Gynaecomastia and the oestrogen to testosterone balance

Gynaecomastia is a benign enlargement of male breast tissue. It comes from a relative shift in the balance of two sex hormones: a relative decrease in testosterone (a male hormone) and a relative increase in oestrogen (a female hormone). Relative matters, because oestrogen does not have to be high in absolute terms; it only has to outweigh testosterone at the breast. A plain way to remember it: in gynaecomastia, oestrogen wins over testosterone.

Although small, male breast tissue can still respond to hormones. Oestrogen stimulates breast tissue to grow, while androgens (male hormones such as testosterone) restrain it. When the balance tips, the tissue that grows is true gland tissue, not simply fat, in a firm, rubbery disc beneath the nipple. It can affect one breast or both, and it can be tender.

It is mainly seen in newborns (from the mother's oestrogen), in puberty, in older men and with heavy alcohol use. In puberty it is transient, which means it usually settles by itself, because oestrogen production peaks before testosterone. In older men and with heavy alcohol use, and in liver disease such as cirrhosis, reduced liver function leaves the liver less able to metabolise (break down) oestrogen. Some drugs can cause it, such as spironolactone (a diuretic, or water tablet, that spares potassium) and anabolic steroids, and so can being overweight.

Gynaecomastia is benign and does not usually raise breast cancer risk. A lump that is hard and fixed, or that comes with skin or nipple changes, still needs assessment.