Prolactin is a 198-amino-acid peptide hormone produced in the anterior pituitary. Its structure resembles that of GH. Prolactin is the principal hormone responsible for lactation. Milk production is stimulated by prolactin when appropriate circulating levels of estrogens, progestins, corticosteroids, and insulin are present. A deficiency of prolactin—which can occur in rare states of pituitary deficiency—is manifested by failure to lactate or by a luteal phase defect. In rare cases of hypothalamic destruction, prolactin levels may be elevated as a result of impaired transport of dopamine (prolactin-inhibiting hormone) to the pituitary. Much more com-monly, however, prolactin is elevated as a result of prolactin-secret-ing adenomas. Hyperprolactinemia produces a syndrome of amenorrhea and galactorrhea in women, and loss of libido and infertility in men. In the case of large tumors (macroadenomas), it can be associated with symptoms of a pituitary mass, including visual changes due to compression of the optic nerves. The hypo-gonadism and infertility associated with hyperprolactinemia result from inhibition of GnRH release.
No preparation of prolactin is available for use in prolactin-deficient patients. For patients with symptomatic hyperprolactine-mia, inhibition of prolactin secretion can be achieved with dopamine agonists, which act in the pituitary to inhibit prolactin release.