The primary cause of gynecomastia is thought to be an imbalance between estrogen and androgen, either due to increased estrogen production, a relative decrease in androgen production, or a combination of both.
Estrogen acts as a breast growth hormone, and therefore excess estradiol in men results in breast enlargement by causing ductal epithelial hyperplasia. Elevated serum estrogen levels in men may be caused by estrogen-producing tumors. Local tissue factors in the breast may also be involved, such as increased aromatase activity, which may cause excessive local estrogen production, decreased estrogen degradation, and changes in estrogen or androgen receptor levels or activity.