Prolactinsecreting Tumors

Incidence

Over half of hormone-producing (functioning) tumors secrete the 198 amino acid lactotroph PRL, the only hormone under inhibitory control by dopamine from the hypothalamus. The clinical syndrome characterized by amenorrhea and galactorrhea was first described by Chiari and colleagues in 1855. PRL was not discovered as a human hormone until 1971.

Clinical Features

Ninety percent of such tumors are in females with secondary amenorrhea as the common presenting feature. Galactorrhea is not always present, perhaps because a permissive level of estrogen may be required for milk production. In men, impotence with decreased sperm count is the endocrinological equivalent. Women, possibly owing to a greater awareness of the effects of hypersecretion, tend to present at a younger age with microadenomas, while men present later in life with visual field disturbances.

Because of the inhibitory control of the hypothalamus, any mass lesion in this area may produce relative hyperprolactinemia. This is termed the "stalk effect". Generally, when this occurs, the levels are under 3000 mU/l (200 ng/ml). Many physiological events can cause hyperpro-lactinemia: pregnancy, lactation, stress, physical activity and nipple stimulation. Pharmacological agents such as dopamine receptor agonists (phenothiazines, metoclopramide), tricyclic

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