The nipple of the breast will obviously discharge milk in a woman who is breastfeeding, and will often leak milk between feeds, particularly when the breast is engorged with milk some hours after a feed. At other times a discharge will indicate some medical problem.
Sex hormone imbalances are the most common cause of abnormal nipple discharges. At almost any time during pregnancy, but particularly late in pregnancy, the higher levels of hormones in the body may stimulate premature breast milk production.
The pituitary gland under the brain sends signals to the ovaries to increase or decrease sex hormone (oestrogen) production. A tumour or cancer of the pituitary gland or ovaries may result in excessive hormone levels and breast milk production.
Newborn infants of both sexes sometimes produce “witch’s milk”, which is a discharge from the nipples in the first few days of life due to high levels of sex hormone passing over to the child from the mother through the placenta during birth. It is a harmless condition that settles quickly.
Other causes of an abnormal nipple discharge include breast cancer that involves the milk ducts (brown or blood stained discharge), kidney failure (may prevent the excretion of the normal amount of oestrogen and the levels of hormone increase), under or over active thyroid gland (hypothyroidism and hyperthyroidism), Cushing syndrome (over production of steroids, or taking large doses of cortisone) and excessive stimulation of a woman’s nipples for a prolonged period of time may result in a reflex which increases oestrogen levels and results in milk production.
Some non-hormonal medications may increase sex hormone production as a side effect. Examples include methyldopa and reserpine (used for serious high blood pressure) and tricyclic antidepressants.