Prolactin is a hormone produced in the pituitary gland, so named because of its role in lactation. It also has other wide-ranging functions in the body, from acting on the reproductive system to influencing behavior and regulating the immune system. Prolactin is a hormone originally named after its function to promote milk production (lactation) in mammals in response to nursing young after birth. It has since been shown to have more than 300 functions in the body. They can be divided into several areas: reproductive, metabolic, fluid regulation (osmoregulation), immune system regulation (immunoregulation) and behavioral functions. In humans, prolactin is produced both in the anterior pituitary gland (anterior lobe of the pituitary gland) and in a number of other places in the body. Lactotroph cells in the pituitary gland produce prolactin, where it is stored and then released into the bloodstream. Human prolactin is also produced in the uterus, immune cells, brain, breast, prostate, skin and fat tissue. One of the main regulators of prolactin production by the pituitary gland is a hormone called dopamine, which is produced by the hypothalamus, the part of the brain just above the pituitary gland. Dopamine limits prolactin production, so the more dopamine there is, the less prolactin is released. Prolactin itself increases dopamine secretion, so this creates a negative feedback loop. Estrogen is another key regulator of prolactin and has been shown to increase the production and secretion of prolactin by the pituitary gland. Studies have shown a slight increase in circulating prolactin in women during the stages of the reproductive cycle when estrogen levels are highest. This is also the case during and after pregnancy, as a higher level of circulating prolactin is required to initiate lactation. In addition to dopamine and estrogen, a whole host of other hormones can both increase and decrease the amount of prolactin released in the body, some examples being thyrotropin-releasing hormone, oxytocin, and antidiuretic hormone. The condition in which prolactin circulates in the blood at increased levels is called hyperprolactinemia. The most common causes of hyperprolactinemia include pregnancy, drugs that reduce the action of dopamine in the body, an underactive thyroid gland, and benign tumors of the pituitary gland (known as prolactinomas). Symptoms may include unwanted milk production, menstrual disturbances, and symptoms due to estrogen deficiency (in women) or testosterone deficiency (in men). The majority of patients with prolactinoma can be successfully treated with drugs that mimic the action of dopamine. The condition in which too little prolactin circulates in the blood is called hypoprolactinemia.This condition is very rare and can occur in people with insufficient pituitary function. A decrease in the amount of secreted prolactin can lead to insufficient production of breast milk after birth. Most people with low prolactin levels have no specific medical problems, although preliminary data suggest they may have a reduced immune response to some infections. References: 1. Al-Chalabi M, Bass AN, Alsalman I. Physiology, Prolactin. StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing; 2021. Accessed 2/14/2022. 2. MedlinePlus. Prolactin Blood Test. Accessed 2/14/2022. 3. MedlinePlus. Prolactin Levels. Accessed 2/14/2022. 4. National Institute of Diabetes and Digestive and Kidney Diseases. Prolactinoma. Accessed 2/14/2022.