At what prolactin level, measured in micrograms per liter (ug/L), should a Magnetic Resonance Imaging (MRI) scan be considered to rule out a pituitary tumor?

From the Guidelines

An MRI scan should be considered to rule out a pituitary tumor when prolactin levels exceed 200 μg/L, as levels above this threshold strongly suggest the presence of a prolactin-secreting pituitary adenoma (prolactinoma) 1. For levels between 20-200 μg/L, clinical judgment is crucial, and an MRI may still be warranted based on symptoms and other findings, such as delayed puberty, galactorrhoea, visual field loss, growth or pubertal arrest, or menstrual disturbance 1. Mild elevations (20-50 μg/L) can occur due to medications, stress, pregnancy, or hypothyroidism, so these causes should be excluded first 1. It is also important to assess baseline macroprolactin levels where serum prolactin is found to be mildly or incidentally elevated, as macroprolactin can circulate with low biological activity and may not require immediate imaging 1. Key considerations in deciding when to perform an MRI include:

• The size of the tumor often correlates with prolactin levels - microadenomas (<10 mm) typically produce levels of 100-200 μg/L, while macroadenomas (>10 mm) can produce levels exceeding 250 μg/L or even into the thousands.
• The presence of symptoms such as headache, menstrual disturbance, short stature, or galactorrhoea, which may indicate a pituitary adenoma.
• The potential for the "hook effect" in very high prolactin levels, which can result in falsely low readings on some assays, requiring sample dilution for accurate measurement. Given the potential for significant morbidity and mortality associated with untreated pituitary tumors, a thorough evaluation, including MRI, is essential when prolactin levels exceed 200 μg/L or when clinical symptoms suggest a pituitary adenoma 1.

From the Research

Prolactin Levels and MRI Consideration

• The decision to consider an MRI scan to rule out a pituitary tumor is based on the level of prolactin in the blood, measured in micrograms per liter (ug/L) [(2,3,4)].
• A prolactin level above 200 ug/L is commonly considered indicative of a prolactin-secreting pituitary adenoma (prolactinoma) 2.
• However, other studies suggest that an MRI should be considered at lower prolactin levels, such as above 100 ng/ml (approximately 100 ug/L), especially in patients with symptoms such as headaches, visual field defects, or galactorrhea [(3,4)].
• The probability of the presence of an adenoma increases with rising serum prolactin levels 3.

Guidelines for Pituitary Imaging

• Pituitary imaging is indicated for the evaluation of hyperprolactinemia, and guidelines suggest that it should be considered in all patients with persistently elevated prolactin levels 4.
• The metoclopramide test may be a useful screening tool to avoid unnecessary MRI studies in patients with moderate hyperprolactinemia 5.
• However, current guidelines recommend prolactin assessment, considering values exceeding 200 ng/mL (approximately 200 ug/L) highly suggestive of prolactinoma 5.

Consideration of Symptoms and Tumor Size

• Symptoms such as infertility, headaches, oligoamenorrhea, galactorrhea, and visual changes may be associated with hyperprolactinemia and pituitary tumors 4.
• Tumor size correlates with the serum prolactin level, but some macroadenomas may be detected in patients with only moderately elevated prolactin values 4.
• Therefore, consideration of symptoms and tumor size is important in determining the need for an MRI scan [(2,3,4)].