal testosterone would be poor. If parents accepted, the boy would be reassigned and renamed as a girl, and surgery performed to remove the testes and construct an artificial vagina. This was based on three now questioned assumptions:
- gender identity and sex differences were solely a matter of social learning rather than biology.
- a male with a penis too small to put into a vagina could not find a satisfactory social and sexual place in society.
- a functionally acceptable vagina could be constructed surgically.
The center most known for this approach (Johns Hopkins Hospital) performed twelve such reassignments between 1960 and 1980, most notably, that of David Reimer, overseen by Dr. John Money. By the mid-1990s reassignment was less often offered, and all three premises had been challenged. Former subjects of such surgery, vocally dissatisfied with adult outcome, played a large part in discouraging this practice. As a result, sexual reassignment is rarely performed today for severe micropenis (although the issue of raising the child as a girl is sometimes still discussed.) Much inaccurate or exaggerated folklore on this topic is available on the internet. (See History of intersex surgery for a more complete discussion of this issue.)

# Micropenis in studies
A relatively high incidence of alleles believed to cause the micropenis condition have been found in a Japanese study of patients with micropenis, consisting of 81 Japanese patients. The presence of a micropenis in these subjects is considered to be due to a higher mutation rate for the SRD5A2 gene, which encodes for the enzyme 5{alpha}-reductase-2 and plays a role in male sex differentiation. The mutation led to decreased expression of the enzyme, which in turn results in penises with erect lengths of -2.5 standard deviations. In the case of the Japanese sample, hormone treatments were also studied and found to be effective, resulting in penis lengths at nearly the average of age-matched Japanese controls (2-3 cm). After extended study of various micropenises, it can be concluded that this condition has severe effects both mentally and physically for the patient.