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What Is Klinefelter Syndrome?

Klinefelter syndrome is a fairly common genetic condition found in males only.

Many boys with Klinefelter syndrome — also known as XXY syndrome — have no signs or symptoms, and some don't even know they have it until later in life.

The XXY condition that causes Klinefelter syndrome can't be changed, but medical treatment and working with therapists can help a boy's development and lessen the condition's effects. Even without treatment, most boys with Klinefelter syndrome grow up to live productive, healthy lives.

What Causes Klinefelter Syndrome?

Boys who have Klinefelter syndrome are born with it. It's also called XXY because they have an extra X in most or all their cells.

Usually, a person has 46 chromosomes in each cell, divided into 23 pairs, which includes two sex chromosomes. Half the chromosomes are inherited from the father and the other half from the mother. The chromosomes contain genes, which determine an individual's characteristics, such as eye color and height. Boys typically have one X chromosome and one Y chromosome, or XY, but boys with XXY syndrome have an extra X chromosome, or XXY.

Klinefelter syndrome is not caused by anything the parents did or did not do. The disorder is a random error in cell division that happens when a parent's reproductive cells are being formed.

If one of these defective cells contributes to a successful pregnancy, the baby will have the XXY condition in some or all his cells. Some boys will even have more than two X chromosomes (XXXY or XXXXY, for example), which increases the risk of severe symptoms and other health concerns.

When a boy is born with the XXY condition in only some cells, it's called mosaic Klinefelter syndrome. Often, boys and men with this condition have milder signs and symptoms than others with the XXY condition.

What Are the Signs & Symptoms of Klinefelter Syndrome?

Not all boys with Klinefelter syndrome will have noticeable symptoms. Other boys can have symptoms that are physically apparent or problems with speech, learning, and development.

Babies with Klinefelter syndrome typically have weak muscles, reduced strength, and quiet personalities. They also can take longer to do things like sit up, crawl, walk, and speak.

Compared with other kids their age, boys with Klinefelter syndrome might have some or all of these symptoms:

  • a taller, less muscular body
  • broader hips and longer legs and arms
  • larger breasts (a condition called gynecomastia)
  • weaker bones
  • a lower energy level
  • smaller penis and testicles
  • delayed or incomplete puberty (some boys won't go through puberty at all)
  • less facial and body hair following puberty

What Problems Can Happen?

Klinefelter syndrome typically causes a boy's testicles to grow at a slower rate than those of other boys. It also prevents the testicles from producing normal amounts of sperm and the hormone testosterone.

Testosterone affects the way a boy develops both physically and sexually. Low hormone levels and problems with sperm production make it difficult or sometimes impossible for a boy with Klinefelter syndrome to father a child later in life.

Many boys with Klinefelter syndrome show symptoms related to their development of social and language skills. They may have trouble paying attention. A lot of boys learn to talk late or have trouble using words to express their emotions. They also can have trouble with things like learning to spell, read, and write.

Socially, they tend to have quiet personalities. They rarely cause trouble and are often more helpful and thoughtful than other boys. They're often shy and sensitive, and many are less self-confident and less active than other boys their age.

Most boys aren't likely to have major health problems, but the condition can bring some other challenges later in life. Klinefelter syndrome puts males at greater risk of breast cancer, some other cancers, and some other diseases like type 2 diabetes, varicose veins and problems with blood vessels, problems with sexual function, and osteoporosis (weak bones) later in life.

Most boys with Klinefelter syndrome can have sex when they become men, usually with the help of testosterone treatment. But problems with their testicles prevent them from making enough normal sperm to father children. Most men with the condition are infertile and can't father a child the usual way. Options for becoming natural parents are limited, but fertility researchers are working on new treatments. By the time someone with Klinefelter syndrome is ready to become a dad, there may be new options available related to saving sperm from the testicles.

How Is Klinefelter Syndrome Diagnosed?

Since Klinefelter syndrome can be hard to notice, many parents don't know their son has it until he grows up or shows delays in puberty. Sometimes, parents who are worried about their son's development consult a doctor, and the diagnosis reveals Klinefelter syndrome. This can help, because the earlier a boy is diagnosed with Klinefelter syndrome, the more effective the treatments usually are.

To diagnose Klinefelter syndrome, doctors usually begin by asking about any learning or behavior issues and examining the boy's testicles and body proportions. They'll check a blood sample for the extra X chromosome. Before birth, the condition may be found through chromosomal analysis or noninvasive prenatal testing (NIPT). NIPT is done on the fluid surrounding the fetus, tissue from the placenta, or the blood from the mother. After birth, doctors can make a diagnosis with a chromosome or from the baby. Doctors also can do hormone testing, usually by taking a blood sample to check for unusual hormone levels.

How Is Klinefelter Syndrome Treated?

There's no way to change the XXY condition if a boy is born with it, but treatments can help relieve some symptoms. As with many conditions, beginning treatment early can make it much more effective.

Testosterone replacement therapy (TRT) works by increasing a boy's testosterone levels into the normal range. Additional testosterone can help a boy with Klinefelter syndrome develop bigger muscles and a deeper voice, as well as promote growth of the penis and facial and body hair. It can also help improve bone density and reduce the growth of a boy's breasts. Testosterone therapy cannot increase the size of a boy's testicles or prevent or reverse infertility, though.

Educational support services can help boys and teens with Klinefelter syndrome keep pace in school. Many benefit from extra help with schoolwork. If your son has Klinefelter syndrome, let his teachers and school nurse know and see what kind of support is available. He may be eligible for an individualized education plan (IEP) or 504 education plan, which both can provide help for kids with special needs.

Speech therapy and physical therapy can help boys with Klinefelter syndrome learn to speak, read, and write better, or improve muscle strength and coordination. Other forms of therapy include behavioral, mental health, and occupational therapy. These can help improve low self-confidence, shyness, and delayed social development.

What Else Should I Know?

Because boys with Klinefelter syndrome can have problems with schoolwork and sports, they may feel like they don't fit in with other kids their age. They're more likely to have low self-esteem, which can make things harder socially and academically. But by the time they're men, most will have normal social relationships with friends, family members, and others.

If your son struggles in school or has trouble making friends, talk to your doctor, school principal, or school counselor. Counselors and therapists can give boys practical skills to help them feel more confident in social settings. And many schools provide educational services that can help your son succeed.

Despite physical differences and other problems, with the right medical care, early intervention, and ongoing support, a boy with Klinefelter syndrome can lead a normal, healthy, and productive life.

Medically reviewed by: Judith L. Ross, MD
Date reviewed: September 2021