Function of Prolactin
Prolactin is a protein hormone whose primary function is to initiate and maintain lactation. 1, 2, 3
Physiological Role of Prolactin
Prolactin is a 23 kDa single chain protein consisting of 199 amino acids that is primarily synthesized and released by lactotrophs in the anterior pituitary gland 2. Its secretion is mainly under inhibitory control by hypothalamic dopamine and regulated through a negative feedback mechanism 2.
The main functions of prolactin include:
- Mammary gland development during pregnancy - Serum prolactin levels rise rapidly during pregnancy with increases in both the size and number of lactotrophs 2
- Milk synthesis initiation - Following clearance of progesterone and estrogen at parturition, prolactin can exert its effect on milk secretion 3
- Maintenance of milk secretion - Nursing stimulates prolactin release from the pituitary which promotes continued milk production 3
Molecular Mechanisms
At the molecular level, prolactin:
- Stabilizes and promotes transcription of casein mRNA
- May stimulate synthesis of alpha-lactalbumin (the regulatory protein of the lactose synthetase enzyme system)
- Increases lipoprotein lipase activity in the mammary gland 3
Hormonal Requirements for Lactation
The minimal hormonal requirements for normal lactation include:
- Prolactin
- Insulin
- Hydrocortisone 3
Prolactin Secretion Patterns
Prolactin secretion is:
- Induced by sleep, stress, sexual intercourse, and suckling stimulus
- Increased by estrogen (explaining higher levels in women, especially during late follicular phase and pregnancy)
- Negatively regulated by dopamine, which is the major physiologic prolactin inhibiting factor (PIF) 4
Clinical Relevance
Abnormal prolactin levels can lead to various clinical conditions:
- Hyperprolactinemia may present with symptoms of hypogonadotropic hypogonadism including menstrual disturbance and infertility 2
- Prolactin measurement should be offered in patients presenting with menstrual disturbances, galactorrhea, visual field loss, or symptoms of hypogonadism 1
- Untreated hyperprolactinemia can lead to bone density loss and increased fracture risk due to hypogonadism 1
Prolactin Forms and Activity
Prolactin exists in three heterogenic forms with varying biological activity:
- The monomer (23 kDa) is found in greatest quantity and is the principal biologically active form
- During lactation, the proportion of monomer in circulation decreases due to selective uptake by the mammary gland
- Over 90% of prolactin in milk is present as the monomer 3
While prolactin is essential for lactation in humans, its other potential functions such as regulation of ovarian function, osmoregulation, and immunoregulation are not fully established 4.
Human Milk Insights: Prolactin levels in breast milk represent the average circulating concentration, though the physiological significance of milk prolactin to the infant remains uncertain 3.
Human Milk Insights: Prolactin levels in breast milk represent the average circulating concentration, though the physiological significance of milk prolactin to the infant remains uncertain 3.