Overview

Klinefelter syndrome is a common condition that results when a person assigned male at birth has an extra copy of the X sex chromosome instead of the typical XY. Klinefelter syndrome is a genetic condition that occurs before birth, but it often isn't diagnosed until adulthood.

Klinefelter syndrome may affect testicular growth. This results in smaller testicles, which can lead to making less of the hormone testosterone. The syndrome also may cause smaller muscle mass, less body and facial hair, and extra breast tissue. The effects of Klinefelter syndrome vary, and not everyone has the same symptoms.

Most people with Klinefelter syndrome produce little or no sperm, but assisted reproductive procedures may make it possible for some people with Klinefelter syndrome to have biological children.

Symptoms

Symptoms of Klinefelter syndrome vary widely. Many children with Klinefelter syndrome show few or only mild symptoms. Most often the condition isn't diagnosed until puberty or adulthood, or it may never be diagnosed. For others, the condition has a noticeable effect on growth or appearance. Klinefelter syndrome may affect development, physical appearance, sexual development and mental health.

Development

  • Slow motor development, such as taking longer than average to sit up, crawl and walk.
  • Speaking later than other babies of the same age.
  • Learning and language problems, such as trouble with reading, writing, spelling or math.

Physical appearance

  • Taller than average height.
  • Longer legs, shorter body, narrower shoulders, broader hips and extra belly fat compared with other children and adults assigned male at birth.
  • After puberty, less muscle mass and less facial and body hair compared with other teens and adults assigned male at birth.
  • Extra breast tissue, called gynecomastia.
  • Low energy levels.

Sexual development

  • Small, firm testicles and a small penis. Babies may be born with testicles that haven't moved from the belly into the scrotum, a condition called undescended testicles.
  • Puberty changes that are delayed, that only include some changes or that don't happen at all.
  • Low sperm count or no sperm.
  • Low sex drive.

Mental health

  • Difficulty expressing thoughts and feelings.
  • Having a hard time engaging in social activities.

When to see a doctor

Talk to your healthcare professional if you notice:

  • Slow development during infancy or childhood. Delays in growth and development can be the first sign of several conditions that need treatment, including Klinefelter syndrome. Though some differences in physical and mental development are expected among children, check with a healthcare professional if you have any concerns.
  • Problems with fertility. Problems with fertility aren't often diagnosed in people with Klinefelter syndrome until they realize they're not able to have a biological child.

Causes

Klinefelter syndrome occurs because of a random change in the egg or the sperm that causes a baby assigned male at birth to be born with an extra X sex chromosome. The condition is not passed down in families.

Klinefelter syndrome can be caused by:

  • One extra copy of the X sex chromosome in each cell (XXY), the most common cause.
  • An extra X sex chromosome in some of the cells. This is called mosaic Klinefelter syndrome and may result in fewer symptoms.
  • More than one extra copy of the X sex chromosome, which is rare and results in a severe form of the syndrome.

In people assigned male at birth, extra copies of genes on the X sex chromosome can interfere with sexual development and fertility.

Risk factors

Klinefelter syndrome occurs because of a random genetic change in the sperm or the egg. The risk of Klinefelter syndrome is not raised by anything parents do or don't do. For people carrying a pregnancy after age 35, the risk is higher but only slightly.

Complications

Klinefelter syndrome may raise the risk of:

  • Anxiety and depression.
  • Social, emotional and behavioral problems, such as low self-esteem.
  • Problem with fertility and sexual function.
  • Thin and brittle bones, a condition called osteoporosis.
  • Heart and blood vessel disease.
  • Breast cancer and some other cancers.
  • Lung disease.
  • Metabolic syndrome, which includes type 2 diabetes, high blood pressure, and high cholesterol and triglycerides.
  • Being overweight.
  • Autoimmune disorders such as lupus and rheumatoid arthritis.
  • Tooth and mouth problems that make dental cavities more likely.
  • Autism spectrum disorder.

Some complications caused by Klinefelter syndrome are the result of low testosterone, also called hypogonadism. Hormone therapy lessens the risk of certain health problems, especially when therapy is started at the beginning of puberty.

By Mayo Clinic Staff