The breasts are made up of a series of structures called lobules, connected to the nipple by a series of ducts, and surrounded by fat and connective tissues. Lactation, the release of milk from the breast, occurs upon the maturation of cells within the lobules. These cells produce milk proteins and fats in response to stimuli from the body, and release the milk via the nipple. Lactation can occur due to a number of underlying causes, and unexplained lactation may indicate disease.
Lactation is a natural part of pregnancy and child rearing, since breast milk provides support and nourishment to the baby. Over the course of pregnancy the breast develops and increases the number of lobules and ducts, which causes the increase in breast size experienced in many women. After this period of breast growth, cells within the lobules mature and begin to generate the milk proteins and fats required for lactation. Lactation as a result of pregnancy occurs in response to normal cycles of circulating hormones, and lactation will cease soon after the child has stopped breast-feeding.
Lactation in a woman who is not pregnant may indicate the presence of diseases such as prolactinomas. A prolactinoma is a benign tumor growing on a structure called the pituitary gland, a part of the brain that controls the release of several hormones throughout the body. Prolactinomas contain cells that produce a hormone called prolactin, which is responsible for breast milk production and lactation.
UCLA reports that patients with prolactinomas will experience a small increase in prolactin, leading to irregular menstruation. As the tumor grows, patients will experience lactation. In severe cases, when the tumor is very large, patients may begin to experience headaches and vision problems. Prolactinomas can be treated by surgery, radiation therapy or drug therapy.
Unexplained lactation can also be caused by hypothyroidism. The thyroid is a gland found in the neck that releases a number of hormones, called thyroid hormones, which perform a range of functions throughout the body. Patients with hypothyroidism commonly have increased levels of prolactin, according to the National Endocrine and Metabolic Diseases Information Service. The increased prolactin levels can lead to lactation in some patients. Patients with hypothyroidism can treat their condition and alleviate the symptoms of increased prolactin by thyroid hormone replacement therapy.