What is XXY (Klinefelter syndrome)?

XXY, also known as Klinefelter syndrome, is a condition that causes physical and learning challenges in affected males. It is caused by a difference in the number of sex chromosomes. Females typically have two X chromosomes, while males typically have one X and Y chromosome. Klinefelter syndrome is caused when a male has two copies of the X chromosome and one Y chromosome. The symptoms of Klinefelter syndrome result from this extra genetic information that impacts the growth and development of an affected individual.

Males with Klinefelter syndrome may be taller than average and typically have small testes, which produce a reduced amount of the hormone testosterone. As adults, many males with Klinefelter syndrome are infertile without fertility treatment. Breast development, delayed puberty, bone problems, weaker muscles, anxiety, and learning disabilities are more common in males with Klinefelter syndrome. Individuals with Klinefelter syndrome are also more likely to develop certain medical conditions as adults, including autoimmune disorders, osteoporosis (weak bones), varicose veins, type 2 diabetes, and heart valve defects.

Each person with Klinefelter syndrome is unique, and the severity of the symptoms varies greatly between individuals. Approximately 75% of individuals with Klinefelter syndrome may never be diagnosed.

How common is XXY (Klinefelter syndrome)?

The incidence of Klinefelter syndrome in the population is 1 in 500 to 1 in 1,000 male births.

How is XXY (Klinefelter syndrome) treated?

There is no single, standard treatment for Klinefelter syndrome. Treatments are based on each individual's physical and intellectual needs as well as their personal strengths and limitations.

Individuals with Klinefelter syndrome may receive care from a team of health professionals. Those who experience learning disabilities or social challenges can benefit from early interventions. Males with Klinefelter syndrome may also benefit from physical therapy if they present with muscle weakness or delayed developmental milestones. Hormone treatment may be used for pubertal development. Newer research has also shown benefit of early hormone replacement treatments in childhood for cognitive, language, and motor development. Some individuals may consider reproductive technologies such as sperm retrieval in adolescence or early adulthood. Although uncommon, some individuals may not require any treatment.

What is the prognosis for an individual with XXY (Klinefelter syndrome)?

The prognosis for an individual with Klinefelter syndrome is quite good with normal or near-normal lifespan.

Other names for

  • Klinefelter syndrome


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