Prolactin is a hormone that is responsible for lactation, the development of certain breast tissue and the production of breast milk. Higher than normal levels of prolactin in the blood can cause certain symptoms, such as irregular periods, infertility and erectile dysfunction. Prolactin (also known as lactotropin) is a hormone that is responsible for lactation, the development of certain breast tissue, and contributes to hundreds of other bodily processes. Prolactin levels are usually low in men, as well as in women who are not breastfeeding and are not pregnant. Most of the prolactin is produced by the pituitary gland. It produces and releases the hormone. The pituitary gland is a small, pea-sized gland located at the base of the brain below the hypothalamus. Dopamine (a brain chemical) and estrogen control the production and release of prolactin from the pituitary gland. The central nervous system, immune system, uterus, and mammary glands are also capable of producing prolactin. The following factors may contribute to the production of prolactin in these tissues: Stimulation of the nipples. Exercise. Stress. Prolactin contributes to hundreds of bodily functions, but the two main ones are: The development of mammary glands in breast tissue and the production of breast milk. Breastfeeding. During pregnancy, the hormones prolactin, estrogen and progesterone stimulate the development of breast tissue and the production of breast milk. Prolactin promotes the growth of a certain type of breast tissue called mammary alveoli, which are the components of the mammary gland where breast milk is produced. Prolactin also stimulates alveolar breast cells to make milk components, including: Lactose (the carbohydrate component of breast milk). Casein (the protein component). Lipids (components that provide energy, essential fatty acids and cholesterol). After childbirth, progesterone levels drop, which increases the number of prolactin receptors in the alveolar cells of the mammary gland. This allows secretion of milk through the nipple. After childbirth, prolactin levels do not remain permanently elevated. Prolactin levels only spike during periods of nipple stimulation by sucking from the baby. As long as the baby continues to nurse, prolactin levels remain elevated. During periods when a woman is not breastfeeding, prolactin levels decrease and accordingly, breast milk production also decreases. Normal ranges for prolactin levels may vary slightly between laboratories. In general, these can be: In men, less than 20 ng/mL (nanograms per milliliter). In women who are not pregnant or breastfeeding: less than 25 ng/mL. For women who are pregnant or breastfeeding: 80 to 400 ng/mL. What causes abnormally low prolactin levels? Prolactin levels are usually low unless a woman is pregnant or breastfeeding, so the only sign of lower than normal prolactin levels is a lack of milk production after giving birth.In these cases, hypopituitarism is often the cause. Hypopituitarism is a rare condition in which there is an absence (deficiency) of one, several, or all of the hormones produced by the pituitary gland. It is usually caused by abnormal pressure on the pituitary gland or damage to the gland itself. References: 1. Al-Chalabi M, Bass AN, Alsalman I. Physiology, Prolactin. StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing; 2021. 2. MedlinePlus. Prolactin Blood Test. 3. MedlinePlus. Prolactin Levels. 4. National Institute of Diabetes and Digestive and Kidney Diseases. Prolactinoma.