Galactorrhea is typically due to a prolactin-secreting pituitary adenoma (prolactinoma). Most tumors when diagnosed in women are microadenomas (< 10 mm in diameter), but a small percentage are macroadenomas (>10 mm in diameter) when diagnosed. The frequency of microadenomas is much lower in men, perhaps because of later recognition. Nonfunctioning pituitary mass lesions also can increase serum prolactin levels by compressing the pituitary stalk and thus reducing the action of dopamine, a prolactin inhibitor.
Hyperprolactinemia and galactorrhea also may be caused by ingestion of certain drugs, including phenothiazines, other antipsychotics, certain antihypertensives (especially alpha-methyldopa), and opioids. Primary hypothyroidism can cause hyperprolactinemia and galactorrhea because increased levels of thyroid-releasing hormone increase secretion of prolactin as well as thyroid-stimulating hormone (TSH). It is unclear why hyperprolactinemia is associated with hypogonadotropism and hypogonadism.
See Galactorrhea in The Manuals for more details.
References
Jameson JL, De Groot LJ (eds): Endocrinology: Adult and Pediatric (pt 2), chapter 7. Philadelphia, Saunders/Elsevier, 2016, pp. 104-128.
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