Normal Serum Prolactin Levels
Normal serum prolactin levels vary by sex and age, with typical reference ranges being approximately <20 μg/L (or <425 mU/L) for non-pregnant adults, though specific laboratory reference ranges should be used as values differ by assay method. 1
Age and Sex-Specific Variations
Prolactin levels are not static across the lifespan and demonstrate important physiological variations:
- Infancy and early childhood: Prolactin levels are highest in the first 2 years of life, then decrease during mid-childhood 2
- Adolescence: Levels rise again during puberty, with girls demonstrating higher concentrations than boys 2
- Adult women: Generally have higher baseline levels than men, with typical upper limits around 20 μg/L in non-pregnant states 1
- Adult men: Basal prolactin levels of at least 5 ng/mL are considered normal, while levels of at least 7 ng/mL are normal for women 3
Pregnancy-Specific Reference Intervals
Prolactin levels increase dramatically during pregnancy and require trimester-specific interpretation 4:
- First trimester: 621.20-3584.00 μIU/mL 4
- Second trimester: 1432.00-5349.68 μIU/mL 4
- Third trimester: 4087.33-9733.65 μIU/mL 4
- Postpartum (24-48 hours after vaginal delivery): 7865.36-10998.86 μIU/mL 4
Clinical Interpretation Thresholds
When evaluating elevated prolactin, specific thresholds help distinguish pathological causes:
- Mild elevation (<100 μg/L or <2000 mU/L): Can occur with non-functioning pituitary adenomas causing stalk compression, medications, stress, or other secondary causes 1, 5, 6
- Moderate elevation (100-1000 μg/L): Strongly suggests a prolactin-producing tumor 5
- Marked elevation (>1000 μg/L or >4000 mU/L): Highly specific for prolactinoma, particularly in pediatric populations 7, 5
Important Caveats for Measurement
A single prolactin measurement taken at any time of day is sufficient for initial assessment, as timing is not critical for diagnosis. 7, 8
However, several factors can confound interpretation:
- Stress-induced elevation: Can raise prolactin up to five times the upper limit of normal 2. For modestly elevated levels, consider repeat sampling 20-60 minutes apart using an indwelling cannula to exclude stress-related elevation 8
- Macroprolactinemia: Accounts for 10-40% of hyperprolactinemia cases in adults and represents biologically inactive prolactin complexes 8. Screen for this in cases of mild or incidental elevation 8
- High-dose hook effect: In patients with large pituitary lesions but paradoxically normal or mildly elevated prolactin, perform serial dilutions to detect this laboratory artifact 8
Sex-Specific Diagnostic Thresholds
Gender influences both basal levels and stimulation test responses 3: