An excessive development of the breast in the male.   The term comes from the Greek words gyne meaning "woman" and mastos meaning "breast."  It is relatively common in adolescence, and usually disappears in a few months.  But in about 10% of young males it does not.  The culprit in gynecomastia is the female hormone estrogen.  Either too little or too much testosterone contributes to this condition.

Further Reading and Resources:

Male Breast Cancer

Medline

Gynecomastia.org

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