The name gynecomastia comes from the Greek “gyne”-womanlike and “mastos”- the breast. The term refers to an enlargement of the male mammary glands which occurs in 30-40 % of men in the adult population. The condition is common in adolescent boys (60-75%), with enlargement of one or both mammary glands, often accompanied by pain.
When enlargement is due to an accumulation of fatty tissue in the male breast, this is known as pseudogynecomastia.  In all cases of gynecomastia, the mammary gland should be closely examined to rule out male breast cancer. 

Physiological (= normal) gynecomastia occurs in male neonates due to the prevalence of female hormones from the mother. This type of gynecomastia disappears within a couple of weeks. The same applies to gynecomastia during puberty, which should have subsided again within 1-2 years. If breast development has not begun to shrink, a visit to an endocrinologist is recommended. In aging gynecomastia, breast enlargement tends to occur after the age of 60, as the production of androgen declines.
Pathological (=abnormal) gynecomastia has various causes. Approximately 60% of gynecomastia are due to drugs (e.g.  spironolaktone, cimetidine, dioxin), hormones and frequently to anabolic steroids.
In testicular causes of gynecomastia, we differentiate between primary and secondary causes. Primary damage to testicles occurs in Klinefelter´s syndrome (a genetic defect with additional x-chromosomes in the affected men), hypothalamic hypogonadism (diminished testicles as a result of insufficient secretion of higher control hormones from the hypothalamus) and the Reifenstein syndrome (a syndrome, combining gynecomastia, deformities of the sexual organs, hypogonadism, and abnormal opening of the urethra-hypospadias). Secondary damage to the testicles caused by other diseases such as testicular cancer, testicular inflammation, injury or previous vasectomy (male sterilisation) may result in a hormone imbalance.
In addition there are tumours and oncological diseases. Most common of these are oestrogen-producing tumours of the adrenal cortex. Amongst other oncological diseases are the hepatocellular carcinoma, lymphoma, lung tumours and tumours in the region of the hypothalamus and the hypophysis. Metabolic disorders which may cause gynecomastia include liver cirrhosis, renal failure and obesity. Hormonal imbalance, most commonly of the thyroid gland, is also a frequent cause of gynecomastia. An increase in breast tissue occurs both in hyperthyroidism (overactive thyroid) and in hypothyroidism (underactive thyroid).
Types of gynecomastia
If the increased male breast tissue is caused mainly by the mammary gland, the condition is a glandular gynecomastia and the glandular tissue should be removed surgically (subcutaneous mastectomy). In the case of an adipose gynecomastia or pseudogynecomastia the breast enlargement is composed mainly of fatty tissue which can be removed by liposuction. The glandular-fat gynecomastia is a mixture of the two types of gynecomastia described above and is treated by a combination of mastectomy and liposuction.