Excessive development of the male mammary glands, even to the functional state; the development of abnormally large breasts in males. May occur in one or both breasts and begins as a small lump beneath the nipple, and the breasts often enlarge unevenly. Developing gynecomastia during puberty is common and usually recedes over a period of months. Gynecomastia can also be caused by chronic liver disease, exposure to anabolic steroids or estrogen, kidney failure, genetics, marijuana use, testosterone dificiency, or side effects from some medications. In rare instances an overactive thyroid, or genetic defects or tumors may be the cause.