Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Hormones
Pituitary Hormones
Lactogenic hormone (prolactin, luteotropic hormone)

Prolactin is considered one of the most "ancient" Pituitary Hormones, as it can be found in the Pituitary Gland of lower terrestrial animals that lack Mammary Glands, and it is also capable of producing a lactogenic effect in mammals. Beyond its primary function (stimulating mammary gland development and Lactation), prolactin has profound biological significance: it promotes the growth of Internal Organs, stimulates corpus luteum secretion (hence its alternative name, "luteotropic hormone"), and exerts renotropic, erythropoietic, and hyperglycemic effects, among others. Excess prolactin—typically caused by prolactin-secreting tumors—leads to Amenorrhea and breast enlargement in women, and impotence in men.

The Structure of prolactin has been decoded in sheep, cattle, and humans. It is a large protein consisting of a single polypeptide chain with three Disulfide Bonds, comprising 199 amino acid residues. Species-specific variations in the Amino Acid Sequence essentially involve only 2–3 residues. Previously, the secretion of lactotropin in the human pituitary was disputed, as it was hypothesized that its function was supposedly performed by somatotropin. Today, compelling evidence confirms the existence of human prolactin, although its concentration in the pituitary is significantly lower than that of Growth Hormone. In women's Blood, prolactin levels rise sharply prior to childbirth, reaching up to 0.2 ng/L compared to a normal baseline of 0.01 ng/L.