47 X-X-Y syndrome; XXY syndrome; XXY trisomy; 47,XXY/46,XY; Mosaic syndrome; Poly-X Klinefelter syndrome
Klinefelter syndrome is a genetic condition that occurs in males when they have an extra X chromosome.
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:
Exams and Tests
Klinefelter syndrome may first be diagnosed when a man comes to the health care provider because of infertility. The following tests may be performed:
Blood tests will be done to check hormone levels including:
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.
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 --
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 Sjogren syndrome
- Breast cancer in men
- Learning disabilities, including dyslexia, which affects reading
- A rare type of tumor called an extragonadal germ cell tumor
- Lung disease
- Varicose veins
When to Contact a Medical Professional
Call for an appointment with 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.
Allan CA, McLachlan RI. Androgen deficiency disorders. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 139.
Matsumoto AM, Bremmer WJ. Testicular disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 19.
Nussbaum RL, McInnes RR, Willard HF. The chromosomal and genomic basis of disease. In: Nussbaum RL, McInnes RR, Willard HF, eds. Thompson & Thompson Genetics in Medicine. 8th ed. Philadelphia, PA: Elsevier; 2016:chap 6.
Last reviewed on: 10/30/2016
Reviewed by: Anna C. Edens Hurst, MD, MS, Assistant 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, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.