Klinefelter syndrome

Klinefelter syndrome is a genetic condition that results when a boy is born with an extra copy of the X chromosome. Klinefelter syndrome is a common genetic condition affecting males.

Klinefelter syndrome adversely affects testicular growth, and this can result in smaller than normal testicles. This can lead to lower production of the sex hormone testosterone. Klinefelter syndrome may also cause reduced muscle mass, reduced body and facial hair, and enlarged breast tissue. The effects of Klinefelter syndrome vary, and not everyone with it develops these signs and symptoms.

Klinefelter syndrome often isn't diagnosed until adulthood. Most men with Klinefelter syndrome produce little or no sperm. But assisted reproductive procedures may make it possible for some men with Klinefelter syndrome to father children.

Symptoms Causes Risk factors Complications

Many boys with Klinefelter syndrome have few noticeable symptoms, and the condition may go undiagnosed until adulthood. For others, the condition has a noticeable effect on growth or appearance. Klinefelter syndrome may also cause speech and learning problems in some boys.

Signs and symptoms of Klinefelter syndrome vary by age and may include:


  • Weak muscles
  • Slow motor development — taking longer than average to sit up, crawl and walk
  • Delay in speaking
  • Quiet, docile personality
  • Problems at birth, such as testicles that haven't descended into the scrotum

Boys and teenagers

  • Taller than average stature
  • Longer legs, shorter torso and broader hips compared with other boys
  • Absent, delayed or incomplete puberty
  • After puberty, less muscular bodies and less facial and body hair compared with other teens
  • Small, firm testicles
  • Small penis
  • Enlarged breast tissue (gynecomastia)
  • Weak bones
  • Low energy levels
  • Shyness
  • Difficulty expressing feelings or socializing
  • Problems with reading, writing, spelling or math
  • Attention problems


  • Infertility
  • Small testicles and penis
  • Taller than average stature
  • Weak bones
  • Decreased facial and body hair
  • Enlarged breast tissue
  • Decreased sex drive

When to see a doctor

See a doctor to rule out Klinefelter syndrome or another health condition if you or your son has:

  • Slow development during infancy or boyhood. If your son seems to be developing more slowly than other boys, see your son's doctor. Common signs and symptoms of Klinefelter syndrome include enlarged breast tissue (gynecomastia), small genitals, and small, firm testicles. Though some variation in physical and mental development is normal, it's best to check with a doctor if you have any concerns.

    Delays in growth and development can be the first sign of a number of conditions that need treatment — including Klinefelter syndrome. If your son does have Klinefelter syndrome, early treatment, including speech therapy, can help prevent or minimize problems.

  • Male infertility. If your partner hasn't gotten pregnant after a year of regular, unprotected sex, see a doctor. Infertility is usually caused by something other than Klinefelter syndrome, but many men aren't diagnosed with the condition until they realize they're unable to father a child.

Klinefelter syndrome occurs as a result of a random error that causes a male to be born with an extra sex chromosome. Of the 46 human chromosomes, the two sex chromosomes determine a person's sex. In females, both sex chromosomes are X (written as XX). Males have an X and a Y sex chromosome (XY).

Most often, Klinefelter syndrome occurs because of one extra copy of the X chromosome in each cell (XXY). Extra copies of genes on the X chromosome can interfere with male sexual development and fertility.

Some males with Klinefelter syndrome have the extra X chromosome only in some of their cells (mosaic Klinefelter syndrome). Rarely, a more severe form of Klinefelter can occur if a male has more than one extra copy of the X chromosome.

Klinefelter syndrome isn't an inherited condition. Rather, the additional sex chromosome results from a random error during the formation of the egg or sperm or after conception.

Klinefelter syndrome stems from a random genetic event. The risk of a child being born with Klinefelter syndrome isn't increased by anything a parent does or doesn't do. For older mothers, the risk is higher but only slightly.

Complications of Klinefelter syndrome can include:

  • Delayed puberty
  • Noticeable physical features, such as sparse facial and body hair, unusually long legs and arms, lack of muscular development, and enlarged breast tissue
  • Learning disabilities, attention problems or social development issues
  • Infertility
  • Weak bones (osteoporosis)
  • Increased risk of varicose veins and other problems with blood vessels
  • Increased risk of breast cancer and cancers of the blood, bone marrow or lymph nodes
  • Increased risk of lung disease
  • Increased risk of autoimmune disorders, such as type 1 diabetes and lupus
  • Increased belly fat, which can lead to other health problems
  • Problems with sexual function

A number of complications caused by Klinefelter syndrome are related to low testosterone (hypogonadism). Testosterone replacement therapy reduces the risk of certain health problems, especially when therapy is started at the beginning of puberty.

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