Normal Prolactin Levels in ng/mL
Normal serum prolactin levels are approximately <20 ng/mL (or <425 mU/L) for non-pregnant adults, with adult women having slightly higher baseline levels than men, though specific laboratory reference ranges should be used as values differ by assay method. 1
Age and Sex-Specific Reference Ranges
Prolactin levels vary significantly with age and sex, being highest in the first 2 years of life, decreasing in mid-childhood, and increasing again in adolescence, with higher levels in girls than boys 1
For adult men, basal prolactin levels of at least 5 ng/mL can be used as a cut-off for normal prolactin reserve 2
For adult women, basal prolactin levels of at least 7 ng/mL can be used as a cut-off for normal prolactin reserve 2
The typical upper limit for non-pregnant adults is around 20 ng/mL, with adult women generally having higher baseline levels than men 1
Clinical Interpretation Thresholds
Mild elevation (<100 ng/mL or <2000 mU/L) can occur with non-functioning pituitary adenomas causing stalk compression, medications, stress, or other secondary causes 1
Levels exceeding 200 ng/mL generally indicate significant pituitary pathology and warrant imaging evaluation 3
In children and adolescents with prolactinomas, prolactin levels usually exceed 4,000 mU/L (approximately 188 ng/mL) 1
Important Diagnostic Considerations
Age-specific and sex-specific reference ranges must be used when interpreting prolactin levels, as concentrations vary substantially across different life stages 1, 4
Stress can elevate prolactin levels up to five times the upper limit of normal, so timing and collection conditions matter despite guidelines stating timing is not critical for diagnosis 1
For modestly elevated levels, consider serial measurements taken 20-60 minutes apart using an indwelling cannula to differentiate stress-related elevation from organic disease 4
The "high-dose hook effect" can cause falsely low measurements in patients with very high prolactin concentrations (typically with large prolactinomas), occurring in approximately 5% of macroprolactinomas—request manual dilution when large pituitary lesions show paradoxically normal or mildly elevated prolactin 5, 6