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Klinefelter Syndrome
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Men

Klinefelter Syndrome

Klinefelter syndrome is a genetic condition occurring when a male is born with an extra X chromosome, which can impact physical development and fertility.

What is Klinefelter syndrome (KS)?

Klinefelter syndrome (KS) is a genetic condition that happens when a male is born with an extra copy of the X chromosome. KS can affect different stages of physical, language, and social development. It also usually causes infertility.

What causes Klinefelter syndrome (KS)?

KS is not inherited. It's caused by a random error that happens when a sperm or egg is formed. This error causes a male to be born with an extra X chromosome.

Chromosomes are tiny "packages" in your cells that contain your genes. Genes carry information, called DNA, that controls what you look like and how your body works. Normally, you're born with a set of 46 chromosomes in each cell, two of which are the sex chromosomes. Females usually have two X chromosomes (XX), and males usually have one X and one Y (XY). In KS, the male usually has two X chromosomes and one Y (XXY).

In rare cases, a male could have XY in some cells and XXY in other cells. This is called mosaic Klinefelter syndrome. Males with mosaic Klinefelter syndrome may have fewer symptoms, depending on the number of XY cells they have in their bodies and where those cells are located.

What are the symptoms of Klinefelter syndrome (KS)?

Some males with KS may have no symptoms or very mild symptoms. So they might not know that they have KS, or they might not get diagnosed with it right away. In other cases, the symptoms can be more severe.

Boys with KS may be taller than other boys their age, with more fat around the belly. After puberty, they may have:

  • Smaller testes and penis
  • Breast growth (called gynecomastia)
  • Less facial and body hair
  • Reduced muscle tone
  • Narrower shoulders and wider hips
  • Weaker bones
  • Decreased sexual interest
  • Lower energy

Many of these symptoms happen because of low testosterone in the body. Testosterone is the main male sex hormone.

Boys with KS may also have learning or language problems. These problems may affect them socially, so they may be shy and quiet and can have trouble fitting in.

Most males with KS are infertile because they make little or no sperm.

How is Klinefelter syndrome (KS) diagnosed?

A genetic test called a karyotype test can diagnose KS. This test can show if there are abnormal chromosomes, including if there is an extra X chromosome.

What are the treatments for Klinefelter syndrome (KS)?

There is no cure for KS, but treatments are available. The sooner the treatment is started, the better. If treatment is started by early puberty, it will likely help in reducing the symptoms.

Treatments for KS may include:

  • Testosterone replacement therapy
  • Surgery to remove or reduce breasts
  • Physical, speech, behavioral, and occupational therapy

In some cases, fertility treatments may help men with KS father children.

Frequently Asked Questions

Most men with classic 47,XXY have very low sperm production, but a small percentage produce viable sperm. Options include testicular sperm extraction (TESE) combined with intracytoplasmic sperm injection (ICSI) during in‑vitro fertilization. If no sperm are found, donor sperm or adoption are alternatives. Hormone therapy (testosterone replacement) can improve secondary sexual characteristics but may further suppress sperm; timing of therapy and fertility planning should be coordinated with a reproductive endocrinologist.
Yes. Early intervention with speech‑language therapy can address delayed language development, while occupational therapy helps fine‑motor skills. Educational support such as individualized education plans (IEPs) focusing on reading, math, and executive‑function strategies is beneficial. Many clinics also offer psychosocial counseling to improve social skills and self‑esteem.
Diagnosis usually begins with a clinical suspicion based on growth, developmental, or fertility concerns. The definitive test is a karyotype analysis (chromosome study) from a blood sample, which counts the number of X and Y chromosomes. Some doctors also use fluorescence in‑situ hybridization (FISH) or microarray testing for faster results. Hormone panels (testosterone, LH, FSH) and a semen analysis are often ordered to assess endocrine function and fertility status.
Testosterone replacement (gel, patch, injection, or pellet) is the standard treatment to address low testosterone, improve muscle mass, bone density, mood, and libido. Typical starting doses are individualized; clinicians monitor blood levels every 3–6 months. Common side effects include acne, sleep apnea worsening, increased red blood cell count (polycythemia), and potential impact on prostate health. Regular follow‑up includes PSA testing, hematocrit, and lipid profile.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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