Harvard Men's Health Watch. Many drugs have been implicated as occasional causes of gynecomastia.
MicroRNA gene expression deregulation in human breast cancer. An overactive thyroid gland can also produce it.
Old age is another occasional cause, probably because testosterone levels tend to decline over the years. The breast enlargement is usually mild and painless, but about a third of men complain of tenderness. Ultimately, it is hoped that such investigations may lead to genetically oriented treatments for male breast cancer.