The concentration of circulating prolactin increases during pregnancy so that by the end of gestation, levels are 10 to 20 times over normal amounts. However, prolactin is prevented from exerting its effect on milk secretion by elevated levels of progesterone. Following clearance of progesterone and estrogen at parturition, copious milk secretion begins. The minimal hormonal requirements for normal lactation to occur are prolactin, insulin and hydrocortisone. Prolactin levels decrease as lactation is established but nursing stimulates prolactin release from the pituitary which promotes continued milk production.
Prolactin is secreted into milk at levels representative of the average circulating concentration.
The physiological significance of milk prolactin to the infant is uncertain. Prolactin exists in three heterogenic forms which possess varying biological activity. The monomer with a molecular weight of 23 kDa is found in greatest quantity and is the principal biologically active form. The pattern of heterogeneity changes during pregnancy to favor even more monomer in proportion to the dimer. However, during lactation, the proportion of the monomer in circulation decreases in response to selective uptake of the monomer by the mammary gland.