Prolactin-secreting tumors, or prolactinomas, are the most common form of pituitary tumors (~40%). Most prolactinomas are small microadenomas (<1 cm), but a few (<10%) are large macroadenomas (>1 cm). Prolactin's primary function is to stimulate lactation, but it also has effects on gonadal function. Elevated concentrations of serum pro-lactin in women typically cause infertility, oligo- or amenorrhea, and decreased libido. Galactorrhea occurs in up to 80% of women. In contrast, the most common symptoms of hyperprolactinemia in men include impotence, infertility, decreased libido, headaches, and visual disturbances. Galactorrhea and gynecomastia are rare in men.2
In general, serum prolactin concentrations parallel tumor size. Macroadenomas are typically associated with concentrations >375 ng/mL.1,2 Some are associated with extremely high concentrations, > 10000 ng/mL. However, up to 15-20% of macroadenomas are associated with serum prolactin concentrations <100 ng/mL. In such cases, compression of a non-prolactin-secreting tumor on the pituitary stalk, which thus interferes with the delivery of hypothalamic dopamine to the adenohypophysis, should be suspected. It can be difficult to distinguish between macroprolactinomas and nonfunctioning macro-adenomas. Precise diagnosis is important because the methods for management of these two entities are quite different. While nonfunctioning macroadenomas respond best to surgical removal, macroprolactinomas respond best to dopamine agonists.
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