MRI of the Pituitary (Sella Turcica)
After excluding medication, thyroid, and renal causes of elevated prolactin, you should order an MRI of the pituitary without gadolinium to evaluate for a prolactinoma or other sellar/parasellar mass 1.
Algorithmic Approach to Imaging
When to Order MRI
The Endocrine Society guidelines are explicit: once you've ruled out the common secondary causes (medications, hypothyroidism, renal failure), imaging is the next step 1. The decision pathway is:
- Prolactin >250 ng/mL: Proceed directly to MRI—this level is highly suggestive of a prolactinoma 2
- Prolactin 100-250 ng/mL: MRI is indicated after excluding secondary causes
- Prolactin <100 ng/mL: Still order MRI if symptomatic (amenorrhea, galactorrhea, hypogonadism) and secondary causes excluded 2
Important Pre-Imaging Considerations
Before ordering the MRI, you must screen for macroprolactin in asymptomatic patients 1. This is critical because approximately 40% of hyperprolactinemic patients have macroprolactinemia—a benign condition where large, biologically inactive prolactin polymers cause falsely elevated readings 1. Polyethylene glycol precipitation is an inexpensive screening method. However, if the patient is symptomatic, proceed with imaging even while awaiting macroprolactin results.
Technical Specifications
MRI without gadolinium is the imaging modality of choice 1. This is more sensitive and specific than CT scanning for detecting pituitary adenomas of all sizes 3. The guideline specifically recommends MRI without contrast, particularly important for pregnant patients but applicable generally 1.
Critical Pitfalls to Avoid
The Hook Effect
If you encounter a large pituitary mass (≥3 cm) with unexpectedly normal or only mildly elevated prolactin (≤250 ng/mL), you must consider the "hook effect" 1, 2. This immunoassay artifact produces falsely low prolactin readings. Order serial dilution of the serum sample (1:100 dilution) to unmask the true prolactin level 1. Missing this can lead to misdiagnosis of a large prolactinoma as a non-functioning adenoma.
Premature Imaging
Recent data shows that over 56% of MRIs for mild hyperprolactinemia are ordered prematurely without proper workup 4. This generates incidental findings of uncertain significance and wastes resources. Ensure you've completed the systematic exclusion of secondary causes first.
Clinical Context Matters
The magnitude of prolactin elevation guides interpretation:
- >250 ng/mL: Virtually diagnostic of prolactinoma; excludes non-functioning adenomas as the cause 2
- <100 ng/mL: Most commonly seen with non-functioning adenomas, medications, macroprolactinemia, or systemic disease 2
- Exception: Up to 25% of microprolactinomas or cystic macroprolactinomas may present with prolactin <100 ng/mL 2
The correlation between tumor size and prolactin level is generally reliable: macroprolactinomas (≥10 mm) typically produce prolactin >250 ng/mL 1.
What MRI Findings Mean
Once imaging is obtained, findings will direct management. Microadenomas (<10 mm) may not require immediate treatment if asymptomatic 1. Macroadenomas typically warrant dopamine agonist therapy. Any mass effect on the optic chiasm or cavernous sinus invasion requires urgent endocrinology referral.