Galactorrhea means a flow of milk from the mammary glands affecting one or both breasts, and occurring outside the normal period of breastfeeding. It is mainly a symptom in connection with excessive quantities of prolactin in the blood: this is called hyperprolactinemia. This increase may occur on its own, through elevated hormone secretion by the pituitary gland, or as a side effect of another disease including renal failure, hyperthyroidism (an increase of thyroid hormones circulating in the blood), or certain medications. Depending on the suspected cause, a blood and brain imaging test can be performed. It is also called galactorrhea when an excessive amount of milk is produced during lactation.