MRI of the Pituitary Gland
The next best test is MRI of the pituitary gland with gadolinium contrast to evaluate for a prolactinoma. 1, 2, 3
Rationale for MRI as the Next Step
With a prolactin level of 125 ng/ml (approximately 2,650 mIU/L), galactorrhea, amenorrhea, and normal TSH, this patient has confirmed pathological hyperprolactinemia that warrants imaging. 2, 3
Why MRI is Indicated Now
Prolactin levels >100 ng/ml strongly suggest a prolactinoma, making MRI the appropriate next diagnostic step after excluding hypothyroidism (which has been done with normal TSH). 4
MRI is superior to CT scanning for detecting pituitary adenomas, providing better definition of small lesions and superior anatomical detail. 5, 4
The degree of prolactin elevation correlates with tumor size: levels in this range (125 ng/ml) typically indicate either a microprolactinoma or small macroadenoma. 1
Technical Specifications for the MRI
Order T1- and T2-weighted coronal sections with gadolinium enhancement for optimal visualization of microadenomas. 4
If a large lesion is found, include sagittal and axial sections to fully characterize the mass and assess for compression of surrounding structures. 4
Critical Pitfall to Avoid: The Hook Effect
Request serial dilutions of the serum sample if MRI reveals a large pituitary mass but prolactin seems disproportionately low for the tumor size. 1, 6
The "hook effect" occurs in approximately 5% of macroprolactinomas, where extremely high prolactin concentrations saturate the immunoassay, producing falsely low measurements. 1, 6
This can lead to misdiagnosis of a prolactinoma as a non-functioning adenoma if not recognized. 6
Additional Considerations Before MRI
Confirm this is not macroprolactinemia if there were any doubt about the clinical picture, though with clear symptoms (galactorrhea and amenorrhea) and this degree of elevation, macroprolactinemia is unlikely. 1, 2
Verify medication history to exclude drug-induced hyperprolactinemia, particularly dopamine antagonists, antipsychotics, and certain antihypertensives. 2, 3
Exclude pregnancy with a pregnancy test, as this is a physiological cause of hyperprolactinemia that would not require MRI. 4
What Happens After MRI
If a prolactinoma is confirmed, cabergoline will be first-line therapy to normalize prolactin levels, shrink the tumor, and restore menstrual function. 1
Visual field testing should be performed if a macroadenoma is found, as compression of the optic chiasm can occur. 1, 7
Serial prolactin measurements will guide treatment response, with the goal of normalization and tumor shrinkage. 3