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

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

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. 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.

Infertility is the most common symptom of Klinefelter syndrome.

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
• Small penis size

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. An infertility specialist may be able to help. Seeing a doctor called an endocrinologist may also be helpful.

These sources can provide more information on Klinefelter syndrome:

• Association for X and Y Chromosome Variations -- genetic.org
• US National Library of Medicine, MedlinePlus -- medlineplus.gov/klinefelterssyndrome.html

Enlarged teeth with a thinning surface is very common in Klinefelter syndrome. This is called taurodontism. This can be seen on dental x-rays.

Klinefelter syndrome also increases the risk of:

• Attention deficient hyperactivity disorder (ADHD)
• Autoimmune disorders, such as lupus, rheumatoid arthritis, and Sjögren syndrome
• Breast cancer in men
• Depression
• Learning disabilities, including dyslexia, which affects reading
• A rare type of tumor called an extragonadal germ cell tumor
• Lung disease
• Osteoporosis
• Varicose veins

Contact your 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.

Published Date : October 27, 2020
Published By : Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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Matsumoto AM, Anawalt BD, Testicular disorders. In: Melmed S, Auchus, RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 19.

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