Klinefelter syndrome occurs when a male has an additional X chromosome, making them 47 XXY. Under normal circumstances, males have XY sex chromosomes and females have XX sex chromosomes.
Rarely, people with Klinefelter syndrome can have even more X chromosomes, such as 48 XXXY or 49 XXXXY. This is associated with more severe features.
Mosaicism can occur, in which some cells of the body contain XXY sex chromosomes, while others contain the normal XY sex chromosomes. This typically results in milder symptoms.
Features
Patients with Klinefelter syndrome appear as normal males until puberty. Features of the condition develop at puberty, including:
- Taller height
- Eunuchoid body shape (long limbs and narrow shoulders)
- Gynaecomastia
- Below-average muscle strength
- Small testicles
- Reduced libido
- Psychological difficulties (e.g., anxiety, shyness ADHD and ASD)
- Infertility (caused by azoospermia, meaning no sperm in the ejaculate)
- Subtle learning difficulties (particularly speech and language)
Management Options
Treatment aims to help with the features of the condition:
- Testosterone replacement improves many of the symptoms
- Advanced IVF techniques have the potential to allow fertility
- Breast reduction surgery for cosmetic purposes
Multidisciplinary team input depends on individual difficulties:
- Speech and language therapy to improve speech and language
- Occupational therapy to assist in day-to-day tasks
- Physiotherapy to strengthen muscles and joints
- Educational support for dyslexia and other learning difficulties
Prognosis
Life expectancy is close to normal. There is an increased risk of:
- Venous thromboembolism
- Metabolic syndrome (obesity, raised cholesterol and type 2 diabetes)
- Breast cancer (compared with other males, but still less than females)
- Osteoporosis
- Anxiety and depression
Last updated December 2025
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