From the Guidelines
Consider imaging for a prolactinoma when prolactin levels are above 2000 mU/l (or 94 μg/l), as recommended by the most recent consensus guideline 1. This threshold is suggested for adult patients with stalk effect, where prolactin levels are reported above the normal range but not higher than 2000 mU/l, even in the absence of corresponding symptoms of hyperprolactinaemia, hypopituitarism, or a pituitary mass. For mild elevations, it is essential to first rule out other causes such as:
- Medications (antipsychotics, metoclopramide, verapamil)
- Hypothyroidism
- Renal failure
- Liver disease
- Chest wall trauma
- Pregnancy If prolactin levels are moderately elevated without clear explanation, imaging may be warranted. The preferred imaging modality is an MRI of the pituitary with contrast, as it provides better soft tissue resolution than CT scans 1. When ordering imaging, also consider checking other pituitary hormones (TSH, free T4, cortisol, LH, FSH) to assess for hypopituitarism, which can occur with larger tumors. It is also crucial to remember that macroprolactinemia (biologically inactive prolactin bound to antibodies) can cause elevated readings without clinical significance. In cases with very large tumors but only modestly elevated prolactin, the "hook effect" should be considered, and serial dilution of the sample can reveal the true, much higher prolactin level. Given the potential for severe primary hypothyroidism to cause hyperprolactinaemia, it is vital to distinguish such pituitary enlargement from a true prolactinoma 1. Ultimately, the decision to pursue imaging should be based on a comprehensive evaluation of the patient's clinical presentation, medical history, and laboratory results.
From the Research
Prolactin Levels and Imaging for Prolactinoma
- Prolactin levels above 200 microg/L are commonly considered diagnostic for the presence of a prolactinoma 2, 3.
- However, prolactinoma cannot be excluded in the presence of lower levels, and prolactin levels > 100 micrograms/L are present in some patients with idiopathic hyperprolactinaemia 3.
- A study suggests that a prolactin level exceeding 200 ng/mL is highly suggestive of prolactinoma, but levels < 100 ng/mL can be found in patients with microprolactinomas 4.
Diagnostic Approaches
- The metoclopramide test can be used as a screening tool to identify patients with moderate hyperprolactinemia for whom MRI studies might be unnecessary 5.
- An increase in PRL (ΔPRLMax) after metoclopramide infusion can help identify patients with an adenoma, and a ΔPRLMax below 220% can identify patients without an adenoma with 71% specificity 5.
Imaging and Follow-up
- Pituitary magnetic resonance imaging (MRI) is necessary to define the presence of a lesion compatible with a pituitary tumour 3.
- However, regular MRI follow-up may be unnecessary in patients with macroprolactinomas and long-term normal prolactin levels on dopamine agonist treatment 6.
- A study found that no significant increase of tumor volume was observed during follow-up in patients with controlled prolactin levels on dopamine agonists, suggesting that MRI follow-up may not be necessary in these patients 6.