Prolactin is a hormone produced primarily by the pituitary gland, a small organ at the base of the brain. This hormone plays a broad role in maintaining the body’s balance. It is involved in over 300 processes in mammals, including humans, and is fundamental for various physiological functions.
The Prolactin Production and Release Pathway
Prolactin is synthesized and secreted by specialized cells called lactotrophs, which are found in the anterior portion of the pituitary gland. While the pituitary gland is the main source, other tissues such as the central nervous system, immune system, uterus, and mammary glands can also produce prolactin.
The hypothalamus, located directly above the pituitary gland, exerts primary control over prolactin secretion. Unlike most other pituitary hormones, prolactin is predominantly under inhibitory control. This inhibition is primarily mediated by dopamine, a neurotransmitter also known as Prolactin-Inhibiting Hormone (PIH), which is released from hypothalamic neurons into the blood vessels connecting the hypothalamus and pituitary.
Dopamine acts to restrain prolactin production; therefore, higher dopamine levels lead to less prolactin release. Conversely, factors that decrease dopamine’s influence, such as nipple stimulation during breastfeeding, can lead to a surge in prolactin. Thyrotropin-Releasing Hormone (TRH) also has a stimulatory effect on prolactin release, and estrogen can increase the sensitivity of lactotroph cells to TRH while decreasing their sensitivity to dopamine’s inhibition.
Primary Functions of Prolactin
Prolactin’s most recognized function is its role in lactation, the process of milk production. After childbirth, prolactin stimulates the mammary glands to synthesize milk components such as lactose, casein, and lipids. This hormone also promotes the development of mammary alveoli, which are the small sacs within the breast tissue responsible for producing and storing milk.
During pregnancy, prolactin works alongside estrogen and progesterone to prepare the breasts for milk production. Following delivery, the number of prolactin receptors on mammary alveolar cells increases, facilitating milk production. The act of suckling by an infant further stimulates prolactin release, creating a positive feedback loop that ensures continued milk supply.
Prolactin also influences reproductive function, particularly in relation to lactation. High concentrations of prolactin during breastfeeding can inhibit the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. This, in turn, reduces the release of other hormones like luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are involved in ovulation and menstrual cycles. Consequently, elevated prolactin levels during lactation often lead to a temporary suppression of ovulation, offering a natural form of birth spacing.
Beyond Lactation: Other Roles of Prolactin
Beyond its well-known role in milk production, prolactin participates in a variety of other bodily processes. This hormone contributes to the regulation of the immune system, acting as an immunomodulator. It influences the activity of various immune cells, contributing to the body’s defense mechanisms.
Prolactin also has a connection to metabolism, affecting how the body processes glucose and lipids. Prolactin levels can influence energy balance and metabolic adaptations, particularly during periods of high energy demand such as nursing.
This hormone also plays a part in maintaining fluid and electrolyte balance. Prolactin receptors are present in the kidneys, and the hormone can modulate the effects of other hormones on renal function, influencing the excretion of water, sodium, and potassium. Furthermore, prolactin has been linked to parental behavior, influencing broader aspects of nurturing and care.
When the Prolactin Pathway Goes Awry
Abnormal prolactin levels can lead to various health conditions, impacting different bodily systems. Hyperprolactinemia, characterized by excessively high levels of prolactin in the blood, is a common disorder. The most frequent cause of hyperprolactinemia is a prolactinoma, a benign (non-cancerous) tumor that forms in the pituitary gland and produces excess prolactin.
Other causes of elevated prolactin include certain medications, such as some antipsychotics, antidepressants, and high blood pressure drugs, which can interfere with dopamine’s inhibitory action. Hypothyroidism, an underactive thyroid gland, can also lead to hyperprolactinemia because low thyroid hormone levels remove the negative feedback on TRH, which in turn stimulates prolactin release. Symptoms of hyperprolactinemia vary but commonly include galactorrhea, which is the milky discharge from the nipples unrelated to pregnancy or breastfeeding, and menstrual irregularities, such as absent or infrequent periods in women. Both men and women may experience infertility and a reduced interest in sex. Men might also experience erectile dysfunction and enlarged breast tissue.
Conversely, hypoprolactinemia, or unusually low prolactin levels, is less common but can also have implications. It has been associated with metabolic dysfunctions, including type 2 diabetes mellitus, fatty liver, and dyslipidemia. Low prolactin can also affect fertility and sexual function in both sexes, and in women, it may lead to difficulties with milk production after childbirth.