Macroprolactinemia presenting like a pituitary tumor.

A serum prolactin level greater than 200 ng/ml is almost pathognomonic of a pituitary tumor in a nonpregnant woman. A patient with a three-year history of documented serum prolactin levels of 350 to 400 ng/ml and no evidence of a pituitary adenoma on computed axial tomographic scanning was recently studied. Detailed evaluation included a 24-hour prolactin profile that revealed blunting of the nocturnal augmentation of prolactin release, low-dose dopamine infusion resulting in normal inhibition of prolactin secretion, and a thyrotropin-releasing hormone bolus that showed a blunted stimulation of prolactin release. Analysis of circulating prolactin by column chromatography of her serum revealed that greater than 85 percent of her circulating immunoreactive prolactin had a molecular weight greater than 100,000 daltons (macroprolactin). This contrasts with other hyperprolactinemic states in which 85 percent of the serum prolactin elutes in a 22,000-dalton peak. Hence, macroprolactinemia is apparently a nonprogressive condition and a novel cause of massive hyperprolactinemia.