Klinefelter syndrome

  • Definition
    • Klinefelter syndrome is a genetic condition that occurs in males when they have an extra X chromosome.

  • Alternative Names
    • 47 X-X-Y syndrome; XXY syndrome; XXY trisomy; 47,XXY/46,XY; Mosaic syndrome; Poly-X Klinefelter syndrome

  • Causes
    • Most people have 46 chromosomes. Chromosomes contain all of your genes and DNA, the building blocks of the body. The 2 sex chromosomes (X and Y) determine if you become a boy or a girl. Girls normally have 2 X chromosomes. Boys normally have 1 X and 1 Y chromosome.

      Klinefelter syndrome results when a boy is born with at least 1 extra X chromosome. Usually, this occurs due to 1 extra X. This is written as XXY.

      Klinefelter syndrome occurs in about 1 out of 500 to 1,000 baby boys. Women who get pregnant after age 35 are slightly more likely to have a boy with this syndrome than younger women.

  • Symptoms
    • Symptoms may include any of the following:

      • Abnormal body proportions (long legs, short trunk, shoulder equal to hip size)
      • Abnormally large breasts (gynecomastia)
      • Infertility
      • Sexual problems
      • Less than normal amount of pubic, armpit, and facial hair
      • Small, firm testicles
      • Tall height
  • Exams and Tests
  • Treatment
    • Testosterone therapy may be prescribed. This can help:

      • Grow body hair
      • Improve appearance of muscles
      • Improve concentration
      • Improve mood and self esteem
      • Increase energy and sex drive
      • Increase strength

      Most men with this syndrome are not able to get a woman pregnant. But, an infertility specialist may be able to help. Seeing a doctor called an endocrinologist may also be helpful.

  • Support Groups
    • These groups can provide more information:

      The American Association for Klinefelter Syndrome Information and Support (AAKSIS) --

      National Institute of Health, National Human Genome Research Institute --

  • Possible Complications
  • When to Contact a Medical Professional
    • Call for an appointment with your health care provider if your son does not develop secondary sexual characteristics at puberty. This includes facial hair growth and a deepening of the voice.

      A genetics counselor can provide information about this condition and direct you to support groups in your area.

  • References
    • Bacino CA. Cytogenetics. In: Kliegman RM, Stanton BF, St. Geme J III, Schor N, Behrman RE, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 76.