Klinefelter syndrome is a genetic disease that causes males to be born with an extra copy of the X chromosome in their cells. Males with this condition have smaller-than-normal testicles that produce less testosterone.

Testosterone is the male hormone that stimulates sexual traits like body hair and muscle growth.

A lack of testosterone causes symptoms like breast growth, a small penis, and less facial and body hair than normal. Many men with this condition won’t be able to father children. Klinefelter syndrome can also delay speech and language development in male children.

Because most of the symptoms stem from low testosterone, many boys with Klinefelter syndrome don’t notice any symptoms until they reach puberty. That’s when testosterone levels normally increase.

Testosterone levels by age »

In young children, the first sign is often a delay in speaking or learning. For example, a baby with Klinefelter syndrome might speak later than his peers.

In older males, common symptoms include:

  • small penis and small, firm testicles
  • little to no sperm production
  • enlarged breasts, also known as gynecomastia
  • very little hair on the face, in the armpits, and around the pubic area
  • increased height
  • long legs with a short trunk
  • lack of muscle tone and strength
  • lack of energy
  • low sex drive
  • increased belly fat
  • problems reading, writing, and communicating
  • infertility
  • anxiety and depression
  • problems interacting socially
  • metabolic disorders like diabetes

Men who only have an extra X chromosome in some of their cells will have milder symptoms. A smaller number of males have more than one extra X chromosome in their cells. The more X chromosomes they have, the more severe their symptoms will be.

Symptoms of more severe Klinefelter types include:

  • major problems with learning and speech
  • poor coordination
  • unique facial features
  • bone problems

Read more: 9 Signs of low testosterone in men »

Klinefelter syndrome is one of the most common chromosome conditions in newborns. It affects between 1 in 500 and 1 in 1,000 newborn males. Variants of the disease with three, four, or more extra X chromosomes are less common. These variants affect 1 in 50,000 or fewer newborns.

It’s possible that Klinefelter syndrome affects even more boys and men than statistics suggest. Sometimes symptoms are so mild that they go unnoticed. Or symptoms can be misdiagnosed being caused by other similar conditions.

Everyone is born with 23 pairs of chromosomes, or 46 chromosomes in total, inside each of their cells. These include two sex chromosomes, X and Y.

  • Females are born with two X chromosomes, XX. These chromosomes give them female sexual traits like breasts and a uterus.
  • Males are born with one X and one Y chromosome, XY. These chromosomes give them male traits such as a penis and testicles.

Boys with Klinefelter syndrome are born with an extra X, causing their cells to have XXY chromosomes. This mistake happens randomly during conception. About half the time the extra chromosome starts in the mother’s egg. The other half of the time it comes from the father’s sperm.

Some boys have more than one extra X chromosome. For example, their chromosome might look like this: XXXXY. Others have different chromosome abnormalities in each cell. For example, some cells might be XXY, while others are XXXY. This is called mosaicism.

This condition is usually random and not caused by anything either parent did. Women who are over age 35 when they get pregnant are slightly more likely to give birth to a baby with Klinefelter syndrome.

A small number of males with Klinefelter syndrome are diagnosed before birth when their mother has one of these tests:

  • Amniocentesis: In this test, a technician removes a small amount of amniotic fluid from the sac surrounding the baby. The fluid is then examined in a lab for chromosome problems.
  • Chorionic villus sampling: Cells from tiny finger-like projections called chorionic villi in the placenta are removed. These cells are then tested for chromosome problems.

Because these tests can increase the risk for a miscarriage, they usually aren’t done unless the baby is at risk for a chromosome problem. Often Klinefelter syndrome isn’t discovered until a boy reaches puberty or later.

If your child is slow to develop, see an endocrinologist. This doctor specializes in diagnosing and treating hormonal conditions.

During the exam, your doctor will ask about any symptoms or development issues you or your child has. Two types of tests are used to diagnose Klinefelter syndrome:

  • Chromosome analysis: Also called karyotyping, this blood test checks for abnormal chromosomes, such as an extra X chromosome.
  • Hormone testing: Blood or urine tests can show low testosterone levels, which are a sign of Klinefelter syndrome.

Mild symptoms often don’t need to be treated. Males with more obvious symptoms should start treatment as early as possible, preferably during puberty. Starting treatment early can prevent some of the symptoms.

One of the main treatments is testosterone replacement therapy. Taking testosterone around the time of puberty will trigger the development of male characteristics that normally happen at puberty, such as:

  • a deep voice
  • hair growth on the face and body
  • increased muscle strength
  • penis growth
  • bone strengthening

Learn more: Hormone replacement therapy for men »

You can take testosterone as a pill, patch, or cream. Or you can get it by injection every two to three weeks.

Other treatments for Klinefelter syndrome include:

  • speech and language therapy
  • physical therapy to improve muscle strength
  • occupational therapy to help you function day to day at school, work, and in your social life
  • behavioral therapy to help gain social skills
  • educational assistance
  • counseling to deal with emotional issues like depression and low self-esteem that stem from the condition
  • surgery (mastectomy) to remove extra breast tissue
  • fertility treatment

Most men with Klinefelter syndrome don’t produce much or any sperm. A lack of sperm can make it hard to father children, but it’s not impossible.

Fertility treatment can help some men become fathers.

If you have a low sperm count, a procedure called intracytoplasmic sperm extraction with intracytoplasmic sperm injection (TESE-ICSI) removes sperm directly from your testicle and injects it into an egg to increase the chances of pregnancy.

It can be hard to live with Klinefelter syndrome. Boys might be embarrassed about the lack of changes in their body during puberty. Men can be upset to know they can’t father children.

A therapist or counselor can help you manage any depression, low self-esteem, or other emotional issues that stem from this condition.

You might also look for a support group, where you can talk with other men who have this condition. You can find support groups through your doctor or on the internet.

Children with Klinefelter syndrome often need extra help in school. Contact your local school district to find out about special programs for kids with disabilities. You might be able to get an Individualized Education Program, which helps tailor a learning program to suit your child’s needs.

Many boys with Klinefelter syndrome have more trouble interacting socially than their peers. An occupational or behavioral therapist can help them learn social skills.

You and your doctor will need to watch out for conditions that are more common in men with Klinefelter syndrome, including:

According to research, Klinefelter syndrome can shorten your life expectancy up to two years. However, you can still live a long, full life with this condition. The earlier you get treatment, the better your outlook will be.