Klinefelter syndrome (47,XXY)
Definition
Most common chromosomal cause of primary hypogonadism and male infertility.
Caused by one or more extra X chromosomes, usually (44 + XXY).
Epidemiology
Incidence: 1 in 600 live male births.
Approximately 80–90% are classic (44 + XXY); the remainder are mosaic or variant forms.
Genetics
Due to meiotic nondisjunction.
Mosaic form: 46,XY/47,XXY (milder phenotype).
Pathophysiology
Seminiferous tubule degeneration and fibrosis.
Decreased Leydig cell function.
Hypergonadotropic hypogonadism.
Hormonal profile
Hormone | Level |
|---|---|
Testosterone | Low |
LH | High |
FSH | High |
Inhibin B | Low |
Clinical features
Small, firm testes (hallmark).
Tall stature with long limbs.
Sparse facial and body hair.
Gynecomastia.
Delayed or incomplete puberty.
Infertility (usually azoospermia).
Reduced muscle mass and libido.
Diagnosis
Karyotyping (gold standard): 47,XXY.
Elevated LH and FSH, low testosterone.
Semen analysis: azoospermia or severe oligospermia.
Management
Testosterone replacement therapy.
Fertility: micro-TESE with intracytoplasmic sperm injection (ICSI).
Treat gynecomastia if symptomatic.
Monitor bone density and metabolic risk.
Complications
Infertility.
Osteoporosis.
Metabolic syndrome and type 2 diabetes.
Venous thromboembolism.
Male breast cancer.
Extragonadal germ cell tumors.
Autoimmune diseases (for example, systemic lupus erythematosus and autoimmune thyroid disease).
High-yield facts
Most common cause of primary testicular failure.
Most common adult presentation: infertility.
Characteristic finding: small firm testes with elevated LH and FSH.
Confirm diagnosis with karyotype analysis.
Academic references
Melmed S, et al. Williams Textbook of Endocrinology. 15th ed. Elsevier; 2024.
Jameson JL, et al. Harrison’s Principles of Internal Medicine. 21st ed. McGraw-Hill; 2022.
Kumar V, Abbas AK, Aster JC. Robbins & Cotran Pathologic Basis of Disease. 11th ed. Elsevier; 2020.
Hoffbrand AV, et al. Essential Haematology. 8th ed. Wiley-Blackwell; 2019.
European Academy of Andrology. Guidelines on Klinefelter syndrome. Andrology. 2020;8(3):545–561.
Gravholt CH, et al. Klinefelter syndrome. Nature Reviews Disease Primers. 2018;4:18018.
Nieschlag E, Behre HM, editors. Andrology: Male Reproductive Health and Dysfunction. 4th ed. Springer; 2023.
McPhee SJ, Papadakis MA. Current Medical Diagnosis & Treatment 2026. McGraw-Hill; 2026.
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