Gynecomastia, or gynaecomastia, is the development of abnormally large mammary glands in males resulting in breast enlargement, which can sometimes cause secretion of milk. The term comes from the Greek gyne (stem gynaik-) meaning "woman" and mastos meaning "breast". The condition can occur physiologically in neonates (due to female hormones from the mother), in adolescence, and in the elderly. In adolescent boys the condition is often a source of distress, but for the large majority of boys whose pubertal gynecomastia is not due to obesity, the breast development shrinks or disappears within a couple of years . The causes of common gynecomastia remain uncertain, although it has generally been attributed to an imbalance of sex hormones or the tissue responsiveness to them; a root cause is rarely determined for individual cases. Breast prominence can result from hypertrophy of breast tissue, chest adipose tissue and skin, and is typically a combination. Breast prominence due solely to excessive adipose is often termed pseudogynecomastia or sometimes lipomastia.

Gynecomastia should be distinguished from work hypertrophy of the pectoralis muscles caused by much exercise, e.g. swimming, bench press.

Causes

Physiologic gynecomastia (also called Turcios Disease) occurs in neonates, at or before puberty and with aging. Many cases of gynecomastia are idiopathic, meaning they have no clear cause. Potential pathologic causes of gynecomastia are: medications including hormones, increased serum estrogen, decreased testosterone production, androgen receptor defects, chronic kidney disease, chronic liver disease, HIV treatment, and other chronic illness. Gynecomastia as a result of spinal cord injury and refeeding after starvation has been reported. In 25% of cases, the cause of the gynecomastia is not known.

Medications cause 10-20% of cases of gynecomastia in post-adolescent adults. These include cimetidine, omeprazole, spironolactone, imatinib mesylate, finasteride and certain antipsychotics. Some act directly on the breast tissue, while others lead to increased secretion of prolactin from the pituitary by blocking the actions of dopamine (prolactin-inhibiting factor/PIF) on the lactotrope cell groups in the anterior pituitary. Androstenedione, used as a performance enhancing food supplement, can lead to breast enlargement by excess estrogen activity. Medications used in the treatment of prostate cancer such as antiandrogens and GnRH analogs can also cause gynecomastia. Marijuana use is also thought by some to be a possible cause; however, published data is contradictory.

Increased estrogen levels can also occur in certain testicular tumors, and in hyperthyroidism. Certain adrenal tumors cause elevated levels of androstenedione which is converted by the enzyme aromatase into estrone, a form of estrogen. Other tumors that secrete hCG can increase estrogen. A decrease in estrogen clearance can occur in liver disease, and this may be the mechanism of gynecomastia in liver cirrhosis. Obesity tends to increase estrogen levels.