When an elevated prolactin level is documented and medications or hypothyroidism as the underlying cause is excluded, knowledge of neuroanatomy and imaging techniques and their interpretation is essential to further evaluation (see Chapter 7). Pituitary hyperprolactinemia is most often caused by a microadenoma or associated with normal imaging findings. These patients can be reassured that the probable course of their condition is benign. Macroadenomas or juxtasellar lesions are less common and require more complex evaluation and treatment, including surgery, radiation, or both. Levels of TSH should be measured in all patients with hyperprolactinemia (Fig. 31.9).
Imaging Techniques In patients with larger microadenomas and macroadenomas, prolactin levels usually are higher than 100 ng/mL. However, levels lower than 100 ng/mL may be associated with smaller microadenomas, macroadenomas that produce a “stalk section” effect, and suprasellar tumors that may be missed on a “coned-down” view of the sella turcica. Modest elevations of prolactin can be associated with microadenomas or macroadenomas, nonlactotroph pituitary tumors, and other central nervous system abnormalities; thus, imaging of the pituitary gland must be considered when otherwise unexplained and persistent prolactin elevation is present. In patients with a clearly identifiable drug-induced or physiologic hyperprolactinemia, imaging is not necessary unless accompanied by symptoms suggesting a mass lesion (headache, visual field deficits). MRI with gadolinium enhancement of the sella and pituitary gland appears to provide the best anatomic detail (287). The cumulative radiation dose from multiple CT scans may cause cataracts, and the “coned-down” views or tomograms of the sella are very insensitive and expose the patient to radiation. For patients with hyperprolactinemia who desire future fertility, MRI is indicated to differentiate a pituitary microadenoma from a macroadenoma and to identify other potential sellar-suprasellar masses. Although rare, when pregnancy-related complications of a pituitary adenoma occur, they occur more frequently in the presence of macroadenomas.
In over 90% of untreated women, microadenomas do not enlarge over a 4-to 6-year period.
The argument that medical therapy will prevent a microadenoma from growing is false. Although prolactin levels correlate with tumor size, both elevations and reductions in prolactin levels may occur without any change in tumor size. If during follow-up a prolactin level rises significantly or central nervous system symptoms (headache, visual changes) are noted, repeat imaging may be indicated. Treatment is discussed below.
Hypothalamic Disorders Dopamine was the first of many substances whose production was demonstrated in the arcuate nucleus. Dopamine-releasing neurons innervate the external zone of the median eminence. When released into the hypophyseal portal system, dopamine inhibits prolactin release in the anterior pituitary. Lesions that disrupt dopamine release can result in hyperprolactinemia. Such lesions may arise from the suprasellar area, pituitary gland, and infundibular stalk, and from adjacent bone, brain, cranial nerves, dura, leptomeninges, nasopharynx, and vessels. Numerous pathologic entities and physiologic conditions in the hypothalamic–pituitary region can disrupt dopamine release and cause hyperprolactinemia.